The Tool Desk
Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →To prevent a change from overwriting a newer version of a resource, send the ETag you received when reading it in an If-Match header. The server applies the change only if that validator still matches; otherwise, it must not perform the requested method, and can report the conflict with 412 Precondition Failed. The client can then retrieve the latest representation and let the application decide how to resolve the difference.
Use the ETag you read to guard a change
An ETag is an opaque validator for a selected representation—not a version number for clients to interpret. Keep the value as returned by the server and send it back unchanged when you want to ensure that the representation has not changed since you read it.
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- Read the resource. Obtain the representation and its ETag from the server.
- Submit the intended change conditionally. Include that ETag in the request’s
If-Matchheader, for exampleIf-Match: "etag-value". - Let the server evaluate the precondition first. Under RFC 9110, an origin server evaluates
If-Matchbefore performing the method and uses strong comparison. If the condition is false, it must not perform the requested method. - Handle a failed precondition. The server may return
412 Precondition Failed. The client can fetch the current representation and show the conflict, merge changes, or ask the user to retry. Those resolution choices belong to the application, not HTTP itself. - Keep the new ETag after success. Validator fields in a successful state-changing response describe the new representation. Save the returned ETag for the next conditional update.
This is optimistic concurrency control: the client proceeds without locking the resource, while the server checks that the client’s view is still current at the point of change.
Choose the condition that matches the task
| Purpose | Condition or response | What it does |
|---|---|---|
| Change only the representation previously observed | If-Match: "etag-value" |
Uses strong comparison. If no supplied tag matches, the method must not be performed; the server may respond with 412. |
| Create only when no current representation exists | If-None-Match: * |
Prevents an unsafe method from inadvertently modifying an existing representation when the client believes none exists. |
| Validate a cached GET representation | If-None-Match: "etag-value" |
Uses weak comparison; a match can produce 304 Not Modified. This is chiefly for cache validation, not the usual lost-update guard. |
| Guard a change when entity tags are unavailable | If-Unmodified-Since: <date> |
Provides a date-based precondition. RFC 9110 describes it for resources that do not supply entity tags. When If-Match is also present, If-Match is evaluated first and the date condition is skipped. |
Do not substitute If-None-Match cache validation for an update guarded by the version you read. If-Match uses strong comparison; If-None-Match uses weak comparison, so they answer different questions.
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Understand lists, wildcards, and comparison
If-Match can contain a list of entity tags; the condition succeeds if at least one matches using strong comparison. A weak ETag therefore cannot satisfy the strong comparison required by If-Match. Treat tags as opaque strings rather than attempting to compare their contents or infer how many edits separate them.
If-Match: * checks whether a current representation exists. It does not assert that the current representation is the exact one the client previously read, so it is not a substitute for sending that read’s specific ETag when preventing lost updates.
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Follow the standard’s precondition order
When several conditional fields are present, RFC 9110 specifies an evaluation order. An origin server evaluates If-Match first. If it is absent, it evaluates If-Unmodified-Since; next comes If-None-Match. A matching If-None-Match on GET or HEAD can produce 304 Not Modified; on other methods, a false condition leads to 412 Precondition Failed. Remaining conditional date and range checks follow. Implementations that accept multiple conditional fields should use this ordering.
What HTTP specifies—and what the application decides
RFC 9110, HTTP Semantics, published by the IETF in June 2022, is the current standards reference here and obsoletes RFC 7232. It states in §13.1.1: “An origin server MUST use the strong comparison function when comparing entity tags for If-Match”. It also describes If-Match as commonly used with state-changing methods such as POST, PUT, and DELETE to prevent accidental overwrites when user agents act in parallel.
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The standard defines the HTTP precondition behavior, not how a particular API, SDK, framework, or server implements it, nor how an application should reconcile conflicting edits. Confirm the endpoint’s actual support for ETags and conditional requests, and choose a user-facing conflict policy appropriate to the data.
Quick Recap
Rank #4
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