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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name:: cache-coherence-patterns-reference
description:: 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.

cache-coherence-patterns-reference

Name: testland/cache-coherence-patterns-reference
Rating: 75.2 (1 reviews)
Author: testland

Overview

Keeping cached values consistent with their source of truth across tiers (browser, CDN, reverse-proxy, application, data store). Wrong coherence shows as stale data; wrong invalidation shows as cache stampedes per cache-stampede-reference. A pure reference consumed by per-tier test skills.

When to use

Designing the cache tiers for a new product / endpoint.
Auditing an existing cache for coherence bugs (stale reads after writes, cross-tenant cache leaks, layered TTLs that fight each other).
PR review of changes to cache headers, Vary, or invalidation triggers.
Investigating "users see stale data" reports.

How to use this reference

Locate the tier(s) the value lives in from the five-tier stack - each tier (browser, CDN, reverse proxy, application, data store) has its own TTL and invalidation mechanism.
Choose the write pattern for the application tier from the cache-writing patterns table (cache-aside, write-through, write-back, write-around, refresh-ahead) based on the read/write mix and how much consistency you need.
Choose the invalidation strategy from the invalidation strategies table (TTL-only, event-driven purge, surrogate keys, version-tagged URLs, soft purge) based on the staleness window you can tolerate.
Set the contract - design the Cache-Control directives, Vary key, and ETag validators per references/rfc-9111-http-caching-directives.md.
Write the per-tier coherence test from the cross-tier problems and test-surface catalog in references/cross-tier-coherence-and-test-surface.md, then re-check the design against the Anti-patterns table below.

The five-tier stack

Tier	Where	Common TTL	Invalidation
Browser	`Cache-Control: private`	minutes-hours	TTL only (or Service Worker code)
CDN	Cloudflare / Fastly / CloudFront / Akamai	seconds-days	Purge API or surrogate-key tag
Reverse proxy	Varnish, nginx	seconds-hours	VCL purge / nginx cache_purge
Application	Redis / Memcached / in-process	seconds-minutes	Direct delete / pub-sub broadcast
Data store	Postgres query cache, RDS read replicas	seconds	Replication-driven

A coherence bug at any tier surfaces at the user. The test surface is layered; each tier needs its own coherence tests.

Cache-writing patterns

For application-tier caches (Redis):

Pattern	Flow	When
Cache-aside (lazy load)	Read miss → read source → populate → return; Write → invalidate cache	Read-heavy, eventual consistency OK
Write-through	Write → write source → write cache (synchronous)	Strong consistency, latency tolerable
Write-back	Write → write cache → async write to source	Burst writes; data-loss risk on cache crash
Write-around	Write → write source (skip cache); reads do cache-aside	Write-heavy with rare re-reads
Refresh-ahead	Background refresh before TTL expires	Predictable read patterns; hot keys

Invalidation strategies

Strategy	Mechanism	Trade-off
TTL-only	Just let it expire	Simple; possibly-stale window = TTL
Event-driven purge	Source-of-truth update fires a delete	Coupling; firehose at high write rate
Surrogate keys (Fastly, Varnish)	Tag responses; purge by tag	Group-invalidation; coordination cost
Version-tagged URLs	`/api/users?_v=42`; new version = new key	Immutable cache; full deploy per change
Soft purge	Mark stale, keep serving until refresh	Used by stale-while-revalidate per `stale-while-revalidate-reference`

Worked example: a multi-tenant dashboard endpoint

Scenario: /api/users serves per-tenant dashboard data, is read-heavy, and must never leak one tenant's rows to another. Walk the four decisions from How to use this reference, then the test.

