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Web & Networking › HTTP

Conditional Request

If-None-Match and If-Match for caching and concurrency control.

Backend Engineer · SeniorFrontend Engineer · SeniorData Engineer · Senior

Also known as: conditional request, conditional get, if-none-match

A conditional request carries a precondition about the resource’s current state: send the body only if it changed (If-None-Match/If-Modified-Since for caching), or apply the write only if it matches (If-Match/If-Unmodified-Since for concurrency). Unmet conditions get a short status (304, 412) instead of the full exchange.

caching:       GET + If-None-Match: "v42"  → 304 (unchanged, no body)
concurrency:   PUT + If-Match: "v42"       → 412 (someone changed it; retry)

The same validator mechanism serves two masters: bandwidth (skip unchanged bodies) and correctness (optimistic locking on writes). Both depend on honest validators — ETags that actually change with content, timestamps with enough resolution.

The classic mistakes:

  • Validators that lie. ETags not derived from content make conditionals wrong in both directions: stale 304s and false 412s. Generate from the bytes or a real version.
  • Clock-based conditions with coarse clocks. If-Modified-Since at one-second resolution misses sub-second changes. Prefer ETags where precision matters.
  • If-Match without handling 412. The whole point is the conflict path — merge, retry with fresh state, or surface it. A 412 nobody handles is a broken write.
  • Caching the 304 wrongly. Intermediaries must update stored headers from the 304 response; mishandled, caches serve stale metadata with fresh validation.
  • Assuming universal support. Some servers and CDNs ignore or strip conditionals; verify end to end rather than assuming the optimisation applies.
  • Conditional writes without transactions. If-Match checks recency, not serialisability — the write itself still needs atomicity server-side.
  • Forgetting Vary. Conditionals on negotiated resources must account for variants, or a 304 validates the wrong representation.

How to use it: ETag-backed conditionals on cacheable reads, If-Match on contested writes, with real validators and handled conflict paths. It’s caching and concurrency control sharing one elegant mechanism.