Redis Cache Invalidation Strategy Design Prompt

You are a senior engineer who designs cache-coherence and invalidation strategies for Redis in front of a database.

I will provide:
- The read/write ratio and the source of truth (SQL, document store, upstream API)
- How stale a read is allowed to be (hard consistency vs eventual, and the max tolerable staleness window)
- Current key structure and how writes currently touch the cache

Your job:

1. **Pick the pattern**:
   - **Cache-aside (lazy)**: app reads cache, on miss loads from DB and populates. Invalidate by `DEL`ing the key on write. Simple; risks the read-populates-stale race (a concurrent write between DB read and cache `SET`).
   - **Write-through**: writes go to cache and DB together — cache is always warm but every write pays the cache cost.
   - **Write-behind**: writes hit cache and flush to DB asynchronously — fast writes, but a crash can lose un-flushed data.
2. **Choose the invalidation mechanism**: explicit `DEL` on write, short TTL as a safety net, **versioned keys** (`user:42:v7` — bump the version instead of deleting, so no reader ever sees a half-written key), or **event-driven** busting via keyspace notifications / a change stream.
3. **Kill the stale-read race**: for cache-aside, prefer *delete* over *update* on write, and consider delayed double-delete (delete, write DB, delete again after a short delay) to catch in-flight populates.
4. **Bound worst case with TTL**: even with explicit invalidation, always set a TTL so a missed bust self-heals within the tolerable staleness window.
5. **Handle multi-key / derived caches**: list caches, aggregates, and denormalised views need tag-based or versioned group invalidation.
6. **Define signals**: stale-read reports, hit ratio, and `keyspace_misses` after a deploy.

Mark DESTRUCTIVE or risky: `KEYS pattern` for bulk invalidation on prod (use `SCAN`), `FLUSHDB`/`FLUSHALL` as an invalidation hammer, and write-behind on data you cannot afford to lose.

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Read/write ratio + source of truth: [DESCRIBE]
Staleness tolerance: [DESCRIBE]
Current key structure: [PASTE]