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+# Knowledge Module Caching Layer Implementation
+
+**Issue:** #79  
+**Branch:** `feature/knowledge-cache`  
+**Status:** ✅ Complete
+
+## Overview
+
+Implemented a comprehensive caching layer for the Knowledge module using Valkey (Redis-compatible), providing significant performance improvements for frequently accessed data.
+
+## Implementation Summary
+
+### 1. Cache Service (`cache.service.ts`)
+
+Created `KnowledgeCacheService` with the following features:
+
+**Core Functionality:**
+- Entry detail caching (by workspace ID and slug)
+- Search results caching (with filter-aware keys)
+- Graph query caching (by entry ID and depth)
+- Configurable TTL (default: 5 minutes)
+- Cache statistics tracking (hits, misses, hit rate)
+- Pattern-based cache invalidation
+
+**Cache Key Structure:**
+```
+knowledge:entry:{workspaceId}:{slug}
+knowledge:search:{workspaceId}:{query}:{filterHash}
+knowledge:graph:{workspaceId}:{entryId}:{maxDepth}
+```
+
+**Configuration:**
+- `KNOWLEDGE_CACHE_ENABLED` - Enable/disable caching (default: true)
+- `KNOWLEDGE_CACHE_TTL` - Cache TTL in seconds (default: 300)
+- `VALKEY_URL` - Valkey connection URL
+
+**Statistics:**
+- Hits/misses tracking
+- Hit rate calculation
+- Sets/deletes counting
+- Statistics reset functionality
+
+### 2. Service Integration
+
+**KnowledgeService (`knowledge.service.ts`):**
+- ✅ Cache-aware `findOne()` - checks cache before DB lookup
+- ✅ Cache invalidation on `create()` - invalidates search/graph caches
+- ✅ Cache invalidation on `update()` - invalidates entry, search, and graph caches
+- ✅ Cache invalidation on `remove()` - invalidates entry, search, and graph caches
+- ✅ Cache invalidation on `restoreVersion()` - invalidates entry, search, and graph caches
+
+**SearchService (`search.service.ts`):**
+- ✅ Cache-aware `search()` - checks cache before executing PostgreSQL query
+- ✅ Filter-aware caching (different results for different filters/pages)
+- ✅ Automatic cache population on search execution
+
+**GraphService (`graph.service.ts`):**
+- ✅ Cache-aware `getEntryGraph()` - checks cache before graph traversal
+- ✅ Depth-aware caching (different cache for different depths)
+- ✅ Automatic cache population after graph computation
+
+### 3. Cache Invalidation Strategy
+
+**Entry-level invalidation:**
+- On create: invalidate workspace search/graph caches
+- On update: invalidate specific entry, workspace search caches, related graph caches
+- On delete: invalidate specific entry, workspace search/graph caches
+- On restore: invalidate specific entry, workspace search/graph caches
+
+**Link-level invalidation:**
+- When entry content changes (potential link changes), invalidate graph caches
+
+**Workspace-level invalidation:**
+- Admin endpoint to clear all caches for a workspace
+
+### 4. REST API Endpoints
+
+**Cache Statistics (`KnowledgeCacheController`):**
+
+```http
+GET /api/knowledge/cache/stats
+```
+Returns cache statistics and enabled status (requires: workspace member)
+
+**Response:**
+```json
+{
+  "enabled": true,
+  "stats": {
+    "hits": 1250,
+    "misses": 180,
+    "sets": 195,
+    "deletes": 15,
+    "hitRate": 0.874
+  }
+}
+```
+
+```http
+POST /api/knowledge/cache/clear
+```
+Clears all caches for the workspace (requires: workspace admin)
+
+```http
+POST /api/knowledge/cache/stats/reset
+```
+Resets cache statistics (requires: workspace admin)
+
+### 5. Testing
+
+Created comprehensive test suite (`cache.service.spec.ts`):
+
+**Test Coverage:**
+- ✅ Cache enabled/disabled configuration
+- ✅ Entry caching (get, set, invalidate)
+- ✅ Search caching with filter differentiation
+- ✅ Graph caching with depth differentiation
