HTTP Caching & Conditional Requests (ETags)

Freshness lets a cache reuse a stored response with no network call at all. Validation lets a cache cheaply confirm a stored response is still good via a round-trip that carries no body. Conflating the two is the tell of a shallow answer — you design with both: aggressive freshness for the common case, validation as the cheap fallback when freshness lapses.

public may be stored by any cache, including shared ones (CDN/proxy). private restricts storage to the end user's own browser — never a shared cache. private is the guard for per-user payloads.

The client sends the stored ETag in If-None-Match. If the representation still matches, the server returns 304 Not Modified with no body — the client reuses its cached copy. Only if it changed does the server return 200 with the full body.

412 Precondition Failed. The client should re-GET the resource (and its new ETag), merge or re-apply its change against the current state, and retry the write with the updated If-Match.

no-store means never write it to any cache — the directive for genuinely sensitive data. no-cache is confusingly named: it does store the response, but must revalidate with the origin before every reuse (expecting a 304). So no-cache still saves bandwidth on unchanged bodies; no-store saves nothing because nothing is kept.

The client GETs the resource and its ETag, then sends If-Match: "<etag>" on the PUT/PATCH. The server applies the write only if the current ETag still matches. If the resource changed underneath, the ETag no longer matches and the server returns 412 Precondition Failed — the write is rejected and the client must re-fetch and reconcile rather than clobber. This is optimistic concurrency control.

A strong validator means byte-for-byte identical. A weak one (W/ prefix) means semantically equivalent — the payload may differ cosmetically (whitespace, a regenerated timestamp) but is "the same" for the user. Strong is required for Range requests and is what you want for concurrency control; weak is fine for cache revalidation of responses that vary only cosmetically.

By default a cache keys on the request URL (plus method). Vary adds named request headers to that key so different header values get different cache entries — e.g. Vary: Accept partitions JSON from XML, Vary: Accept-Encoding separates gzip from brotli. Get it wrong and the cache serves the wrong representation — or worse, the wrong user's data.

Two families: content hashes (hash the serialized body) — exact but you pay CPU to render and hash on every request, even ones that end in a 304; or version metadata (a row's updated_at or a monotonic version column) — cheap to read, strong if the version is reliable, and computable without rendering the full body. At scale, prefer a stored version/sequence number so you can answer a conditional request without materializing the representation. Watch out for hashing that differs across nodes (key ordering, serializer version) — it'll break revalidation and tank your hit rate.

One value, two jobs: read it back as If-None-Match and you get a 304 cache validation; send it back as If-Match on a write and you get optimistic locking. Elegant because the server maintains a single version notion. The tension: concurrency wants a strong, exact validator (any change must fail the write), while cache revalidation tolerates a weak one — so if you serve weak ETags for cosmetic-variance caching, don't reuse them for If-Match, or you'll accept writes against a "semantically equal" but actually-different revision.

The edge entry is still within its s-maxage freshness window, so the CDN serves it without asking the origin — by design. Fixes: shorten s-maxage and lean on revalidation; issue an explicit purge/ban to the CDN on write (eventual, per-POP propagation, so allow a moment); or use stale-while-revalidate so the next request triggers a background refresh. Pick based on how tight your staleness SLA is — instant correctness means active purge plus a short TTL as a safety net.

A strong answer covers: classify resources first — catalog/product pages are public-stable, price/inventory are public-volatile, the cart/account is per-user → for public-stable, set public with a generous s-maxage at the CDN and a shorter max-age in the browser so you can purge the edge while clients return soon, and emit strong ETags so lapsed entries revalidate with a cheap 304 rather than re-shipping the body → add stale-while-revalidate so users never wait on origin during a refresh → for volatile price/inventory, short TTL or validation-only plus an active purge on write where instant correctness matters → per-user data: private, no-store, kept off the shared cache → set Vary: Accept, Accept-Encoding precisely and never over-vary (high-cardinality headers shred hit rate) → name the trade-off: long edge TTL maximizes offload but risks stale-after-write unless you can purge; ETag validation never serves wrong data but costs a round-trip → measure cache hit rate and origin offload as the success metric, and guard against a thundering herd on simultaneous expiry with jittered TTLs or stale-while-revalidate.

A strong answer covers: the hazard is the lost update — concurrent read-modify-write over a stateless protocol → on GET, return the document with a strong, exact ETag for that revision (a stored version/sequence column, not a body hash, so you can answer without materializing the representation) → require every PUT/PATCH to carry If-Match: <etag>; if the current version no longer matches, reject with 412 Precondition Failed so the client must re-fetch and reconcile rather than clobber → enforce the check atomically at the store — UPDATE … WHERE id=? AND version=? and check rows-affected, or a transaction — because the HTTP layer only communicates the precondition, it doesn't enforce it → offer race-free create via If-None-Match: * → reuse the same version as an If-None-Match validator for 304 caching (one value, two jobs), but do not reuse a weak ETag for If-Match or you'll accept a write against a merely "semantically equal" revision → name the trade-off: optimistic is lock-free and cheap on the happy path but thrashes under frequent conflicts, where a short-lived scoped lock with a lease/TTL is the fallback → surface conflicts to the UI for merge, and log 412 rates as a signal that contention is too high.