v4.1.0

feat(readme): expand README with storage integrity, WAL, query planner, session & transaction docs; update test script to enable verbose logging and increase timeout
v4.0.0
2026-02-01 16:05:00 +00:00 · 2026-02-01 16:05:00 +00:00 · 2026-02-01 16:02:03 +00:00 · 2026-02-01 16:02:03 +00:00
21 changed files with 2124 additions and 105 deletions
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,27 @@
 # Changelog

+## 2026-02-01 - 4.1.0 - feat(readme)
+expand README with storage integrity, WAL, query planner, session & transaction docs; update test script to enable verbose logging and increase timeout
+
+- Updated npm test script to run tstest with --verbose, --logfile and --timeout 60 to improve test output and avoid timeouts.
+- Extensive README additions: file storage adapter examples with checksum options, write-ahead logging (WAL) usage and recovery, query planner examples, index and query execution details, session and transaction examples and features.
+- Wire protocol / features table updated to include Transactions and Sessions and added admin commands (dbStats, collStats).
+- Architecture diagram and component list updated to include QueryPlanner, SessionEngine, TransactionEngine and WAL; storage layer annotated with checksums and WAL.
+- Minor example import tweak: MongoClient import now includes Db type in test examples.
+
+## 2026-02-01 - 4.0.0 - BREAKING CHANGE(storage,engine,server)
+add session & transaction management, index/query planner, WAL and checksum support; integrate index-accelerated queries and update storage API (findByIds) to enable index optimizations
+
+- Add SessionEngine with session lifecycle, auto-abort of transactions on expiry and session tracking in CommandRouter and AdminHandler.
+- Introduce TransactionEngine integrations in CommandRouter and AdminHandler; handlers now support start/commit/abort transaction workflows.
+- Add IndexEngine enhancements including a simple B-tree and hash map optimizations; integrate index usage into Find/Count/Insert/Update/Delete handlers for index-accelerated queries and index maintenance on mutations.
+- Add QueryPlanner to choose IXSCAN vs COLLSCAN and provide explain plans.
+- Add WAL (write-ahead log) for durability, with LSNs, checkpoints and recovery APIs.
+- Add checksum utilities and FileStorageAdapter support for checksums (enableChecksums/strictChecksums), with verification on read and optional strict failure behavior.
+- IStorageAdapter interface changed to include findByIds; MemoryStorageAdapter and FileStorageAdapter implement findByIds to support index lookups.
+- Exported API additions: WAL, QueryPlanner, SessionEngine, checksum utilities; CommandRouter now caches IndexEngines and exposes transaction/session engines.
+- Breaking change: the IStorageAdapter interface change requires third-party storage adapters to implement the new findByIds method.
+
 ## 2026-02-01 - 3.0.0 - BREAKING CHANGE(tsmdb)
 rename CongoDB to TsmDB and relocate/rename wire-protocol server implementation and public exports

--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
  "name": "@push.rocks/smartmongo",
-  "version": "3.0.0",
+  "version": "4.1.0",
  "private": false,
  "description": "A module for creating and managing a local MongoDB instance for testing purposes.",
  "main": "dist_ts/index.js",
@@ -9,7 +9,7 @@
  "author": "Lossless GmbH",
  "license": "MIT",
  "scripts": {
-    "test": "(tstest test/)",
+    "test": "(tstest test/. --verbose --logfile --timeout 60)",
    "build": "(tsbuild --web)",
    "buildDocs": "tsdoc"
  },
--- a/readme.md
+++ b/readme.md
@@ -134,8 +134,8 @@ await server.start();
 console.log(server.getConnectionUri()); // mongodb://127.0.0.1:27017

 // Server properties
-console.log(server.running);           // true
-console.log(server.getUptime());       // seconds
+console.log(server.running);              // true
+console.log(server.getUptime());          // seconds
 console.log(server.getConnectionCount()); // active connections

 await server.stop();
@@ -279,7 +279,7 @@ console.log(result.deletedCount);   // 1

 ### Storage Adapters

-TsmDB supports pluggable storage:
+TsmDB supports pluggable storage with data integrity features:

 ```typescript
 // In-memory (default) - fast, data lost on stop
@@ -292,13 +292,118 @@ const server = new tsmdb.TsmdbServer({
  persistIntervalMs: 30000  // Save every 30 seconds
 });

-// File-based - persistent storage
-const server = new tsmdb.TsmdbServer({
-  storage: 'file',
-  storagePath: './data/tsmdb'
+// File-based - persistent storage with optional checksums
+import { FileStorageAdapter } from '@push.rocks/smartmongo/tsmdb';
+
+const adapter = new FileStorageAdapter('./data/tsmdb', {
+  enableChecksums: true,   // CRC32 checksums for data integrity
+  strictChecksums: false   // Log warnings vs throw on mismatch
 });
 ```

+## ⚡ Performance & Reliability Features
+
+TsmDB includes enterprise-grade features for robustness:
+
+### 🔍 Index-Accelerated Queries
+
+Indexes are automatically used to accelerate queries. Instead of scanning all documents, TsmDB uses:
+
+- **Hash indexes** for equality queries (`$eq`, `$in`)
+- **B-tree indexes** for range queries (`$gt`, `$gte`, `$lt`, `$lte`)
+
+```typescript
+// Create an index
+await collection.createIndex({ email: 1 });
+await collection.createIndex({ age: 1 });
+
+// These queries will use the index (fast!)
+await collection.findOne({ email: 'alice@example.com' }); // Uses hash lookup
+await collection.find({ age: { $gte: 18, $lt: 65 } });    // Uses B-tree range scan
+```
+
+### 📊 Query Planner
+
+TsmDB includes a query planner that analyzes queries and selects optimal execution strategies:
+
+```typescript
+import { tsmdb } from '@push.rocks/smartmongo';
+
+// For debugging, you can access the query planner
+const planner = new tsmdb.QueryPlanner(indexEngine);
+const plan = planner.createPlan(filter);
+
+console.log(plan);
+// {
+//   type: 'IXSCAN',           // or 'IXSCAN_RANGE', 'COLLSCAN'
+//   indexName: 'email_1',
+//   estimatedCost: 1,
+//   selectivity: 0.001
+// }
+```
+
+### 📝 Write-Ahead Logging (WAL)
+
+For durability, TsmDB supports write-ahead logging:
+
+```typescript
+import { tsmdb } from '@push.rocks/smartmongo';
+
+const wal = new tsmdb.WAL('./data/wal.log');
+await wal.initialize();
+
+// WAL entries include:
+// - LSN (Log Sequence Number)
+// - Timestamp
+// - Operation type (insert, update, delete, checkpoint)
+// - Document data (BSON serialized)
+// - CRC32 checksum
+
+// Recovery support
+const entries = await wal.getEntriesAfter(lastCheckpointLsn);
+```
+
+### 🔐 Session Management
+
+TsmDB tracks client sessions with automatic timeout and transaction linking:
+
+```typescript
+// Sessions are automatically managed when using the MongoDB driver
+const session = client.startSession();
+
+try {
+  session.startTransaction();
+  await collection.insertOne({ name: 'Alice' }, { session });
+  await collection.updateOne({ name: 'Bob' }, { $inc: { balance: 100 } }, { session });
+  await session.commitTransaction();
+} catch (error) {
+  await session.abortTransaction();
+} finally {
+  session.endSession();
+}
+
+// Session features:
+// - Automatic session timeout (30 minutes default)
+// - Transaction auto-abort on session expiry
+// - Session activity tracking
+```
+
+### ✅ Data Integrity Checksums
+
+File-based storage supports CRC32 checksums to detect corruption:
+
+```typescript
+import { FileStorageAdapter } from '@push.rocks/smartmongo/tsmdb';
+
+const adapter = new FileStorageAdapter('./data', {
+  enableChecksums: true,
+  strictChecksums: true  // Throw error on corruption (vs warning)
+});
+
+// Documents are checksummed on write, verified on read
+// Checksums are automatically stripped before returning to client
+```
+
 ### 📋 Supported Wire Protocol Commands

 | Category | Commands |
@@ -307,7 +412,9 @@ const server = new tsmdb.TsmdbServer({
 | **CRUD** | `find`, `insert`, `update`, `delete`, `findAndModify`, `getMore`, `killCursors` |
 | **Aggregation** | `aggregate`, `count`, `distinct` |
 | **Indexes** | `createIndexes`, `dropIndexes`, `listIndexes` |
-| **Admin** | `ping`, `listDatabases`, `listCollections`, `drop`, `dropDatabase`, `create`, `serverStatus`, `buildInfo` |
+| **Transactions** | `startTransaction`, `commitTransaction`, `abortTransaction` |
+| **Sessions** | `startSession`, `endSessions` |
+| **Admin** | `ping`, `listDatabases`, `listCollections`, `drop`, `dropDatabase`, `create`, `serverStatus`, `buildInfo`, `dbStats`, `collStats` |

 TsmDB supports MongoDB wire protocol versions 0-21, compatible with MongoDB 3.6 through 7.0 drivers.

@@ -317,7 +424,7 @@ TsmDB supports MongoDB wire protocol versions 0-21, compatible with MongoDB 3.6

 ```typescript
 import { tsmdb } from '@push.rocks/smartmongo';
-import { MongoClient } from 'mongodb';
+import { MongoClient, Db } from 'mongodb';

 let server: tsmdb.TsmdbServer;
 let client: MongoClient;
@@ -421,21 +528,37 @@ export default tap.start();
                          ▼
 ┌─────────────────────────────────────────────────────────────┐
 │                       Engines                               │
-│  ┌───────────┐ ┌────────────┐ ┌────────────┐ ┌───────────┐ │
-│  │  Query    │ │   Update   │ │Aggregation │ │   Index   │ │
-│  │  Engine   │ │   Engine   │ │  Engine    │ │  Engine   │ │
-│  └───────────┘ └────────────┘ └────────────┘ └───────────┘ │
+│  ┌─────────┐ ┌────────┐ ┌───────────┐ ┌───────┐ ┌───────┐ │
+│  │ Query   │ │ Update │ │Aggregation│ │ Index │ │Session│ │
+│  │ Planner │ │ Engine │ │  Engine   │ │Engine │ │Engine │ │
+│  └─────────┘ └────────┘ └───────────┘ └───────┘ └───────┘ │
+│                   ┌──────────────────────┐                  │
+│                   │ Transaction Engine   │                  │
+│                   └──────────────────────┘                  │
 └─────────────────────────┬───────────────────────────────────┘
                          │
                          ▼
 ┌─────────────────────────────────────────────────────────────┐
-│                   Storage Adapters                          │
-│       ┌──────────────────┐  ┌──────────────────┐           │
-│       │  MemoryStorage   │  │   FileStorage    │           │
-│       └──────────────────┘  └──────────────────┘           │
+│                   Storage Layer                              │
+│  ┌──────────────────┐  ┌──────────────────┐  ┌──────────┐  │
+│  │  MemoryStorage   │  │   FileStorage    │  │   WAL    │  │
+│  │                  │  │  (+ Checksums)   │  │          │  │
+│  └──────────────────┘  └──────────────────┘  └──────────┘  │
 └─────────────────────────────────────────────────────────────┘
 ```

+### Key Components
+
+| Component | Description |
+|-----------|-------------|
+| **WireProtocol** | Parses MongoDB OP_MSG binary protocol |
+| **CommandRouter** | Routes commands to appropriate handlers |
+| **QueryPlanner** | Analyzes queries and selects execution strategy |
+| **IndexEngine** | Manages B-tree and hash indexes |
+| **SessionEngine** | Tracks client sessions and timeouts |
+| **TransactionEngine** | Handles ACID transaction semantics |
+| **WAL** | Write-ahead logging for durability |
+
 ## License and Legal Information

