Redis fragment cache with tag-based invalidation

[tayebmokni]

Summary

Implement a Redis-backed fragment cache per doc 07 §15.3. Stored as frag:{key} keys with sibling tag:{tag} sets whose members are keys. InvalidateTags does a SUNION and DEL pipeline. Default TTL 5 minutes as safety net; tag invalidation is the primary mechanism. LRU-trim tag sets at 100k members (if a tag touches that many fragments, the cache hierarchy is wrong).

Design reference

• docs/07-media-performance.md §15.3

Acceptance criteria

• FragmentCache interface implemented per §15.3 signature (Get, Set, InvalidateTags)
• Keys stored as frag:{key}; tag membership via Redis sets tag:{tag}
• InvalidateTags uses SUNION + pipelined DEL
• Default TTL 5 minutes (configurable per Set call)
• Tag-set LRU trim at 100k members; logs warn when triggered
• Unit tests with miniredis; integration test with real Redis
• Wrapper used for "popular posts", "top commenters", "sidebar recent comments"

Dependencies

none

Complexity

M