testland/cache-coherence-patterns-reference
Pure-reference catalog of cache-coherence patterns across the request path. Defines the five-tier cache stack (browser → CDN → reverse-proxy → application → data store), the per-tier cache-writing patterns (cache-aside, write-through, write-back, write-around, refresh-ahead), and the canonical invalidation strategies (TTL-only, event-driven purge, surrogate keys, version-tagged URLs, soft purge), plus an anti-pattern table and a worked multi-tenant coherence-test example. Deep detail - the RFC 9111 Cache-Control / Vary / ETag directive tables and the cross-tier coherence + per-tier test surface - lives in references/. Use for pattern selection, Cache-Control header design, and coherence audits; use a cache-key-collision check when the question is whether two requests in an existing system collide on a concrete key scheme. Consumed by redis-cache-tests, cdn-cache-purge-tests, varnish-test-vtc-syntax, browser-cache-control-tests, and the cache-key-collision check.
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cross-tier-coherence-and-test-surface.mdreferences/
Cross-tier coherence problems and the per-tier test surface
Deep reference for cache-coherence-patterns-reference SKILL.md. Consult
when auditing an existing multi-tier cache for coherence bugs and when
deciding what to test at each tier.
Cross-tier coherence problems
A coherence bug at any tier surfaces at the user; the failure usually lives in the seam between two tiers, not inside one.
| Problem | Where | Detection |
|---|---|---|
| Browser caches stale page after server purge | Browser ignores must-revalidate, or no must-revalidate | E2E test: write → reload → see old |
| CDN serves stale after origin update | Purge didn't propagate or s-maxage too long | E2E: write → purge → read at CDN edge |
| Different Vary at browser vs CDN | CDN strips headers; cache keys diverge | Header-comparison test |
| Layered TTL inversion | s-maxage < max-age → CDN refreshes more often than browser; browser eventually outpaces CDN | Audit the TTL stack |
Vary: Cookie without normalised cookies | Tracker cookies fragment cache; near-zero hit rate | Inspect Vary; normalise |
| Tenant-scoped data with shared Vary | Cross-tenant leak per cross-tenant-data-leak-tests | Add Authorization to Vary or use private |
Testable behaviours by tier
Each tier needs its own coherence tests; the categories below map to the per-tier test skills that consume this reference.
| Tier | Test categories |
|---|---|
| Browser | Cache-Control respected (max-age, no-cache, must-revalidate); ETag round-trip; Vary honoured |
| CDN | Edge hit/miss vs origin; purge API works end-to-end; s-maxage overrides max-age |
| Reverse proxy | VCL purge (varnish-test-vtc-syntax); grace-mode behaviour |
| Application | Cache-aside write-then-invalidate; key collisions |
| Data store | Replication lag (separate concern; out of scope here) |