 This repository contains open-source code licensed under the MIT License. A copy of the license can be found in the [LICENSE](./LICENSE) file.
--- a/ts/00_commitinfo_data.ts
+++ b/ts/00_commitinfo_data.ts
@@ -3,6 +3,6 @@
 */
 export const commitinfo = {
  name: '@push.rocks/smartmongo',
-  version: '3.0.0',
+  version: '4.1.0',
  description: 'A module for creating and managing a local MongoDB instance for testing purposes.'
 }
--- a/ts/tsmdb/engine/IndexEngine.ts
+++ b/ts/tsmdb/engine/IndexEngine.ts
@@ -1,5 +1,89 @@
 import * as plugins from '../tsmdb.plugins.js';
 import type { IStorageAdapter } from '../storage/IStorageAdapter.js';
+
+// Simple B-Tree implementation for range queries
+// Since sorted-btree has ESM/CJS interop issues, we use a simple custom implementation
+class SimpleBTree<K, V> {
+  private entries: Map<string, { key: K; value: V }> = new Map();
+  private sortedKeys: K[] = [];
+  private comparator: (a: K, b: K) => number;
+
+  constructor(_unused?: undefined, comparator?: (a: K, b: K) => number) {
+    this.comparator = comparator || ((a: K, b: K) => {
+      if (a < b) return -1;
+      if (a > b) return 1;
+      return 0;
+    });
+  }
+
+  private keyToString(key: K): string {
+    return JSON.stringify(key);
+  }
+
+  set(key: K, value: V): boolean {
+    const keyStr = this.keyToString(key);
+    const existed = this.entries.has(keyStr);
+    this.entries.set(keyStr, { key, value });
+
+    if (!existed) {
+      // Insert in sorted order
+      const idx = this.sortedKeys.findIndex(k => this.comparator(k, key) > 0);
+      if (idx === -1) {
+        this.sortedKeys.push(key);
+      } else {
+        this.sortedKeys.splice(idx, 0, key);
+      }
+    }
+    return !existed;
+  }
+
+  get(key: K): V | undefined {
+    const entry = this.entries.get(this.keyToString(key));
+    return entry?.value;
+  }
+
+  delete(key: K): boolean {
+    const keyStr = this.keyToString(key);
+    if (this.entries.has(keyStr)) {
+      this.entries.delete(keyStr);
+      const idx = this.sortedKeys.findIndex(k => this.comparator(k, key) === 0);
+      if (idx !== -1) {
+        this.sortedKeys.splice(idx, 1);
+      }
+      return true;
+    }
+    return false;
+  }
+
+  forRange(
+    lowKey: K | undefined,
+    highKey: K | undefined,
+    lowInclusive: boolean,
+    highInclusive: boolean,
+    callback: (value: V, key: K) => void
+  ): void {
+    for (const key of this.sortedKeys) {
+      // Check low bound
+      if (lowKey !== undefined) {
+        const cmp = this.comparator(key, lowKey);
+        if (cmp < 0) continue;
+        if (cmp === 0 && !lowInclusive) continue;
+      }
+
+      // Check high bound
+      if (highKey !== undefined) {
+        const cmp = this.comparator(key, highKey);
+        if (cmp > 0) break;
+        if (cmp === 0 && !highInclusive) break;
+      }
+
+      const entry = this.entries.get(this.keyToString(key));
+      if (entry) {
+        callback(entry.value, key);
+      }
+    }
+  }
+}
 import type {
  Document,
  IStoredDocument,
@@ -11,7 +95,61 @@ import { TsmdbDuplicateKeyError, TsmdbIndexError } from '../errors/TsmdbErrors.j
 import { QueryEngine } from './QueryEngine.js';

 /**
- * Index data structure for fast lookups
+ * Comparator for B-Tree that handles mixed types consistently
+ */
+function indexKeyComparator(a: any, b: any): number {
+  // Handle null/undefined
+  if (a === null || a === undefined) {
+    if (b === null || b === undefined) return 0;
+    return -1;
+  }
+  if (b === null || b === undefined) return 1;
+
+  // Handle arrays (compound keys)
+  if (Array.isArray(a) && Array.isArray(b)) {
+    for (let i = 0; i < Math.max(a.length, b.length); i++) {
+      const cmp = indexKeyComparator(a[i], b[i]);
+      if (cmp !== 0) return cmp;
+    }
+    return 0;
+  }
+
+  // Handle ObjectId
+  if (a instanceof plugins.bson.ObjectId && b instanceof plugins.bson.ObjectId) {
+    return a.toHexString().localeCompare(b.toHexString());
+  }
+
+  // Handle Date
+  if (a instanceof Date && b instanceof Date) {
+    return a.getTime() - b.getTime();
+  }
+
+  // Handle different types - use type ordering (null < number < string < object)
+  const typeOrder = (v: any): number => {
+    if (v === null || v === undefined) return 0;
+    if (typeof v === 'number') return 1;
+    if (typeof v === 'string') return 2;
+    if (typeof v === 'boolean') return 3;
+    if (v instanceof Date) return 4;
+    if (v instanceof plugins.bson.ObjectId) return 5;
+    return 6;
+  };
+
+  const typeA = typeOrder(a);
+  const typeB = typeOrder(b);
+  if (typeA !== typeB) return typeA - typeB;
+
+  // Same type comparison
+  if (typeof a === 'number') return a - b;
+  if (typeof a === 'string') return a.localeCompare(b);
+  if (typeof a === 'boolean') return (a ? 1 : 0) - (b ? 1 : 0);
+
+  // Fallback to string comparison
+  return String(a).localeCompare(String(b));
+}
+
+/**
+ * Index data structure using B-Tree for range queries
 */
 interface IIndexData {
  name: string;
@@ -19,8 +157,10 @@ interface IIndexData {
  unique: boolean;
  sparse: boolean;
  expireAfterSeconds?: number;
-  // Map from index key value to document _id(s)
-  entries: Map<string, Set<string>>;
+  // B-Tree for ordered index lookups (supports range queries)
+  btree: SimpleBTree<any, Set<string>>;
+  // Hash map for fast equality lookups
+  hashMap: Map<string, Set<string>>;
 }

 /**
@@ -55,7 +195,8 @@ export class IndexEngine {
        unique: indexSpec.unique || false,
        sparse: indexSpec.sparse || false,
        expireAfterSeconds: indexSpec.expireAfterSeconds,
-        entries: new Map(),
+        btree: new SimpleBTree<any, Set<string>>(undefined, indexKeyComparator),
+        hashMap: new Map(),
      };

      // Build index entries
@@ -63,10 +204,20 @@ export class IndexEngine {
        const keyValue = this.extractKeyValue(doc, indexSpec.key);
        if (keyValue !== null || !indexData.sparse) {
          const keyStr = JSON.stringify(keyValue);
-          if (!indexData.entries.has(keyStr)) {
-            indexData.entries.set(keyStr, new Set());
+
+          // Add to hash map
+          if (!indexData.hashMap.has(keyStr)) {
+            indexData.hashMap.set(keyStr, new Set());
+          }
+          indexData.hashMap.get(keyStr)!.add(doc._id.toHexString());
+
+          // Add to B-tree
+          const existing = indexData.btree.get(keyValue);
+          if (existing) {
+            existing.add(doc._id.toHexString());
+          } else {
+            indexData.btree.set(keyValue, new Set([doc._id.toHexString()]));
          }
-          indexData.entries.get(keyStr)!.add(doc._id.toHexString());
        }
      }

@@ -100,7 +251,8 @@ export class IndexEngine {
      unique: options?.unique || false,
      sparse: options?.sparse || false,
      expireAfterSeconds: options?.expireAfterSeconds,
-      entries: new Map(),
+      btree: new SimpleBTree<any, Set<string>>(undefined, indexKeyComparator),
+      hashMap: new Map(),
    };

    // Build index from existing documents
@@ -115,7 +267,7 @@ export class IndexEngine {

      const keyStr = JSON.stringify(keyValue);

-      if (indexData.unique && indexData.entries.has(keyStr)) {
+      if (indexData.unique && indexData.hashMap.has(keyStr)) {
        throw new TsmdbDuplicateKeyError(
          `E11000 duplicate key error index: ${this.dbName}.${this.collName}.$${name}`,
          key as Record<string, 1>,
@@ -123,10 +275,19 @@ export class IndexEngine {
        );
      }

-      if (!indexData.entries.has(keyStr)) {
-        indexData.entries.set(keyStr, new Set());
+      // Add to hash map
+      if (!indexData.hashMap.has(keyStr)) {
+        indexData.hashMap.set(keyStr, new Set());
+      }
+      indexData.hashMap.get(keyStr)!.add(doc._id.toHexString());
+
+      // Add to B-tree
+      const existing = indexData.btree.get(keyValue);
+      if (existing) {
+        existing.add(doc._id.toHexString());
+      } else {
+        indexData.btree.set(keyValue, new Set([doc._id.toHexString()]));
      }
-      indexData.entries.get(keyStr)!.add(doc._id.toHexString());
    }

    // Store index
@@ -213,7 +374,7 @@ export class IndexEngine {

      // Check unique constraint
      if (indexData.unique) {
-        const existing = indexData.entries.get(keyStr);
+        const existing = indexData.hashMap.get(keyStr);
        if (existing && existing.size > 0) {
          throw new TsmdbDuplicateKeyError(
            `E11000 duplicate key error collection: ${this.dbName}.${this.collName} index: ${name}`,
@@ -223,10 +384,19 @@ export class IndexEngine {
        }
      }

-      if (!indexData.entries.has(keyStr)) {
-        indexData.entries.set(keyStr, new Set());
+      // Add to hash map
+      if (!indexData.hashMap.has(keyStr)) {
+        indexData.hashMap.set(keyStr, new Set());
+      }
+      indexData.hashMap.get(keyStr)!.add(doc._id.toHexString());
+
+      // Add to B-tree
+      const btreeSet = indexData.btree.get(keyValue);
+      if (btreeSet) {
+        btreeSet.add(doc._id.toHexString());
+      } else {
+        indexData.btree.set(keyValue, new Set([doc._id.toHexString()]));
      }
-      indexData.entries.get(keyStr)!.add(doc._id.toHexString());
    }
  }

@@ -245,11 +415,21 @@ export class IndexEngine {
      // Remove old entry if key changed
      if (oldKeyStr !== newKeyStr) {
        if (oldKeyValue !== null || !indexData.sparse) {
-          const oldSet = indexData.entries.get(oldKeyStr);
-          if (oldSet) {
-            oldSet.delete(oldDoc._id.toHexString());
-            if (oldSet.size === 0) {
-              indexData.entries.delete(oldKeyStr);
+          // Remove from hash map
+          const oldHashSet = indexData.hashMap.get(oldKeyStr);
+          if (oldHashSet) {
+            oldHashSet.delete(oldDoc._id.toHexString());
+            if (oldHashSet.size === 0) {
+              indexData.hashMap.delete(oldKeyStr);
+            }
+          }
+
+          // Remove from B-tree
+          const oldBtreeSet = indexData.btree.get(oldKeyValue);
+          if (oldBtreeSet) {
+            oldBtreeSet.delete(oldDoc._id.toHexString());
+            if (oldBtreeSet.size === 0) {
+              indexData.btree.delete(oldKeyValue);
            }
          }
        }
@@ -258,7 +438,7 @@ export class IndexEngine {
        if (newKeyValue !== null || !indexData.sparse) {
          // Check unique constraint
          if (indexData.unique) {
-            const existing = indexData.entries.get(newKeyStr);
+            const existing = indexData.hashMap.get(newKeyStr);
            if (existing && existing.size > 0) {
              throw new TsmdbDuplicateKeyError(
                `E11000 duplicate key error collection: ${this.dbName}.${this.collName} index: ${name}`,
@@ -268,10 +448,19 @@ export class IndexEngine {
            }
          }

-          if (!indexData.entries.has(newKeyStr)) {
-            indexData.entries.set(newKeyStr, new Set());
+          // Add to hash map
+          if (!indexData.hashMap.has(newKeyStr)) {
+            indexData.hashMap.set(newKeyStr, new Set());
+          }
+          indexData.hashMap.get(newKeyStr)!.add(newDoc._id.toHexString());
+
+          // Add to B-tree
+          const newBtreeSet = indexData.btree.get(newKeyValue);
+          if (newBtreeSet) {
+            newBtreeSet.add(newDoc._id.toHexString());
+          } else {
+            indexData.btree.set(newKeyValue, new Set([newDoc._id.toHexString()]));
          }
-          indexData.entries.get(newKeyStr)!.add(newDoc._id.toHexString());
        }
      }
    }
@@ -291,11 +480,22 @@ export class IndexEngine {
      }

      const keyStr = JSON.stringify(keyValue);
-      const set = indexData.entries.get(keyStr);
-      if (set) {
-        set.delete(doc._id.toHexString());
-        if (set.size === 0) {
-          indexData.entries.delete(keyStr);
+
+      // Remove from hash map
+      const hashSet = indexData.hashMap.get(keyStr);
+      if (hashSet) {
+        hashSet.delete(doc._id.toHexString());
+        if (hashSet.size === 0) {
+          indexData.hashMap.delete(keyStr);
+        }
+      }
+
+      // Remove from B-tree
+      const btreeSet = indexData.btree.get(keyValue);
+      if (btreeSet) {
+        btreeSet.delete(doc._id.toHexString());
+        if (btreeSet.size === 0) {
+          indexData.btree.delete(keyValue);
        }
      }
    }
@@ -309,8 +509,8 @@ export class IndexEngine {
      return null;
    }