+- ✅ Cache statistics tracking
+- ✅ Workspace cache clearing
+- ✅ Cache miss/hit behavior
+- ✅ Pattern-based invalidation
+
+**Test Scenarios:** 13 test cases covering all major functionality
+
+### 6. Documentation
+
+**Updated README.md:**
+- Added "Caching" section with overview
+- Configuration examples
+- Cache invalidation strategy explanation
+- Performance benefits (estimated 80-99% improvement)
+- API endpoint documentation
+
+**Updated .env.example:**
+- Added `KNOWLEDGE_CACHE_ENABLED` configuration
+- Added `KNOWLEDGE_CACHE_TTL` configuration
+- Included helpful comments
+
+## Files Modified/Created
+
+### New Files:
+- ✅ `apps/api/src/knowledge/services/cache.service.ts` (381 lines)
+- ✅ `apps/api/src/knowledge/services/cache.service.spec.ts` (296 lines)
+
+### Modified Files:
+- ✅ `apps/api/src/knowledge/knowledge.service.ts` - Added cache integration
+- ✅ `apps/api/src/knowledge/services/search.service.ts` - Added cache integration
+- ✅ `apps/api/src/knowledge/services/graph.service.ts` - Added cache integration
+- ✅ `apps/api/src/knowledge/knowledge.controller.ts` - Added cache endpoints
+- ✅ `apps/api/src/knowledge/knowledge.module.ts` - Added cache service provider
+- ✅ `apps/api/src/knowledge/services/index.ts` - Exported cache service
+- ✅ `apps/api/package.json` - Added ioredis dependency
+- ✅ `.env.example` - Added cache configuration
+- ✅ `README.md` - Added cache documentation
+
+## Performance Impact
+
+**Expected Performance Improvements:**
+- Entry retrieval: 10-50ms → 2-5ms (80-90% improvement)
+- Search queries: 100-300ms → 2-5ms (95-98% improvement)
+- Graph traversals: 200-500ms → 2-5ms (95-99% improvement)
+
+**Cache Hit Rates:**
+- Expected: 70-90% for active workspaces
+- Measured via `/api/knowledge/cache/stats` endpoint
+
+## Configuration
+
+### Environment Variables
+
+```bash
+# Enable/disable caching (useful for development)
+KNOWLEDGE_CACHE_ENABLED=true
+
+# Cache TTL in seconds (default: 5 minutes)
+KNOWLEDGE_CACHE_TTL=300
+
+# Valkey connection
+VALKEY_URL=redis://localhost:6379
+```
+
+### Development Mode
+
+Disable caching during development:
+```bash
+KNOWLEDGE_CACHE_ENABLED=false
+```
+
+## Git History
+
+```bash
+# Commits:
+576d2c3 - chore: add ioredis dependency for cache service
+90abe2a - feat: add knowledge module caching layer (closes #79)
+
+# Branch: feature/knowledge-cache
+# Remote: origin/feature/knowledge-cache
+```
+
+## Next Steps
+
+1. ✅ Merge to develop branch
+2. ⏳ Monitor cache hit rates in production
+3. ⏳ Tune TTL values based on usage patterns
+4. ⏳ Consider adding cache warming for frequently accessed entries
+5. ⏳ Add cache metrics to monitoring dashboard
+
+## Deliverables Checklist
+
+- ✅ Caching service integrated with Valkey
+- ✅ Entry detail cache (GET /api/knowledge/entries/:slug)
+- ✅ Search results cache
+- ✅ Graph query cache
+- ✅ TTL configuration (5 minutes default, configurable)
+- ✅ Cache invalidation on update/delete
+- ✅ Cache invalidation on entry changes
+- ✅ Cache invalidation on link changes
+- ✅ Caching wrapped around KnowledgeService methods
+- ✅ Cache statistics endpoint
+- ✅ Environment variables for cache TTL
+- ✅ Option to disable cache for development
+- ✅ Cache hit/miss metrics
+- ✅ Tests for cache behavior
+- ✅ Documentation in README
+
+## Notes
+
+- Cache gracefully degrades - errors don't break the application
+- Cache can be completely disabled via environment variable
+- Statistics are in-memory (reset on service restart)
+- Pattern-based invalidation uses Redis SCAN (safe for large datasets)
+- All cache operations are async and non-blocking
+
+---
+
+**Implementation Complete:** All deliverables met ✅  
+**Ready for:** Code review and merge to develop