api: YAML JSON TOON Database
version: 1.0.0
format: json
dataset:
  id: 1002
  slug: http-conditional-requests
  title: HTTP Conditional Requests and Validators
  description: "Conditional request headers, comparison rules, and response outcomes for safe caching and lost-update protection."
  summary: "Conditional request headers, comparison rules, and response outcomes for safe caching and lost-update protection. Each row includes a practical application and a failure to check before using it."
  use_when[2]: Avoid downloading unchanged reference files.,Prevent an edit from overwriting someone else’s update.
  caveats[2]: A 304 response has no representation body.,"Validators identify a selected representation, including negotiated variants."
  related[2]: http-cache-directives,http-methods-reference
  category: HTTP Status Codes
  category_slug: http-status-codes
  tags: "http-status-codes,http conditional requests,reference,engineering"
  view_count: 0
  created_at: 1791798640
  updated_at: 1791798640
  reviewed_at: 2026-10-12
  sources[2]{title,url}:
    IETF HTTP semantics,"https://httpwg.org/specs/rfc9110.html"
    IETF HTTP caching,"https://httpwg.org/specs/rfc9111.html"
  reading[1]: http-caching-with-etags-and-cache-control
  provenance: "Original editorial reference notes by YJTOON, informed by the linked primary documentation. External specifications retain their own licenses."
  entry_count: 8
  fields[4]{section,field,type,example}:
    data,header,string,ETag
    data,example,string,"ETag: \"revision-7\""
    data,purpose,string,Identify a selected representation.
    data,check,string,The value is opaque; clients must not construct…
data[8]{header,example,purpose,check}:
  ETag,"ETag: \"revision-7\"",Identify a selected representation.,The value is opaque; clients must not construct it.
  Last-Modified,"Last-Modified: Mon, 12 Oct 2026 08:00:00 GMT",Expose a modification timestamp.,Second-level precision may miss rapid edits.
  If-None-Match,"If-None-Match: \"revision-7\"",Validate a cached representation.,Matching GET or HEAD produces 304; other methods can produce 412.
  If-Modified-Since,"If-Modified-Since: Mon, 12 Oct 2026 08:00:00 GMT",Validate using a date.,If-None-Match takes precedence when both are present.
  If-Match,"If-Match: \"revision-7\"",Apply a write only to the expected revision.,Use strong entity-tag comparison; mismatch produces 412.
  If-Unmodified-Since,"If-Unmodified-Since: Mon, 12 Oct 2026 08:00:00 GMT",Protect a write with a timestamp.,If-Match takes precedence; timestamps are less precise.
  If-Range,"If-Range: \"revision-7\"",Resume a range only if the representation is unchanged.,A mismatch sends the whole representation instead.
  Weak ETag,"ETag: W/\"summary-7\"",Mark semantic equivalence without byte identity.,A weak validator cannot satisfy strong If-Match comparison.