-    // Get filter fields
-    const filterFields = new Set(this.getFilterFields(filter));
+    // Get filter fields and operators
+    const filterInfo = this.analyzeFilter(filter);

    // Score each index
    let bestIndex: { name: string; data: IIndexData } | null = null;
@@ -320,12 +520,21 @@ export class IndexEngine {
      const indexFields = Object.keys(indexData.key);
      let score = 0;

-      // Count how many index fields are in the filter
+      // Count how many index fields can be used
      for (const field of indexFields) {
-        if (filterFields.has(field)) {
-          score++;
+        const info = filterInfo.get(field);
+        if (!info) break;
+
+        // Equality is best
+        if (info.equality) {
+          score += 2;
+        } else if (info.range) {
+          // Range queries can use B-tree
+          score += 1;
+        } else if (info.in) {
+          score += 1.5;
        } else {
-          break; // Index fields must be contiguous
+          break;
        }
      }

@@ -344,7 +553,46 @@ export class IndexEngine {
  }

  /**
-   * Use index to find candidate document IDs
+   * Analyze filter to extract field operators
+   */
+  private analyzeFilter(filter: Document): Map<string, { equality: boolean; range: boolean; in: boolean; ops: Record<string, any> }> {
+    const result = new Map<string, { equality: boolean; range: boolean; in: boolean; ops: Record<string, any> }>();
+
+    for (const [key, value] of Object.entries(filter)) {
+      if (key.startsWith('$')) continue;
+
+      const info = { equality: false, range: false, in: false, ops: {} as Record<string, any> };
+
+      if (typeof value !== 'object' || value === null || value instanceof plugins.bson.ObjectId || value instanceof Date) {
+        info.equality = true;
+        info.ops['$eq'] = value;
+      } else {
+        const ops = value as Record<string, any>;
+        if (ops.$eq !== undefined) {
+          info.equality = true;
+          info.ops['$eq'] = ops.$eq;
+        }
+        if (ops.$in !== undefined) {
+          info.in = true;
+          info.ops['$in'] = ops.$in;
+        }
+        if (ops.$gt !== undefined || ops.$gte !== undefined || ops.$lt !== undefined || ops.$lte !== undefined) {
+          info.range = true;
+          if (ops.$gt !== undefined) info.ops['$gt'] = ops.$gt;
+          if (ops.$gte !== undefined) info.ops['$gte'] = ops.$gte;
+          if (ops.$lt !== undefined) info.ops['$lt'] = ops.$lt;
+          if (ops.$lte !== undefined) info.ops['$lte'] = ops.$lte;
+        }
+      }
+
+      result.set(key, info);
+    }
+
+    return result;
+  }
+
+  /**
+   * Use index to find candidate document IDs (supports range queries with B-tree)
   */
  async findCandidateIds(filter: Document): Promise<Set<string> | null> {
    await this.initialize();
@@ -352,25 +600,58 @@ export class IndexEngine {
    const index = this.selectIndex(filter);
    if (!index) return null;

-    // Try to use the index for equality matches
+    const filterInfo = this.analyzeFilter(filter);
    const indexFields = Object.keys(index.data.key);
-    const equalityValues: Record<string, any> = {};

-    for (const field of indexFields) {
-      const filterValue = this.getFilterValue(filter, field);
-      if (filterValue === undefined) break;
+    // For single-field indexes with range queries, use B-tree
+    if (indexFields.length === 1) {
+      const field = indexFields[0];
+      const info = filterInfo.get(field);

-      // Only use equality matches for index lookup
-      if (typeof filterValue === 'object' && filterValue !== null) {
-        if (filterValue.$eq !== undefined) {
-          equalityValues[field] = filterValue.$eq;
-        } else if (filterValue.$in !== undefined) {
+      if (info) {
+        // Handle equality using hash map (faster)
+        if (info.equality) {
+          const keyStr = JSON.stringify(info.ops['$eq']);
+          return index.data.hashMap.get(keyStr) || new Set();
+        }
+
+        // Handle $in using hash map
+        if (info.in) {
+          const results = new Set<string>();
+          for (const val of info.ops['$in']) {
+            const keyStr = JSON.stringify(val);
+            const ids = index.data.hashMap.get(keyStr);
+            if (ids) {
+              for (const id of ids) {
+                results.add(id);
+              }
+            }
+          }
+          return results;
+        }
+
+        // Handle range queries using B-tree
+        if (info.range) {
+          return this.findRangeCandidates(index.data, info.ops);
+        }
+      }
+    } else {
+      // For compound indexes, use hash map with partial key matching
+      const equalityValues: Record<string, any> = {};
+
+      for (const field of indexFields) {
+        const info = filterInfo.get(field);
+        if (!info) break;
+
+        if (info.equality) {
+          equalityValues[field] = info.ops['$eq'];
+        } else if (info.in) {
          // Handle $in with multiple lookups
          const results = new Set<string>();
-          for (const val of filterValue.$in) {
+          for (const val of info.ops['$in']) {
            equalityValues[field] = val;
            const keyStr = JSON.stringify(this.buildKeyValue(equalityValues, index.data.key));
-            const ids = index.data.entries.get(keyStr);
+            const ids = index.data.hashMap.get(keyStr);
            if (ids) {
              for (const id of ids) {
                results.add(id);
@@ -379,19 +660,57 @@ export class IndexEngine {
          }
          return results;
        } else {
-          break; // Non-equality operator, stop here
+          break; // Non-equality/in operator, stop here
        }
-      } else {
-        equalityValues[field] = filterValue;
+      }
+
+      if (Object.keys(equalityValues).length > 0) {
+        const keyStr = JSON.stringify(this.buildKeyValue(equalityValues, index.data.key));
+        return index.data.hashMap.get(keyStr) || new Set();
      }
    }

-    if (Object.keys(equalityValues).length === 0) {
-      return null;
+    return null;
+  }
+
+  /**
+   * Find candidates using B-tree range scan
+   */
+  private findRangeCandidates(indexData: IIndexData, ops: Record<string, any>): Set<string> {
+    const results = new Set<string>();
+
+    let lowKey: any = undefined;
+    let highKey: any = undefined;
+    let lowInclusive = true;
+    let highInclusive = true;
+
+    if (ops['$gt'] !== undefined) {
+      lowKey = ops['$gt'];
+      lowInclusive = false;
+    }
+    if (ops['$gte'] !== undefined) {
+      lowKey = ops['$gte'];
+      lowInclusive = true;
+    }
+    if (ops['$lt'] !== undefined) {
+      highKey = ops['$lt'];
+      highInclusive = false;
+    }
+    if (ops['$lte'] !== undefined) {
+      highKey = ops['$lte'];
+      highInclusive = true;
    }

-    const keyStr = JSON.stringify(this.buildKeyValue(equalityValues, index.data.key));
-    return index.data.entries.get(keyStr) || new Set();
+    // Use B-tree range iteration
+    indexData.btree.forRange(lowKey, highKey, lowInclusive, highInclusive, (value, key) => {
+      if (value) {
+        for (const id of value) {
+          results.add(id);
+        }
+      }
+    });
+
+    return results;
  }

