# 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