Tiers. The response flows browser → CDN → application (Redis) → data store. Because the payload is per-user, it must not sit in a shared cache: the browser tier gets Cache-Control: private and the CDN is bypassed for this route (or keyed per tenant), not left on the shared edge.
Write pattern. Reads dominate and eventual consistency after a profile edit is acceptable, so the application tier uses cache-aside: a read miss loads from the data store and populates Redis; a write invalidates the tenant's Redis key.
Invalidation. A profile edit is an urgent update, so TTL-only is not enough - pair the TTL with event-driven purge so the write fires a delete of the tenant's cache key immediately.
Contract. Set Vary: Authorization so each tenant gets a separate cache entry (the fix for the cross-tenant leak in the Anti-patterns table), and add a content-hash ETag so an unchanged reload returns 304 Not Modified instead of the full body.

Coherence test (the browser-tier "write → reload → see old" case):

Arrange: tenant A loads /api/users; the response is cached.
Act: tenant A edits a user - which must fire the Redis purge - then reloads the page.
Assert (staleness): the reload shows the edited value, not the pre-write state. A failure here means the write path skipped the invalidate - the "cache-aside without write-then-invalidate" anti-pattern.
Assert (isolation): a request from tenant B with a different Authorization header never returns tenant A's cached rows, proving the Vary: Authorization split holds.

Anti-patterns

Anti-pattern	Why it fails	Fix
`Cache-Control: public` on per-user data	Shared cache leaks data	Use `private` for user-specific
Missing `Vary: Authorization`	Cross-tenant leak	Add to Vary or set `private`
`s-maxage` longer than session lifetime	Logged-out users see another user's data	Match TTL to security window
TTL but no purge	Stale-window = TTL even for urgent updates	Implement purge API + use surrogate keys
ETag generated per-request from `now()`	Defeats the validation	Stable ETag from content hash
`no-cache` instead of `no-store` for sensitive data	Browser still stores; just revalidates	`no-store, no-cache, must-revalidate, private`
Browser TTL = CDN TTL = origin TTL	Multi-tier amplifies staleness instead of layering it	Origin lowest, CDN longer, browser shortest
Cache-aside without write-then-invalidate	Reads see pre-write state for TTL window	Always invalidate on write
`Vary: *`	Disables shared cache entirely	Use specific headers
Single Cache-Control for HTML + JSON + assets	One-size doesn't fit; HTML often short, assets long	Per-route directives

Deep references

The contract layer and the audit-and-test surface live in two companion references so this file stays a decision surface:

RFC 9111 directive tables - Cache-Control response directives, Vary key derivation, and ETag / If-None-Match revalidation: references/rfc-9111-http-caching-directives.md.
Cross-tier coherence problems + per-tier test surface - the seam bugs to audit for and what to test at each tier: references/cross-tier-coherence-and-test-surface.md.

Limitations

RFC 9111 governs HTTP caches only. Application-tier caches (Redis) use their own semantics; coherence is application- enforced.
Doesn't specify replication. Read replicas, multi-region CDN have their own coherence layer.
No global invalidation. Cross-tier purge requires coordination; no built-in protocol.
Cache-Control parsing has implementation drift. Some CDNs ignore directives they don't recognise; verify per vendor.

References

RFC 9111 HTTP Caching: www.rfc-editor.org/rfc/rfc9111.html; full directive tables in references/rfc-9111-http-caching-directives.md.
RFC 8246 immutable: www.rfc-editor.org/rfc/rfc8246.html.
Cross-tier coherence problems + per-tier test surface (with their cross-references): references/cross-tier-coherence-and-test-surface.md.
RFC 5861 stale-while-revalidate / stale-if-error (companion): stale-while-revalidate-reference.
Companion catalog: cache-stampede-reference.
Cross-tenant leaks via cache: cross-tenant-data-leak-tests.
Consumed by: redis-cache-tests, cdn-cache-purge-tests, varnish-test-vtc-syntax, browser-cache-control-tests.

Workspace: testland
Visibility: Public
Created: 8 days ago
Last updated: 7 days ago
Repository: https://github.com/testland/qa
Publish Source: GitHub
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