  // ============================================================================
--- a/ts/tsmdb/engine/QueryPlanner.ts
+++ b/ts/tsmdb/engine/QueryPlanner.ts
@@ -0,0 +1,393 @@
+import * as plugins from '../tsmdb.plugins.js';
+import type { Document, IStoredDocument } from '../types/interfaces.js';
+import { IndexEngine } from './IndexEngine.js';
+
+/**
+ * Query execution plan types
+ */
+export type TQueryPlanType = 'IXSCAN' | 'COLLSCAN' | 'FETCH' | 'IXSCAN_RANGE';
+
+/**
+ * Represents a query execution plan
+ */
+export interface IQueryPlan {
+  /** The type of scan used */
+  type: TQueryPlanType;
+  /** Index name if using an index */
+  indexName?: string;
+  /** Index key specification */
+  indexKey?: Record<string, 1 | -1 | string>;
+  /** Whether the query can be fully satisfied by the index */
+  indexCovering: boolean;
+  /** Estimated selectivity (0-1, lower is more selective) */
+  selectivity: number;
+  /** Whether range operators are used */
+  usesRange: boolean;
+  /** Fields used from the index */
+  indexFieldsUsed: string[];
+  /** Filter conditions that must be applied post-index lookup */
+  residualFilter?: Document;
+  /** Explanation for debugging */
+  explanation: string;
+}
+
+/**
+ * Filter operator analysis
+ */
+interface IFilterOperatorInfo {
+  field: string;
+  operators: string[];
+  equality: boolean;
+  range: boolean;
+  in: boolean;
+  exists: boolean;
+  regex: boolean;
+  values: Record<string, any>;
+}
+
+/**
+ * QueryPlanner - Analyzes queries and selects optimal execution plans
+ */
+export class QueryPlanner {
+  private indexEngine: IndexEngine;
+
+  constructor(indexEngine: IndexEngine) {
+    this.indexEngine = indexEngine;
+  }
+
+  /**
+   * Generate an execution plan for a query filter
+   */
+  async plan(filter: Document): Promise<IQueryPlan> {
+    await this.indexEngine['initialize']();
+
+    // Empty filter = full collection scan
+    if (!filter || Object.keys(filter).length === 0) {
+      return {
+        type: 'COLLSCAN',
+        indexCovering: false,
+        selectivity: 1.0,
+        usesRange: false,
+        indexFieldsUsed: [],
+        explanation: 'No filter specified, full collection scan required',
+      };
+    }
+
+    // Analyze the filter
+    const operatorInfo = this.analyzeFilter(filter);
+
+    // Get available indexes
+    const indexes = await this.indexEngine.listIndexes();
+
+    // Score each index
+    let bestPlan: IQueryPlan | null = null;
+    let bestScore = -1;
+
+    for (const index of indexes) {
+      const plan = this.scoreIndex(index, operatorInfo, filter);
+      if (plan.selectivity < 1.0) {
+        const score = this.calculateScore(plan);
+        if (score > bestScore) {
+          bestScore = score;
+          bestPlan = plan;
+        }
+      }
+    }
+
+    // If no suitable index found, fall back to collection scan
+    if (!bestPlan || bestScore <= 0) {
+      return {
+        type: 'COLLSCAN',
+        indexCovering: false,
+        selectivity: 1.0,
+        usesRange: false,
+        indexFieldsUsed: [],
+        explanation: 'No suitable index found for this query',
+      };
+    }
+
+    return bestPlan;
+  }
+
+  /**
+   * Analyze filter to extract operator information per field
+   */
+  private analyzeFilter(filter: Document, prefix = ''): Map<string, IFilterOperatorInfo> {
+    const result = new Map<string, IFilterOperatorInfo>();
+
+    for (const [key, value] of Object.entries(filter)) {
+      // Skip logical operators at the top level
+      if (key.startsWith('$')) {
+        if (key === '$and' && Array.isArray(value)) {
+          // Merge $and conditions
+          for (const subFilter of value) {
+            const subInfo = this.analyzeFilter(subFilter, prefix);
+            for (const [field, info] of subInfo) {
+              if (result.has(field)) {
+                // Merge operators
+                const existing = result.get(field)!;
+                existing.operators.push(...info.operators);
+                existing.equality = existing.equality || info.equality;
+                existing.range = existing.range || info.range;
+                existing.in = existing.in || info.in;
+                Object.assign(existing.values, info.values);
+              } else {
+                result.set(field, info);
+              }
+            }
+          }
+        }
+        continue;
+      }
+
+      const fullKey = prefix ? `${prefix}.${key}` : key;
+      const info: IFilterOperatorInfo = {
+        field: fullKey,
+        operators: [],
+        equality: false,
+        range: false,
+        in: false,
+        exists: false,
+        regex: false,
+        values: {},
+      };
+
+      if (typeof value !== 'object' || value === null || value instanceof plugins.bson.ObjectId || value instanceof Date) {
+        // Direct equality
+        info.equality = true;
+        info.operators.push('$eq');
+        info.values['$eq'] = value;
+      } else if (Array.isArray(value)) {
+        // Array equality (rare, but possible)
+        info.equality = true;
+        info.operators.push('$eq');
+        info.values['$eq'] = value;
+      } else {
+        // Operator object
+        for (const [op, opValue] of Object.entries(value)) {
+          if (op.startsWith('$')) {
+            info.operators.push(op);
+            info.values[op] = opValue;
+
+            switch (op) {
+              case '$eq':
+                info.equality = true;
+                break;
+              case '$ne':
+              case '$not':
+                // These can use indexes but with low selectivity
+                break;
+              case '$in':
+                info.in = true;
+                break;
+              case '$nin':
+                // Can't efficiently use indexes
+                break;
+              case '$gt':
+              case '$gte':
+              case '$lt':
+              case '$lte':
+                info.range = true;
+                break;
+              case '$exists':
+                info.exists = true;
+                break;
+              case '$regex':
+                info.regex = true;
+                break;
+            }
+          } else {
+            // Nested object - recurse
+            const nestedInfo = this.analyzeFilter({ [op]: opValue }, fullKey);
+            for (const [nestedField, nestedFieldInfo] of nestedInfo) {
+              result.set(nestedField, nestedFieldInfo);
+            }
+          }
+        }
+      }
+
+      if (info.operators.length > 0) {
+        result.set(fullKey, info);
+      }
+    }
+
+    return result;
+  }
+
+  /**
+   * Score an index for the given filter
+   */
+  private scoreIndex(
+    index: { name: string; key: Record<string, any>; unique?: boolean; sparse?: boolean },
+    operatorInfo: Map<string, IFilterOperatorInfo>,
+    filter: Document
+  ): IQueryPlan {
+    const indexFields = Object.keys(index.key);
+    const usedFields: string[] = [];
+    let usesRange = false;
+    let canUseIndex = true;
+    let selectivity = 1.0;
+    let residualFilter: Document | undefined;
+
+    // Check each index field in order
+    for (const field of indexFields) {
+      const info = operatorInfo.get(field);
+      if (!info) {
+        // Index field not in filter - stop here
+        break;
+      }
+
+      usedFields.push(field);
+
+      // Calculate selectivity based on operator
+      if (info.equality) {
+        // Equality has high selectivity
+        selectivity *= 0.01; // Assume 1% match
+      } else if (info.in) {
+        // $in selectivity depends on array size
+        const inValues = info.values['$in'];
+        if (Array.isArray(inValues)) {
+          selectivity *= Math.min(0.5, inValues.length * 0.01);
+        } else {
+          selectivity *= 0.1;
+        }
+      } else if (info.range) {
+        // Range queries have moderate selectivity
+        selectivity *= 0.25;
+        usesRange = true;
+        // After range, can't use more index fields efficiently
+        break;
+      } else if (info.exists) {
+        // $exists can use sparse indexes
+        selectivity *= 0.5;
+      } else {
+        // Other operators may not be indexable
+        canUseIndex = false;
+        break;
+      }
+    }
+
+    if (!canUseIndex || usedFields.length === 0) {
+      return {
+        type: 'COLLSCAN',
+        indexCovering: false,
+        selectivity: 1.0,
+        usesRange: false,
+        indexFieldsUsed: [],
+        explanation: `Index ${index.name} cannot be used for this query`,
+      };
+    }
+
+    // Build residual filter for conditions not covered by index
+    const coveredFields = new Set(usedFields);
+    const residualConditions: Record<string, any> = {};
+    for (const [field, info] of operatorInfo) {
+      if (!coveredFields.has(field)) {
+        // This field isn't covered by the index
+        if (info.equality) {
+          residualConditions[field] = info.values['$eq'];
+        } else {
+          residualConditions[field] = info.values;
+        }
+      }
+    }
+
+    if (Object.keys(residualConditions).length > 0) {
+      residualFilter = residualConditions;
+    }
+
+    // Unique indexes have better selectivity for equality
+    if (index.unique && usedFields.length === indexFields.length) {
+      selectivity = Math.min(selectivity, 0.001); // At most 1 document
+    }
+
+    return {
+      type: usesRange ? 'IXSCAN_RANGE' : 'IXSCAN',
+      indexName: index.name,
+      indexKey: index.key,
+      indexCovering: Object.keys(residualConditions).length === 0,
+      selectivity,
+      usesRange,
+      indexFieldsUsed: usedFields,
+      residualFilter,
+      explanation: `Using index ${index.name} on fields [${usedFields.join(', ')}]`,
+    };
+  }
+
+  /**
+   * Calculate overall score for a plan (higher is better)
+   */
+  private calculateScore(plan: IQueryPlan): number {
+    let score = 0;
+
+    // Lower selectivity is better (fewer documents to fetch)
+    score += (1 - plan.selectivity) * 100;
+
+    // Index covering queries are best
+    if (plan.indexCovering) {
+      score += 50;
+    }
+
+    // More index fields used is better
+    score += plan.indexFieldsUsed.length * 10;
+
+    // Equality scans are better than range scans
+    if (!plan.usesRange) {
+      score += 20;
+    }
+
+    return score;
+  }
+
+  /**
+   * Explain a query - returns detailed plan information
+   */
+  async explain(filter: Document): Promise<{
+    queryPlanner: {
+      plannerVersion: number;
+      namespace: string;
+      indexFilterSet: boolean;
+      winningPlan: IQueryPlan;
+      rejectedPlans: IQueryPlan[];
+    };
+  }> {
+    await this.indexEngine['initialize']();
+
+    // Analyze the filter
+    const operatorInfo = this.analyzeFilter(filter);
+
+    // Get available indexes
+    const indexes = await this.indexEngine.listIndexes();
+
+    // Score all indexes
+    const plans: IQueryPlan[] = [];
+
+    for (const index of indexes) {
+      const plan = this.scoreIndex(index, operatorInfo, filter);
+      plans.push(plan);
+    }
+
+    // Add collection scan as fallback
+    plans.push({
+      type: 'COLLSCAN',
+      indexCovering: false,
+      selectivity: 1.0,
+      usesRange: false,
+      indexFieldsUsed: [],
+      explanation: 'Full collection scan',
+    });
+
+    // Sort by score (best first)
+    plans.sort((a, b) => this.calculateScore(b) - this.calculateScore(a));
+
+    return {
+      queryPlanner: {
+        plannerVersion: 1,
+        namespace: `${this.indexEngine['dbName']}.${this.indexEngine['collName']}`,
+        indexFilterSet: false,
+        winningPlan: plans[0],
+        rejectedPlans: plans.slice(1),
+      },
+    };
+  }
+}
--- a/ts/tsmdb/engine/SessionEngine.ts
+++ b/ts/tsmdb/engine/SessionEngine.ts
@@ -0,0 +1,292 @@
+import * as plugins from '../tsmdb.plugins.js';
+import type { TransactionEngine } from './TransactionEngine.js';
+
+/**
+ * Session state
+ */
+export interface ISession {
+  /** Session ID (UUID) */
+  id: string;
+  /** Timestamp when the session was created */
+  createdAt: number;
+  /** Timestamp of the last activity */
+  lastActivityAt: number;
+  /** Current transaction ID if any */
+  txnId?: string;
+  /** Transaction number for ordering */
+  txnNumber?: number;
+  /** Whether the session is in a transaction */
+  inTransaction: boolean;
+  /** Session metadata */
+  metadata?: Record<string, any>;
+}
+
+/**
+ * Session engine options
+ */
+export interface ISessionEngineOptions {
+  /** Session timeout in milliseconds (default: 30 minutes) */
+  sessionTimeoutMs?: number;
+  /** Interval to check for expired sessions in ms (default: 60 seconds) */
+  cleanupIntervalMs?: number;
+}
+
+/**
+ * Session engine for managing client sessions
+ * - Tracks session lifecycle (create, touch, end)
+ * - Links sessions to transactions
+ * - Auto-aborts transactions on session expiry
+ */
+export class SessionEngine {
+  private sessions: Map<string, ISession> = new Map();
+  private sessionTimeoutMs: number;
+  private cleanupInterval?: ReturnType<typeof setInterval>;
+  private transactionEngine?: TransactionEngine;
+
+  constructor(options?: ISessionEngineOptions) {
+    this.sessionTimeoutMs = options?.sessionTimeoutMs ?? 30 * 60 * 1000; // 30 minutes default
+    const cleanupIntervalMs = options?.cleanupIntervalMs ?? 60 * 1000; // 1 minute default
+
+    // Start cleanup interval
+    this.cleanupInterval = setInterval(() => {
+      this.cleanupExpiredSessions();
+    }, cleanupIntervalMs);
+  }
+
+  /**
+   * Set the transaction engine to use for auto-abort
+   */
+  setTransactionEngine(engine: TransactionEngine): void {
+    this.transactionEngine = engine;
+  }
+
+  /**
+   * Start a new session
+   */
+  startSession(sessionId?: string, metadata?: Record<string, any>): ISession {
+    const id = sessionId ?? new plugins.bson.UUID().toHexString();
+    const now = Date.now();
+
+    const session: ISession = {
+      id,
+      createdAt: now,
+      lastActivityAt: now,
+      inTransaction: false,
+      metadata,
+    };
+
+    this.sessions.set(id, session);
+    return session;
+  }
+
+  /**
+   * Get a session by ID
+   */
+  getSession(sessionId: string): ISession | undefined {
+    const session = this.sessions.get(sessionId);
+    if (session && this.isSessionExpired(session)) {
+      // Session expired, clean it up
+      this.endSession(sessionId);
+      return undefined;
+    }
+    return session;
+  }
+
+  /**
+   * Touch a session to update last activity time
+   */
+  touchSession(sessionId: string): boolean {
+    const session = this.sessions.get(sessionId);
+    if (!session) return false;
+
+    if (this.isSessionExpired(session)) {
+      this.endSession(sessionId);
+      return false;
+    }
+
+    session.lastActivityAt = Date.now();
+    return true;
+  }
+
+  /**
+   * End a session explicitly
+   * This will also abort any active transaction
+   */
+  async endSession(sessionId: string): Promise<boolean> {
+    const session = this.sessions.get(sessionId);
+    if (!session) return false;
+
+    // If session has an active transaction, abort it
+    if (session.inTransaction && session.txnId && this.transactionEngine) {
+      try {
+        await this.transactionEngine.abortTransaction(session.txnId);
+      } catch (e) {
+        // Ignore abort errors during cleanup
+      }
+    }
+
+    this.sessions.delete(sessionId);
+    return true;
+  }
+
+  /**
+   * Start a transaction in a session
+   */
+  startTransaction(sessionId: string, txnId: string, txnNumber?: number): boolean {
+    const session = this.sessions.get(sessionId);
+    if (!session) return false;
+
+    if (this.isSessionExpired(session)) {
+      this.endSession(sessionId);
+      return false;
+    }
+
+    session.txnId = txnId;
+    session.txnNumber = txnNumber;
+    session.inTransaction = true;
+    session.lastActivityAt = Date.now();
+
+    return true;
+  }
+
+  /**
+   * End a transaction in a session (commit or abort)
+   */
+  endTransaction(sessionId: string): boolean {
+    const session = this.sessions.get(sessionId);
+    if (!session) return false;
+
+    session.txnId = undefined;
+    session.txnNumber = undefined;
+    session.inTransaction = false;
+    session.lastActivityAt = Date.now();
+
+    return true;
+  }
+
+  /**
+   * Get transaction ID for a session
+   */
+  getTransactionId(sessionId: string): string | undefined {
+    const session = this.sessions.get(sessionId);
+    return session?.txnId;
+  }
+
+  /**
+   * Check if session is in a transaction
+   */
+  isInTransaction(sessionId: string): boolean {
+    const session = this.sessions.get(sessionId);
+    return session?.inTransaction ?? false;
+  }
+
+  /**
+   * Check if a session is expired
+   */
+  isSessionExpired(session: ISession): boolean {
+    return Date.now() - session.lastActivityAt > this.sessionTimeoutMs;
+  }
+
+  /**
+   * Cleanup expired sessions
+   * This is called periodically by the cleanup interval
+   */
+  private async cleanupExpiredSessions(): Promise<void> {
+    const expiredSessions: string[] = [];
+
+    for (const [id, session] of this.sessions) {
+      if (this.isSessionExpired(session)) {
+        expiredSessions.push(id);
+      }
+    }
+
+    // End all expired sessions (this will also abort their transactions)
+    for (const sessionId of expiredSessions) {
+      await this.endSession(sessionId);
+    }
+  }
+
+  /**
+   * Get all active sessions
+   */
+  listSessions(): ISession[] {
+    const activeSessions: ISession[] = [];
+    for (const session of this.sessions.values()) {
+      if (!this.isSessionExpired(session)) {
+        activeSessions.push(session);
+      }
+    }
+    return activeSessions;
+  }
+
+  /**
+   * Get session count
+   */
+  getSessionCount(): number {
+    return this.sessions.size;
+  }
+
+  /**
+   * Get sessions with active transactions
+   */
+  getSessionsWithTransactions(): ISession[] {
+    return this.listSessions().filter(s => s.inTransaction);
+  }
+
+  /**
+   * Refresh session timeout
+   */
+  refreshSession(sessionId: string): boolean {
+    return this.touchSession(sessionId);
+  }
+
+  /**
+   * Close the session engine and cleanup
+   */
+  close(): void {
+    if (this.cleanupInterval) {
+      clearInterval(this.cleanupInterval);
+      this.cleanupInterval = undefined;
+    }
+
+    // Clear all sessions
+    this.sessions.clear();
+  }
+
+  /**
+   * Get or create a session for a given session ID
+   * Useful for handling MongoDB driver session requests
+   */
+  getOrCreateSession(sessionId: string): ISession {
+    let session = this.getSession(sessionId);
+    if (!session) {
+      session = this.startSession(sessionId);
+    } else {
+      this.touchSession(sessionId);
+    }
+    return session;
+  }
+
+  /**
+   * Extract session ID from MongoDB lsid (logical session ID)
+   */
+  static extractSessionId(lsid: any): string | undefined {
+    if (!lsid) return undefined;
+
+    // MongoDB session ID format: { id: UUID }
+    if (lsid.id) {
+      if (lsid.id instanceof plugins.bson.UUID) {
+        return lsid.id.toHexString();
+      }
+      if (typeof lsid.id === 'string') {
+        return lsid.id;
+      }
+      if (lsid.id.$binary?.base64) {
+        // Binary UUID format
+        return Buffer.from(lsid.id.$binary.base64, 'base64').toString('hex');
+      }
+    }
+
+    return undefined;
+  }
+}
--- a/ts/tsmdb/index.ts
+++ b/ts/tsmdb/index.ts
@@ -19,6 +19,8 @@ export type { IStorageAdapter } from './storage/IStorageAdapter.js';
 export { MemoryStorageAdapter } from './storage/MemoryStorageAdapter.js';
 export { FileStorageAdapter } from './storage/FileStorageAdapter.js';
 export { OpLog } from './storage/OpLog.js';
+export { WAL } from './storage/WAL.js';
+export type { IWalEntry, TWalOperation } from './storage/WAL.js';

