The reverse index approach (Redis sets tracking HMAC keys per context)
was over-engineered. Scoped purge doesn't need an index — recompute
the HMAC key from the invalidation params and DELETE directly. One
Redis command, no TOCTOU race, no atomicity concern, no stale members.
Broad purge uses key-prefix scan (keys are now "ctx:{context}:{hmac}").
This is rare (Tier 3 fallback) and acceptable as a SCAN operation.
Eliminated from both Python and TypeScript:
- All SET/SADD/SMEMBERS/SREM index operations
- CacheBackend.get_index, remove_from_index, delete_index, delete_indexes_by_prefix
- build_index_keys function
- Pipeline transaction complexity
- TOCTOU race condition (was critical, now impossible)
Backend interface is now 5 methods: get, set, delete, delete_by_prefix, clear.
Redis tests updated — prefix isolation test added, connection leak fixed.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Ryth Azhur
7f5542e305