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Report collapsed requests and storage

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Several waiting requests can share one forward request, but reuse can also fail.

DIAGRAMRead RFC section 2.6 ↗
RFC 9211 · § 2.6EXAMPLE
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Report collapsed requests and storageRequestA and requestB are separate waiting requests at the same cache. A starts one forward request; B joins that pending request without a new send. Eligible response from next hop is delivered to both and stored, reporting fwd plus collapsed and stored true for B. Separate failure case starts a new forward request for B and eventual response reports collapsed false; no implication that final HTTP response failed.One forward request can serve several waitersRequest ARequest BCacheNext hopB still needs a responsenew request BSeparate failed-sharing case · new request neededEdgeCache; fwd=uri-miss; collapsed=?0Report after new request completes · false ≠ HTTP error
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Make a new request when the shared result cannot be reused.

Separate B case: the attempted sharing did not yield a usable response for B, so a new forward request is made. After that request completes, the resulting response reports collapsed=?0. This resets the earlier success example.

Source for this drawing: RFC 9211 · Read RFC section 2.6 ↗

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New to this? Start hereA quick picture + helpful background

A request starts a conversation.

A client asks for a resource. A server sends a response. The method, address, headers, and status explain what that conversation means.

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RFC 9111 Caching RFC 9110 HTTP

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KEY POINT

collapsed reports whether sharing succeeded, not whether forwarding disappeared.

  • A cache can combine concurrent requests into one forward request. collapsed is a Boolean meaningful only when fwd is present.
  • collapsed true means the response was successfully reused for requests combined together. If an attempted collapse fails, collapsed=?0 reports that a new request had to be made. If absent, the request was not collapsed with others.
  • In Structured Fields parameter syntax, bare ;collapsed and ;stored serialize true. ?0 explicitly represents false. These are Booleans, not text strings such as "true".
  • stored reports whether the resulting response was stored. Collapsing and storage are separate: a report can contain fwd, collapsed and stored together.
  • Concurrent request labels A/B identify two fictional requests, not users or credentials. The example omits detailed eligibility checks; sharing must still satisfy HTTP caching rules.
DETAILS

The absent/true/false cases are distinct. A failed collapse is not an HTTP failure code or proof that the final request failed; it says sharing could not satisfy that request.

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