 // Export engines
 export { QueryEngine } from './engine/QueryEngine.js';
@@ -26,6 +28,10 @@ export { UpdateEngine } from './engine/UpdateEngine.js';
 export { AggregationEngine } from './engine/AggregationEngine.js';
 export { IndexEngine } from './engine/IndexEngine.js';
 export { TransactionEngine } from './engine/TransactionEngine.js';
+export { QueryPlanner } from './engine/QueryPlanner.js';
+export type { IQueryPlan, TQueryPlanType } from './engine/QueryPlanner.js';
+export { SessionEngine } from './engine/SessionEngine.js';
+export type { ISession, ISessionEngineOptions } from './engine/SessionEngine.js';

 // Export server (the main entry point for using TsmDB)
 export { TsmdbServer } from './server/TsmdbServer.js';
@@ -35,3 +41,6 @@ export type { ITsmdbServerOptions } from './server/TsmdbServer.js';
 export { WireProtocol } from './server/WireProtocol.js';
 export { CommandRouter } from './server/CommandRouter.js';
 export type { ICommandHandler, IHandlerContext, ICursorState } from './server/CommandRouter.js';
+
+// Export utilities
+export * from './utils/checksum.js';
--- a/ts/tsmdb/server/CommandRouter.ts
+++ b/ts/tsmdb/server/CommandRouter.ts
@@ -2,6 +2,9 @@ import * as plugins from '../tsmdb.plugins.js';
 import type { IStorageAdapter } from '../storage/IStorageAdapter.js';
 import type { IParsedCommand } from './WireProtocol.js';
 import type { TsmdbServer } from './TsmdbServer.js';
+import { IndexEngine } from '../engine/IndexEngine.js';
+import { TransactionEngine } from '../engine/TransactionEngine.js';
+import { SessionEngine } from '../engine/SessionEngine.js';

 // Import handlers
 import { HelloHandler } from './handlers/HelloHandler.js';
@@ -22,6 +25,16 @@ export interface IHandlerContext {
  database: string;
  command: plugins.bson.Document;
  documentSequences?: Map<string, plugins.bson.Document[]>;
+  /** Get or create an IndexEngine for a collection */
+  getIndexEngine: (collName: string) => IndexEngine;
+  /** Transaction engine instance */
+  transactionEngine: TransactionEngine;
+  /** Current transaction ID (if in a transaction) */
+  txnId?: string;
+  /** Session ID (from lsid) */
+  sessionId?: string;
+  /** Session engine instance */
+  sessionEngine: SessionEngine;
 }

 /**
@@ -43,12 +56,54 @@ export class CommandRouter {
  private cursors: Map<bigint, ICursorState> = new Map();
  private cursorIdCounter: bigint = BigInt(1);

+  // Index engine cache: db.collection -> IndexEngine
+  private indexEngines: Map<string, IndexEngine> = new Map();
+
+  // Transaction engine (shared across all handlers)
+  private transactionEngine: TransactionEngine;
+
+  // Session engine (shared across all handlers)
+  private sessionEngine: SessionEngine;
+
  constructor(storage: IStorageAdapter, server: TsmdbServer) {
    this.storage = storage;
    this.server = server;
+    this.transactionEngine = new TransactionEngine(storage);
+    this.sessionEngine = new SessionEngine();
+    // Link session engine to transaction engine for auto-abort on session expiry
+    this.sessionEngine.setTransactionEngine(this.transactionEngine);
    this.registerHandlers();
  }

+  /**
+   * Get or create an IndexEngine for a database.collection
+   */
+  getIndexEngine(dbName: string, collName: string): IndexEngine {
+    const key = `${dbName}.${collName}`;
+    let engine = this.indexEngines.get(key);
+    if (!engine) {
+      engine = new IndexEngine(dbName, collName, this.storage);
+      this.indexEngines.set(key, engine);
+    }
+    return engine;
+  }
+
+  /**
+   * Clear index engine cache for a collection (used when collection is dropped)
+   */
+  clearIndexEngineCache(dbName: string, collName?: string): void {
+    if (collName) {
+      this.indexEngines.delete(`${dbName}.${collName}`);
+    } else {
+      // Clear all engines for the database
+      for (const key of this.indexEngines.keys()) {
+        if (key.startsWith(`${dbName}.`)) {
+          this.indexEngines.delete(key);
+        }
+      }
+    }
+  }
+
  /**
   * Register all command handlers
   */
@@ -120,6 +175,29 @@ export class CommandRouter {
  async route(parsedCommand: IParsedCommand): Promise<plugins.bson.Document> {
    const { commandName, command, database, documentSequences } = parsedCommand;

+    // Extract session ID from lsid using SessionEngine helper
+    let sessionId = SessionEngine.extractSessionId(command.lsid);
+    let txnId: string | undefined;
+
+    // If we have a session ID, register/touch the session
+    if (sessionId) {
+      this.sessionEngine.getOrCreateSession(sessionId);
+    }
+
+    // Check if this starts a new transaction
+    if (command.startTransaction && sessionId) {
+      txnId = this.transactionEngine.startTransaction(sessionId);
+      this.sessionEngine.startTransaction(sessionId, txnId, command.txnNumber);
+    } else if (sessionId && this.sessionEngine.isInTransaction(sessionId)) {
+      // Continue existing transaction
+      txnId = this.sessionEngine.getTransactionId(sessionId);
+      // Verify transaction is still active
+      if (txnId && !this.transactionEngine.isActive(txnId)) {
+        this.sessionEngine.endTransaction(sessionId);
+        txnId = undefined;
+      }
+    }
+
    // Create handler context
    const context: IHandlerContext = {
      storage: this.storage,
@@ -127,6 +205,11 @@ export class CommandRouter {
      database,
      command,
      documentSequences,
+      getIndexEngine: (collName: string) => this.getIndexEngine(database, collName),
+      transactionEngine: this.transactionEngine,
+      sessionEngine: this.sessionEngine,
+      txnId,
+      sessionId,
    };

    // Find handler
@@ -164,6 +247,32 @@ export class CommandRouter {
      };
    }
  }
+
+  /**
+   * Close the command router and cleanup resources
+   */
+  close(): void {
+    // Close session engine (stops cleanup interval, clears sessions)
+    this.sessionEngine.close();
+    // Clear cursors
+    this.cursors.clear();
+    // Clear index engine cache
+    this.indexEngines.clear();
+  }
+
+  /**
+   * Get session engine (for administrative purposes)
+   */
+  getSessionEngine(): SessionEngine {
+    return this.sessionEngine;
+  }
+
+  /**
+   * Get transaction engine (for administrative purposes)
+   */
+  getTransactionEngine(): TransactionEngine {
+    return this.transactionEngine;
+  }
 }

 /**
--- a/ts/tsmdb/server/TsmdbServer.ts
+++ b/ts/tsmdb/server/TsmdbServer.ts
@@ -154,6 +154,9 @@ export class TsmdbServer {
    }
    this.connections.clear();

+    // Close command router (cleans up session engine, cursors, etc.)
+    this.commandRouter.close();
+
    // Close storage
    await this.storage.close();

--- a/ts/tsmdb/server/handlers/AdminHandler.ts
+++ b/ts/tsmdb/server/handlers/AdminHandler.ts
@@ -1,5 +1,6 @@
 import * as plugins from '../../tsmdb.plugins.js';
 import type { ICommandHandler, IHandlerContext } from '../CommandRouter.js';
+import { SessionEngine } from '../../engine/SessionEngine.js';

 /**
 * AdminHandler - Handles administrative commands
@@ -237,10 +238,12 @@ export class AdminHandler implements ICommandHandler {
   * Handle serverStatus command
   */
  private async handleServerStatus(context: IHandlerContext): Promise<plugins.bson.Document> {
-    const { server } = context;
+    const { server, sessionEngine } = context;

    const uptime = server.getUptime();
    const connections = server.getConnectionCount();
+    const sessions = sessionEngine.listSessions();
+    const sessionsWithTxn = sessionEngine.getSessionsWithTransactions();

    return {
      ok: 1,
@@ -263,6 +266,26 @@ export class AdminHandler implements ICommandHandler {
        totalCreated: connections,
        active: connections,
      },
+      logicalSessionRecordCache: {
+        activeSessionsCount: sessions.length,
+        sessionsCollectionJobCount: 0,
+        lastSessionsCollectionJobDurationMillis: 0,
+        lastSessionsCollectionJobTimestamp: new Date(),
+        transactionReaperJobCount: 0,
+        lastTransactionReaperJobDurationMillis: 0,
+        lastTransactionReaperJobTimestamp: new Date(),
+      },
+      transactions: {
+        retriedCommandsCount: 0,
+        retriedStatementsCount: 0,
+        transactionsCollectionWriteCount: 0,
+        currentActive: sessionsWithTxn.length,
+        currentInactive: 0,
+        currentOpen: sessionsWithTxn.length,
+        totalStarted: sessionsWithTxn.length,
+        totalCommitted: 0,
+        totalAborted: 0,
+      },
      network: {
        bytesIn: 0,
        bytesOut: 0,
@@ -409,6 +432,17 @@ export class AdminHandler implements ICommandHandler {
   * Handle endSessions command
   */
  private async handleEndSessions(context: IHandlerContext): Promise<plugins.bson.Document> {
+    const { command, sessionEngine } = context;
+
+    // End each session in the array
+    const sessions = command.endSessions || [];
+    for (const sessionSpec of sessions) {
+      const sessionId = SessionEngine.extractSessionId(sessionSpec);
+      if (sessionId) {
+        await sessionEngine.endSession(sessionId);
+      }
+    }
+
    return { ok: 1 };
  }

