TYPO3 v12 Cache Tag Invalidation Breaks with Non‑Taggable PSR‑6 Backends

Goal

Guarantee that flushByTag() reliably invalidates cached entries for core features such as page cache, content element caching, and Extbase persistence.

Constraints and Uncertainty

• TYPO3 v12 migrated the Caching Framework to PSR‑6/PSR‑16, making TaggableCacheInterface a hard requirement for tag‑based flushing.
• Native RedisBackend implements tags via Redis sets, but many generic PSR‑6 adapters (e.g., league/flysystem, cloud SDKs) do not.
• When a backend lacks TaggableCacheInterface, flushByTag() either throws UnsupportedOperationException or is silently ignored, depending on CacheFactory configuration.
• No built‑in health check, fallback, or deprecation warning alerts administrators to the loss of tag invalidation.

Open Questions

1. Which backend configuration changes ensure TaggableCacheInterface compliance for managed Redis services (AWS ElastiCache, Azure Cache for Redis)?
2. How can administrators detect silent tag‑invalidation failures in production without custom instrumentation?
3. Is a core‑level fallback or health‑check mechanism planned for future TYPO3 releases?