Edge cache

[nanaabdul1172]

closes #150

Files Created/Modified
NEW:
cache-control.interceptor.ts

Created interceptors that add Cache-Control and Cache-Tag headers
NearbyCacheControlInterceptor for nearby gist queries (uses geohash-based tags)
GistIdCacheControlInterceptor for single gist queries (uses ID-based tags)
Headers: Cache-Control: s-maxage=60, stale-while-revalidate=120
MODIFIED:
gists.controller.ts

Applied NearbyCacheControlInterceptor to GET /gists endpoint
Applied GistIdCacheControlInterceptor to GET /gists/:id endpoint
Cache tags generated dynamically based on request parameters
MODIFIED:
gists.service.ts

Updated invalidateNearbyCache() to call CDN purge when new gists are posted
Purges cache tags like gist:nearby:{geohash_cell}
MODIFIED:
cache.service.ts

Added purgeCdnTags() method supporting Fastly, Vercel, and Cloudflare
Gracefully degrades if CDN config not present
MODIFIED:
gists.module.ts

Registered interceptors as providers for dependency injection
MODIFIED:
.env.example

Added CDN_PURGE_ENDPOINT and CDN_PURGE_TOKEN configuration
NEW:
CACHE_CONTROL_IMPLEMENTATION.md

Comprehensive documentation of the implementation
✅ Acceptance Criteria Met
✅ Response bears both Cache-Control and Cache-Tag headers
✅ Post a new gist → purge the tag → CDNs revalidate
✅ Implementation in
gists.controller.ts
✅ Created
cache-control.interceptor.ts
🎯 How It Works
GET requests receive headers:

Cache-Control: s-maxage=60, stale-while-revalidate=120
Cache-Tag: gist:nearby:{geohash} (for nearby queries)
Cache-Tag: gist:one:{id} (for single gist queries)
POST requests trigger:

Redis cache invalidation (existing behavior)
CDN purge via API call to configured endpoint
Tag-based purging ensures only affected caches are invalidated
Cache tags use geohash for nearby queries, so all queries in the same ~153m cell share cache and purge together

🔧 Configuration
Add to .env (optional, degrades gracefully if not set):

CDN_PURGE_ENDPOINT=https://api.fastly.com/service/{service_id}/purge
CDN_PURGE_TOKEN=your_api_token
✅ Verified
No TypeScript compilation errors
All dependencies properly injected
GeoService correctly used for geohash computation
Backward compatible (works without CDN configuration)