@@ -416,16 +450,87 @@ export class AdminHandler implements ICommandHandler {
   * Handle abortTransaction command
   */
  private async handleAbortTransaction(context: IHandlerContext): Promise<plugins.bson.Document> {
-    // Transactions are not fully supported, but acknowledge the command
-    return { ok: 1 };
+    const { transactionEngine, sessionEngine, txnId, sessionId } = context;
+
+    if (!txnId) {
+      return {
+        ok: 0,
+        errmsg: 'No transaction started',
+        code: 251,
+        codeName: 'NoSuchTransaction',
+      };
+    }
+
+    try {
+      await transactionEngine.abortTransaction(txnId);
+      transactionEngine.endTransaction(txnId);
+      // Update session state
+      if (sessionId) {
+        sessionEngine.endTransaction(sessionId);
+      }
+      return { ok: 1 };
+    } catch (error: any) {
+      return {
+        ok: 0,
+        errmsg: error.message || 'Abort transaction failed',
+        code: error.code || 1,
+        codeName: error.codeName || 'UnknownError',
+      };
+    }
  }

  /**
   * Handle commitTransaction command
   */
  private async handleCommitTransaction(context: IHandlerContext): Promise<plugins.bson.Document> {
-    // Transactions are not fully supported, but acknowledge the command
-    return { ok: 1 };
+    const { transactionEngine, sessionEngine, txnId, sessionId } = context;
+
+    if (!txnId) {
+      return {
+        ok: 0,
+        errmsg: 'No transaction started',
+        code: 251,
+        codeName: 'NoSuchTransaction',
+      };
+    }
+
+    try {
+      await transactionEngine.commitTransaction(txnId);
+      transactionEngine.endTransaction(txnId);
+      // Update session state
+      if (sessionId) {
+        sessionEngine.endTransaction(sessionId);
+      }
+      return { ok: 1 };
+    } catch (error: any) {
+      // If commit fails, transaction should be aborted
+      try {
+        await transactionEngine.abortTransaction(txnId);
+        transactionEngine.endTransaction(txnId);
+        if (sessionId) {
+          sessionEngine.endTransaction(sessionId);
+        }
+      } catch {
+        // Ignore abort errors
+      }
+
+      if (error.code === 112) {
+        // Write conflict
+        return {
+          ok: 0,
+          errmsg: error.message || 'Write conflict during commit',
+          code: 112,
+          codeName: 'WriteConflict',
+        };
+      }
+
+      return {
+        ok: 0,
+        errmsg: error.message || 'Commit transaction failed',
+        code: error.code || 1,
+        codeName: error.codeName || 'UnknownError',
+      };
+    }
  }

  /**
--- a/ts/tsmdb/server/handlers/DeleteHandler.ts
+++ b/ts/tsmdb/server/handlers/DeleteHandler.ts
@@ -1,5 +1,6 @@
 import * as plugins from '../../tsmdb.plugins.js';
 import type { ICommandHandler, IHandlerContext } from '../CommandRouter.js';
+import type { IStoredDocument } from '../../types/interfaces.js';
 import { QueryEngine } from '../../engine/QueryEngine.js';

 /**
@@ -47,6 +48,8 @@ export class DeleteHandler implements ICommandHandler {
      return { ok: 1, n: 0 };
    }

+    const indexEngine = context.getIndexEngine(collection);
+
    for (let i = 0; i < deletes.length; i++) {
      const deleteSpec = deletes[i];
      const filter = deleteSpec.q || deleteSpec.filter || {};
@@ -56,8 +59,15 @@ export class DeleteHandler implements ICommandHandler {
      const deleteAll = limit === 0;

      try {
-        // Get all documents
-        const documents = await storage.findAll(database, collection);
+        // Try to use index-accelerated query
+        const candidateIds = await indexEngine.findCandidateIds(filter);
+
+        let documents: IStoredDocument[];
+        if (candidateIds !== null) {
+          documents = await storage.findByIds(database, collection, candidateIds);
+        } else {
+          documents = await storage.findAll(database, collection);
+        }

        // Apply filter
        const matchingDocs = QueryEngine.filter(documents, filter);
@@ -69,6 +79,11 @@ export class DeleteHandler implements ICommandHandler {
        // Determine which documents to delete
        const docsToDelete = deleteAll ? matchingDocs : matchingDocs.slice(0, 1);

+        // Update indexes for deleted documents
+        for (const doc of docsToDelete) {
+          await indexEngine.onDelete(doc as any);
+        }
+
        // Delete the documents
        const idsToDelete = docsToDelete.map(doc => doc._id);
        const deleted = await storage.deleteByIds(database, collection, idsToDelete);
--- a/ts/tsmdb/server/handlers/FindHandler.ts
+++ b/ts/tsmdb/server/handlers/FindHandler.ts
@@ -1,5 +1,6 @@
 import * as plugins from '../../tsmdb.plugins.js';
 import type { ICommandHandler, IHandlerContext, ICursorState } from '../CommandRouter.js';
+import type { IStoredDocument } from '../../types/interfaces.js';
 import { QueryEngine } from '../../engine/QueryEngine.js';

 /**
@@ -45,7 +46,7 @@ export class FindHandler implements ICommandHandler {
   * Handle find command
   */
  private async handleFind(context: IHandlerContext): Promise<plugins.bson.Document> {
-    const { storage, database, command } = context;
+    const { storage, database, command, getIndexEngine } = context;

    const collection = command.find;
    const filter = command.filter || {};
@@ -70,11 +71,22 @@ export class FindHandler implements ICommandHandler {
      };
    }

-    // Get all documents
-    let documents = await storage.findAll(database, collection);
+    // Try to use index-accelerated query
+    const indexEngine = getIndexEngine(collection);
+    const candidateIds = await indexEngine.findCandidateIds(filter);

-    // Apply filter
-    documents = QueryEngine.filter(documents, filter);
+    let documents: IStoredDocument[];
+    if (candidateIds !== null) {
+      // Index hit - fetch only candidate documents
+      documents = await storage.findByIds(database, collection, candidateIds);
+      // Still apply filter for any conditions the index couldn't fully satisfy
+      documents = QueryEngine.filter(documents, filter);
+    } else {
+      // No suitable index - full collection scan
+      documents = await storage.findAll(database, collection);
+      // Apply filter
+      documents = QueryEngine.filter(documents, filter);
+    }

    // Apply sort
    if (sort) {
@@ -233,7 +245,7 @@ export class FindHandler implements ICommandHandler {
   * Handle count command
   */
  private async handleCount(context: IHandlerContext): Promise<plugins.bson.Document> {
-    const { storage, database, command } = context;
+    const { storage, database, command, getIndexEngine } = context;

    const collection = command.count;
    const query = command.query || {};
@@ -246,11 +258,20 @@ export class FindHandler implements ICommandHandler {
      return { ok: 1, n: 0 };
    }

-    // Get all documents
-    let documents = await storage.findAll(database, collection);
+    // Try to use index-accelerated query
+    const indexEngine = getIndexEngine(collection);
+    const candidateIds = await indexEngine.findCandidateIds(query);

-    // Apply filter
-    documents = QueryEngine.filter(documents, query);
+    let documents: IStoredDocument[];
+    if (candidateIds !== null) {
+      // Index hit - fetch only candidate documents
+      documents = await storage.findByIds(database, collection, candidateIds);
+      documents = QueryEngine.filter(documents, query);
+    } else {
+      // No suitable index - full collection scan
+      documents = await storage.findAll(database, collection);
+      documents = QueryEngine.filter(documents, query);
+    }

    // Apply skip
    if (skip > 0) {
@@ -269,7 +290,7 @@ export class FindHandler implements ICommandHandler {
   * Handle distinct command
   */
  private async handleDistinct(context: IHandlerContext): Promise<plugins.bson.Document> {
-    const { storage, database, command } = context;
+    const { storage, database, command, getIndexEngine } = context;

    const collection = command.distinct;
    const key = command.key;
@@ -290,8 +311,16 @@ export class FindHandler implements ICommandHandler {
      return { ok: 1, values: [] };
    }

-    // Get all documents
-    const documents = await storage.findAll(database, collection);
+    // Try to use index-accelerated query
+    const indexEngine = getIndexEngine(collection);
+    const candidateIds = await indexEngine.findCandidateIds(query);
+
+    let documents: IStoredDocument[];
+    if (candidateIds !== null) {
+      documents = await storage.findByIds(database, collection, candidateIds);
+    } else {
+      documents = await storage.findAll(database, collection);
+    }

    // Get distinct values
    const values = QueryEngine.distinct(documents, key, query);
--- a/ts/tsmdb/server/handlers/InsertHandler.ts
+++ b/ts/tsmdb/server/handlers/InsertHandler.ts
@@ -1,5 +1,6 @@
 import * as plugins from '../../tsmdb.plugins.js';
 import type { ICommandHandler, IHandlerContext } from '../CommandRouter.js';
+import type { IStoredDocument } from '../../types/interfaces.js';

 /**
 * InsertHandler - Handles insert commands
@@ -42,6 +43,8 @@ export class InsertHandler implements ICommandHandler {
    // Ensure collection exists
    await storage.createCollection(database, collection);

+    const indexEngine = context.getIndexEngine(collection);
+
    // Insert documents
    for (let i = 0; i < documents.length; i++) {
      const doc = documents[i];
@@ -52,6 +55,9 @@ export class InsertHandler implements ICommandHandler {
          doc._id = new plugins.bson.ObjectId();
        }

+        // Check index constraints before insert (doc now has _id)
+        await indexEngine.onInsert(doc as IStoredDocument);
+
        await storage.insertOne(database, collection, doc);
        insertedCount++;
      } catch (error: any) {
--- a/ts/tsmdb/server/handlers/UpdateHandler.ts
+++ b/ts/tsmdb/server/handlers/UpdateHandler.ts
@@ -1,5 +1,6 @@
 import * as plugins from '../../tsmdb.plugins.js';
 import type { ICommandHandler, IHandlerContext } from '../CommandRouter.js';
+import type { IStoredDocument } from '../../types/interfaces.js';
 import { QueryEngine } from '../../engine/QueryEngine.js';
 import { UpdateEngine } from '../../engine/UpdateEngine.js';

@@ -69,6 +70,8 @@ export class UpdateHandler implements ICommandHandler {
    // Ensure collection exists
    await storage.createCollection(database, collection);

+    const indexEngine = context.getIndexEngine(collection);
+
    for (let i = 0; i < updates.length; i++) {
      const updateSpec = updates[i];
      const filter = updateSpec.q || updateSpec.filter || {};
@@ -78,8 +81,15 @@ export class UpdateHandler implements ICommandHandler {
      const arrayFilters = updateSpec.arrayFilters;

      try {
-        // Get all documents
-        let documents = await storage.findAll(database, collection);
+        // Try to use index-accelerated query
+        const candidateIds = await indexEngine.findCandidateIds(filter);
+
+        let documents: IStoredDocument[];
+        if (candidateIds !== null) {
+          documents = await storage.findByIds(database, collection, candidateIds);
+        } else {
+          documents = await storage.findAll(database, collection);
+        }

        // Apply filter
        let matchingDocs = QueryEngine.filter(documents, filter);
@@ -99,6 +109,8 @@ export class UpdateHandler implements ICommandHandler {
            Object.assign(updatedDoc, update.$setOnInsert);
          }

+          // Update index for the new document
+          await indexEngine.onInsert(updatedDoc);
          await storage.insertOne(database, collection, updatedDoc);
          totalUpserted++;
          upserted.push({ index: i, _id: updatedDoc._id });
@@ -113,6 +125,8 @@ export class UpdateHandler implements ICommandHandler {
            // Check if document actually changed
            const changed = JSON.stringify(doc) !== JSON.stringify(updatedDoc);
            if (changed) {
+              // Update index
+              await indexEngine.onUpdate(doc as any, updatedDoc);
              await storage.updateById(database, collection, doc._id, updatedDoc);
              totalModified++;
            }
@@ -186,8 +200,17 @@ export class UpdateHandler implements ICommandHandler {
    // Ensure collection exists
    await storage.createCollection(database, collection);

-    // Get matching documents
-    let documents = await storage.findAll(database, collection);
+    // Try to use index-accelerated query
+    const indexEngine = context.getIndexEngine(collection);
+    const candidateIds = await indexEngine.findCandidateIds(query);
+
+    let documents: IStoredDocument[];
+    if (candidateIds !== null) {
+      documents = await storage.findByIds(database, collection, candidateIds);
+    } else {
+      documents = await storage.findAll(database, collection);
+    }
+
    let matchingDocs = QueryEngine.filter(documents, query);

    // Apply sort if specified
@@ -203,6 +226,8 @@ export class UpdateHandler implements ICommandHandler {
        return { ok: 1, value: null };
      }

+      // Update index for delete
+      await indexEngine.onDelete(doc as any);
      await storage.deleteById(database, collection, doc._id);

      let result = doc;
@@ -231,6 +256,8 @@ export class UpdateHandler implements ICommandHandler {
        // Update existing
        originalDoc = { ...doc };
        resultDoc = UpdateEngine.applyUpdate(doc, update, arrayFilters);
+        // Update index
+        await indexEngine.onUpdate(doc as any, resultDoc as any);
        await storage.updateById(database, collection, doc._id, resultDoc as any);
      } else {
        // Upsert
@@ -243,6 +270,8 @@ export class UpdateHandler implements ICommandHandler {
          Object.assign(resultDoc, update.$setOnInsert);
        }

+        // Update index for insert
+        await indexEngine.onInsert(resultDoc as any);
        await storage.insertOne(database, collection, resultDoc);
      }

--- a/ts/tsmdb/storage/FileStorageAdapter.ts
+++ b/ts/tsmdb/storage/FileStorageAdapter.ts
@@ -1,6 +1,17 @@
 import * as plugins from '../tsmdb.plugins.js';
 import type { IStorageAdapter } from './IStorageAdapter.js';
 import type { IStoredDocument, IOpLogEntry, Document } from '../types/interfaces.js';
+import { calculateDocumentChecksum, verifyChecksum } from '../utils/checksum.js';
+
+/**
+ * File storage adapter options
+ */
+export interface IFileStorageAdapterOptions {
+  /** Enable checksum verification for data integrity */
+  enableChecksums?: boolean;
+  /** Throw error on checksum mismatch (default: false, just log warning) */
+  strictChecksums?: boolean;
+}

 /**
 * File-based storage adapter for TsmDB
@@ -11,9 +22,13 @@ export class FileStorageAdapter implements IStorageAdapter {
  private opLogCounter = 0;
  private initialized = false;
  private fs = new plugins.smartfs.SmartFs(new plugins.smartfs.SmartFsProviderNode());
+  private enableChecksums: boolean;
+  private strictChecksums: boolean;

-  constructor(basePath: string) {
+  constructor(basePath: string, options?: IFileStorageAdapterOptions) {
    this.basePath = basePath;
+    this.enableChecksums = options?.enableChecksums ?? false;
+    this.strictChecksums = options?.strictChecksums ?? false;
  }

  // ============================================================================
@@ -68,6 +83,45 @@ export class FileStorageAdapter implements IStorageAdapter {
    return doc;
  }

+  /**
+   * Verify document checksum and handle errors
+   */
+  private verifyDocumentChecksum(doc: any): boolean {
+    if (!this.enableChecksums || !doc._checksum) {
+      return true;
+    }
+
+    const isValid = verifyChecksum(doc);
+    if (!isValid) {
+      const errorMsg = `Checksum mismatch for document ${doc._id}`;
+      if (this.strictChecksums) {
+        throw new Error(errorMsg);
+      } else {
+        console.warn(`WARNING: ${errorMsg}`);
+      }
+    }
+    return isValid;
+  }
+
+  /**
+   * Add checksum to document before storing
+   */
+  private prepareDocumentForStorage(doc: any): any {
+    if (!this.enableChecksums) {
+      return doc;
+    }
+    const checksum = calculateDocumentChecksum(doc);
+    return { ...doc, _checksum: checksum };
+  }
+
+  /**
+   * Remove internal checksum field before returning to user
+   */
+  private cleanDocumentForReturn(doc: any): IStoredDocument {
+    const { _checksum, ...cleanDoc } = doc;
+    return this.restoreObjectIds(cleanDoc);
+  }
+
  // ============================================================================
  // Initialization
  // ============================================================================
@@ -233,7 +287,9 @@ export class FileStorageAdapter implements IStorageAdapter {
      throw new Error(`Duplicate key error: _id ${idStr}`);
    }

-    docs.push(storedDoc);
+    // Add checksum if enabled
+    const docToStore = this.prepareDocumentForStorage(storedDoc);
+    docs.push(docToStore);
    await this.writeJsonFile(collPath, docs);
    return storedDoc;
  }
@@ -258,7 +314,9 @@ export class FileStorageAdapter implements IStorageAdapter {
      }

      existingIds.add(idStr);
-      docs.push(storedDoc);
+      // Add checksum if enabled
+      const docToStore = this.prepareDocumentForStorage(storedDoc);
+      docs.push(docToStore);
      results.push(storedDoc);
    }

@@ -270,10 +328,33 @@ export class FileStorageAdapter implements IStorageAdapter {
    await this.createCollection(dbName, collName);
    const collPath = this.getCollectionPath(dbName, collName);
    const docs = await this.readJsonFile<any[]>(collPath, []);
-    return docs.map(doc => this.restoreObjectIds(doc));
+    return docs.map(doc => {
+      // Verify checksum if enabled
+      this.verifyDocumentChecksum(doc);
+      // Clean and return document without internal checksum field
+      return this.cleanDocumentForReturn(doc);
+    });
+  }
+
+  async findByIds(dbName: string, collName: string, ids: Set<string>): Promise<IStoredDocument[]> {
+    await this.createCollection(dbName, collName);
+    const collPath = this.getCollectionPath(dbName, collName);
+    const docs = await this.readJsonFile<any[]>(collPath, []);
+    const results: IStoredDocument[] = [];
+    for (const doc of docs) {
+      // Verify checksum if enabled
+      this.verifyDocumentChecksum(doc);
+      // Clean and restore document
+      const cleaned = this.cleanDocumentForReturn(doc);
+      if (ids.has(cleaned._id.toHexString())) {
+        results.push(cleaned);
+      }
+    }
+    return results;
  }

  async findById(dbName: string, collName: string, id: plugins.bson.ObjectId): Promise<IStoredDocument | null> {
+    // Use findAll which already handles checksum verification
    const docs = await this.findAll(dbName, collName);
    const idStr = id.toHexString();
    return docs.find(d => d._id.toHexString() === idStr) || null;
@@ -291,7 +372,9 @@ export class FileStorageAdapter implements IStorageAdapter {

    if (idx === -1) return false;

-    docs[idx] = doc;
+    // Add checksum if enabled
+    const docToStore = this.prepareDocumentForStorage(doc);
+    docs[idx] = docToStore;
    await this.writeJsonFile(collPath, docs);
    return true;
  }
--- a/ts/tsmdb/storage/IStorageAdapter.ts
+++ b/ts/tsmdb/storage/IStorageAdapter.ts
@@ -90,6 +90,12 @@ export interface IStorageAdapter {
   */
  findAll(dbName: string, collName: string): Promise<IStoredDocument[]>;

+  /**
+   * Find documents by a set of _id strings (hex format)
+   * Used for index-accelerated queries
+   */
+  findByIds(dbName: string, collName: string, ids: Set<string>): Promise<IStoredDocument[]>;
+
  /**
   * Find a document by _id
   */
--- a/ts/tsmdb/storage/MemoryStorageAdapter.ts
+++ b/ts/tsmdb/storage/MemoryStorageAdapter.ts
@@ -196,6 +196,18 @@ export class MemoryStorageAdapter implements IStorageAdapter {
    return Array.from(collection.values());
  }

+  async findByIds(dbName: string, collName: string, ids: Set<string>): Promise<IStoredDocument[]> {
+    const collection = this.ensureCollection(dbName, collName);
+    const results: IStoredDocument[] = [];
+    for (const id of ids) {
+      const doc = collection.get(id);
+      if (doc) {
+        results.push(doc);
+      }
+    }
+    return results;
+  }
+
  async findById(dbName: string, collName: string, id: plugins.bson.ObjectId): Promise<IStoredDocument | null> {
    const collection = this.ensureCollection(dbName, collName);
    return collection.get(id.toHexString()) || null;
--- a/ts/tsmdb/storage/WAL.ts
+++ b/ts/tsmdb/storage/WAL.ts
@@ -0,0 +1,375 @@
+import * as plugins from '../tsmdb.plugins.js';
+import type { Document, IStoredDocument } from '../types/interfaces.js';
+
+/**
+ * WAL entry operation types
+ */
+export type TWalOperation = 'insert' | 'update' | 'delete' | 'checkpoint' | 'begin' | 'commit' | 'abort';
+
+/**
+ * WAL entry structure
+ */
+export interface IWalEntry {
+  /** Log Sequence Number - monotonically increasing */
+  lsn: number;
+  /** Timestamp of the operation */
+  timestamp: number;
+  /** Operation type */
+  operation: TWalOperation;
+  /** Database name */
+  dbName: string;
+  /** Collection name */
+  collName: string;
+  /** Document ID (hex string) */
+  documentId: string;
+  /** Document data (BSON serialized, base64 encoded) */
+  data?: string;
+  /** Previous document data for updates (for rollback) */
+  previousData?: string;
+  /** Transaction ID if part of a transaction */
+  txnId?: string;
+  /** CRC32 checksum of the entry (excluding this field) */
+  checksum: number;
+}
+
+/**
+ * Checkpoint record
+ */
+interface ICheckpointRecord {
+  lsn: number;
+  timestamp: number;
+  lastCommittedLsn: number;
+}
+
+/**
+ * Write-Ahead Log (WAL) for durability and crash recovery
+ *
+ * The WAL ensures durability by writing operations to a log file before
+ * they are applied to the main storage. On crash recovery, uncommitted
+ * operations can be replayed to restore the database to a consistent state.
+ */
+export class WAL {
+  private walPath: string;
+  private currentLsn: number = 0;
+  private lastCheckpointLsn: number = 0;
+  private entries: IWalEntry[] = [];
+  private isInitialized: boolean = false;
+  private fs = new plugins.smartfs.SmartFs(new plugins.smartfs.SmartFsProviderNode());
+
+  // In-memory uncommitted entries per transaction
+  private uncommittedTxns: Map<string, IWalEntry[]> = new Map();
+
+  // Checkpoint interval (number of entries between checkpoints)
+  private checkpointInterval: number = 1000;
+
+  constructor(walPath: string, options?: { checkpointInterval?: number }) {
+    this.walPath = walPath;
+    if (options?.checkpointInterval) {
+      this.checkpointInterval = options.checkpointInterval;
+    }
+  }
+
+  /**
+   * Initialize the WAL, loading existing entries and recovering if needed
+   */
+  async initialize(): Promise<{ recoveredEntries: IWalEntry[] }> {
+    if (this.isInitialized) {
+      return { recoveredEntries: [] };
+    }
+
+    // Ensure WAL directory exists
+    const walDir = this.walPath.substring(0, this.walPath.lastIndexOf('/'));
+    if (walDir) {
+      await this.fs.directory(walDir).recursive().create();
+    }
+
+    // Try to load existing WAL
+    const exists = await this.fs.file(this.walPath).exists();
+    if (exists) {
+      const content = await this.fs.file(this.walPath).encoding('utf8').read();
+      const lines = (content as string).split('\n').filter(line => line.trim());
+
+      for (const line of lines) {
+        try {
+          const entry = JSON.parse(line) as IWalEntry;
+          // Verify checksum
+          if (this.verifyChecksum(entry)) {
+            this.entries.push(entry);
+            if (entry.lsn > this.currentLsn) {
+              this.currentLsn = entry.lsn;
+            }
+            if (entry.operation === 'checkpoint') {
+              this.lastCheckpointLsn = entry.lsn;
+            }
+          }
+        } catch {
+          // Skip corrupted entries
+          console.warn('Skipping corrupted WAL entry');
+        }
+      }
+    }
+
+    this.isInitialized = true;
+
+    // Return entries after last checkpoint that need recovery
+    const recoveredEntries = this.entries.filter(
+      e => e.lsn > this.lastCheckpointLsn &&
+           (e.operation === 'insert' || e.operation === 'update' || e.operation === 'delete')
+    );
+
+    return { recoveredEntries };
+  }
+
+  /**
+   * Log an insert operation
+   */
+  async logInsert(dbName: string, collName: string, doc: IStoredDocument, txnId?: string): Promise<number> {
+    return this.appendEntry({
+      operation: 'insert',
+      dbName,
+      collName,
+      documentId: doc._id.toHexString(),
+      data: this.serializeDocument(doc),
+      txnId,
+    });
+  }
+
+  /**
+   * Log an update operation
+   */
+  async logUpdate(
+    dbName: string,
+    collName: string,
+    oldDoc: IStoredDocument,
+    newDoc: IStoredDocument,
+    txnId?: string
+  ): Promise<number> {
+    return this.appendEntry({
+      operation: 'update',
+      dbName,
+      collName,
+      documentId: oldDoc._id.toHexString(),
+      data: this.serializeDocument(newDoc),
+      previousData: this.serializeDocument(oldDoc),
+      txnId,
+    });
+  }
+
+  /**
+   * Log a delete operation
+   */
+  async logDelete(dbName: string, collName: string, doc: IStoredDocument, txnId?: string): Promise<number> {
+    return this.appendEntry({
+      operation: 'delete',
+      dbName,
+      collName,
+      documentId: doc._id.toHexString(),
+      previousData: this.serializeDocument(doc),
+      txnId,
+    });
+  }
+
+  /**
+   * Log transaction begin
+   */
+  async logBeginTransaction(txnId: string): Promise<number> {
+    this.uncommittedTxns.set(txnId, []);
+    return this.appendEntry({
+      operation: 'begin',
+      dbName: '',
+      collName: '',
+      documentId: '',
+      txnId,
+    });
+  }
+
+  /**
+   * Log transaction commit
+   */
+  async logCommitTransaction(txnId: string): Promise<number> {
+    this.uncommittedTxns.delete(txnId);
+    return this.appendEntry({
+      operation: 'commit',
+      dbName: '',
+      collName: '',
+      documentId: '',
+      txnId,
+    });
+  }
+
+  /**
+   * Log transaction abort
+   */
+  async logAbortTransaction(txnId: string): Promise<number> {
+    this.uncommittedTxns.delete(txnId);
+    return this.appendEntry({
+      operation: 'abort',
+      dbName: '',
+      collName: '',
+      documentId: '',
+      txnId,
+    });
+  }
+
+  /**
+   * Get entries to roll back for an aborted transaction
+   */
+  getTransactionEntries(txnId: string): IWalEntry[] {
+    return this.entries.filter(e => e.txnId === txnId);
+  }
+
+  /**
+   * Create a checkpoint - marks a consistent point in the log
+   */
+  async checkpoint(): Promise<number> {
+    const lsn = await this.appendEntry({
+      operation: 'checkpoint',
+      dbName: '',
+      collName: '',
+      documentId: '',
+    });
+    this.lastCheckpointLsn = lsn;
+
+    // Truncate old entries (keep only entries after checkpoint)
+    await this.truncate();
+
+    return lsn;
+  }
+
+  /**
+   * Truncate the WAL file, removing entries before the last checkpoint
+   */
+  private async truncate(): Promise<void> {
+    // Keep entries after last checkpoint
+    const newEntries = this.entries.filter(e => e.lsn >= this.lastCheckpointLsn);
+    this.entries = newEntries;
+
+    // Rewrite the WAL file
+    const lines = this.entries.map(e => JSON.stringify(e)).join('\n');
+    await this.fs.file(this.walPath).encoding('utf8').write(lines);
+  }
+
+  /**
+   * Get current LSN
+   */
+  getCurrentLsn(): number {
+    return this.currentLsn;
+  }
+
+  /**
+   * Get entries after a specific LSN (for recovery)
+   */
+  getEntriesAfter(lsn: number): IWalEntry[] {
+    return this.entries.filter(e => e.lsn > lsn);
+  }
+
+  /**
+   * Close the WAL
+   */
+  async close(): Promise<void> {
+    if (this.isInitialized) {
+      // Final checkpoint before close
+      await this.checkpoint();
+    }
+    this.isInitialized = false;
+  }
+
+  // ============================================================================
+  // Private Methods
+  // ============================================================================
+
+  private async appendEntry(
+    partial: Omit<IWalEntry, 'lsn' | 'timestamp' | 'checksum'>
+  ): Promise<number> {
+    await this.initialize();
+
+    this.currentLsn++;
+    const entry: IWalEntry = {
+      ...partial,
+      lsn: this.currentLsn,
+      timestamp: Date.now(),
+      checksum: 0, // Will be calculated
+    };
+
+    // Calculate checksum
+    entry.checksum = this.calculateChecksum(entry);
+
+    // Track in transaction if applicable
+    if (partial.txnId && this.uncommittedTxns.has(partial.txnId)) {
+      this.uncommittedTxns.get(partial.txnId)!.push(entry);
+    }
+
+    // Add to in-memory log
+    this.entries.push(entry);
+
+    // Append to file (append mode for durability)
+    await this.fs.file(this.walPath).encoding('utf8').append(JSON.stringify(entry) + '\n');
+
+    // Check if we need a checkpoint
+    if (this.entries.length - this.lastCheckpointLsn >= this.checkpointInterval) {
+      await this.checkpoint();
+    }
+
+    return entry.lsn;
+  }
+
+  private serializeDocument(doc: Document): string {
+    // Serialize document to BSON and encode as base64
+    const bsonData = plugins.bson.serialize(doc);
+    return Buffer.from(bsonData).toString('base64');
+  }
+
+  private deserializeDocument(data: string): Document {
+    // Decode base64 and deserialize from BSON
+    const buffer = Buffer.from(data, 'base64');
+    return plugins.bson.deserialize(buffer);
+  }
+
+  private calculateChecksum(entry: IWalEntry): number {
+    // Simple CRC32-like checksum
+    const str = JSON.stringify({
+      lsn: entry.lsn,
+      timestamp: entry.timestamp,
+      operation: entry.operation,
+      dbName: entry.dbName,
+      collName: entry.collName,
+      documentId: entry.documentId,
+      data: entry.data,
+      previousData: entry.previousData,
+      txnId: entry.txnId,
+    });
+
+    let crc = 0xFFFFFFFF;
+    for (let i = 0; i < str.length; i++) {
+      crc ^= str.charCodeAt(i);
+      for (let j = 0; j < 8; j++) {
+        crc = (crc >>> 1) ^ (crc & 1 ? 0xEDB88320 : 0);
+      }
+    }
+    return (~crc) >>> 0;
+  }
+
+  private verifyChecksum(entry: IWalEntry): boolean {
+    const savedChecksum = entry.checksum;
+    entry.checksum = 0;
+    const calculatedChecksum = this.calculateChecksum(entry);
+    entry.checksum = savedChecksum;
+    return calculatedChecksum === savedChecksum;
+  }
+
+  /**
+   * Recover document from WAL entry
+   */
+  recoverDocument(entry: IWalEntry): IStoredDocument | null {
+    if (!entry.data) return null;
+    return this.deserializeDocument(entry.data) as IStoredDocument;
+  }
+
+  /**
+   * Recover previous document state from WAL entry (for rollback)
+   */
+  recoverPreviousDocument(entry: IWalEntry): IStoredDocument | null {
+    if (!entry.previousData) return null;
+    return this.deserializeDocument(entry.previousData) as IStoredDocument;
+  }
+}
--- a/ts/tsmdb/utils/checksum.ts
+++ b/ts/tsmdb/utils/checksum.ts
@@ -0,0 +1,88 @@
+/**
+ * CRC32 checksum utilities for data integrity
+ */
+
+// CRC32 lookup table
+const CRC32_TABLE: number[] = [];
+
+// Initialize the CRC32 table
+function initCRC32Table(): void {
+  if (CRC32_TABLE.length > 0) return;
+
+  for (let i = 0; i < 256; i++) {
+    let crc = i;
+    for (let j = 0; j < 8; j++) {
+      crc = (crc & 1) ? (0xEDB88320 ^ (crc >>> 1)) : (crc >>> 1);
+    }
+    CRC32_TABLE[i] = crc >>> 0;
+  }
+}
+
+/**
+ * Calculate CRC32 checksum for a string
+ */
+export function calculateCRC32(data: string): number {
+  initCRC32Table();
+
+  let crc = 0xFFFFFFFF;
+  for (let i = 0; i < data.length; i++) {
+    const byte = data.charCodeAt(i) & 0xFF;
+    crc = CRC32_TABLE[(crc ^ byte) & 0xFF] ^ (crc >>> 8);
+  }
+  return (~crc) >>> 0;
+}
+
+/**
+ * Calculate CRC32 checksum for a Buffer
+ */
+export function calculateCRC32Buffer(data: Buffer): number {
+  initCRC32Table();
+
+  let crc = 0xFFFFFFFF;
+  for (let i = 0; i < data.length; i++) {
+    crc = CRC32_TABLE[(crc ^ data[i]) & 0xFF] ^ (crc >>> 8);
+  }
+  return (~crc) >>> 0;
+}
+
+/**
+ * Calculate checksum for a document (serialized as JSON)
+ */
+export function calculateDocumentChecksum(doc: Record<string, any>): number {
+  // Exclude _checksum field from calculation
+  const { _checksum, ...docWithoutChecksum } = doc;
+  const json = JSON.stringify(docWithoutChecksum);
+  return calculateCRC32(json);
+}
+
+/**
+ * Add checksum to a document
+ */
+export function addChecksum<T extends Record<string, any>>(doc: T): T & { _checksum: number } {
+  const checksum = calculateDocumentChecksum(doc);
+  return { ...doc, _checksum: checksum };
+}
+
+/**
+ * Verify checksum of a document
+ * Returns true if checksum is valid or if document has no checksum
+ */
+export function verifyChecksum(doc: Record<string, any>): boolean {
+  if (!('_checksum' in doc)) {
+    // No checksum to verify
+    return true;
+  }
+
+  const storedChecksum = doc._checksum;
+  const calculatedChecksum = calculateDocumentChecksum(doc);
+
+  return storedChecksum === calculatedChecksum;
+}
+
+/**
+ * Remove checksum from a document
+ */
+export function removeChecksum<T extends Record<string, any>>(doc: T): Omit<T, '_checksum'> {
+  const { _checksum, ...docWithoutChecksum } = doc;
+  return docWithoutChecksum as Omit<T, '_checksum'>;
+}
--- a/ts/tsmdb/utils/index.ts
+++ b/ts/tsmdb/utils/index.ts
@@ -0,0 +1 @@
+export * from './checksum.js';
Author	SHA1	Message	Date
Juergen Kunz	e3dc19aa7c	v4.1.0 Some checks failed Default (tags) / security (push) Successful in 29s Details Default (tags) / test (push) Failing after 4m0s Details Default (tags) / release (push) Has been skipped Details Default (tags) / metadata (push) Has been skipped Details	2026-02-01 16:05:00 +00:00
Juergen Kunz	316af45b5e	feat(readme): expand README with storage integrity, WAL, query planner, session & transaction docs; update test script to enable verbose logging and increase timeout	2026-02-01 16:05:00 +00:00
Juergen Kunz	6932059965	v4.0.0 Some checks failed Default (tags) / security (push) Successful in 43s Details Default (tags) / test (push) Failing after 4m0s Details Default (tags) / release (push) Has been skipped Details Default (tags) / metadata (push) Has been skipped Details	2026-02-01 16:02:03 +00:00
Juergen Kunz	bd1764159e	BREAKING CHANGE(storage,engine,server): add session & transaction management, index/query planner, WAL and checksum support; integrate index-accelerated queries and update storage API (findByIds) to enable index optimizations	2026-02-01 16:02:03 +00:00