How does If-Match prevent lost updates?
The client sends the ETag of the version it edited. If someone changed the resource meanwhile, the ETag no longer matches and the server answers 412 instead of silently overwriting their work.
HTTP 412 PRECONDITION FAILED
An If-Match or If-Unmodified-Since condition was false. Used for optimistic locking. 412 belongs to the 4xx client error class and is defined in RFC 9110.
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About HTTP 412
What Does HTTP 412 Precondition Failed Mean?
An If-Match or If-Unmodified-Since condition was false. Used for optimistic locking. The code is registered with IANA and defined in RFC 9110.
Client error: the request has a problem, so it cannot be fulfilled as sent. Every code from 400 to 499 shares that general meaning, so a client that does not recognise 412 must treat it like 400.
When Do You See a 412 Response?
412 Precondition Failed means a conditional header in the request evaluated to false. The usual case is optimistic locking: a client sends If-Match with the ETag it last saw, but the resource has changed since, so the server refuses to overwrite someone else's update. If-Unmodified-Since works the same way with dates. It also appears when If-None-Match: * is used to create a resource only if it does not already exist.
How Do You Fix a 412?
Clients should GET the resource again to obtain the current ETag and content, reapply or merge their change, and resend with the new validator. Servers should include the current ETag in the 412 response so the client can see what changed. Make sure ETags are strong when used with If-Match, since weak ETags never satisfy If-Match comparisons.
What Does a 412 Response Look Like?
HTTP/1.1 412 Precondition Failed
ETag: "v18"
Content-Type: application/json412 is not cacheable by default: a browser or CDN stores it only when Cache-Control or Expires explicitly allows it.
To inspect a real one without sending it anywhere:
- Copy the response headers from your browser Network tab, or from
curl -i. - Paste them into the Header decoder to see every header explained.
- Check caching in the Cache lane.
Which Status Codes Are Related to 412?
- 409 Conflict: The request conflicts with the current state, such as an edit conflict or a duplicate.
- 428 Precondition Required: The server requires a conditional request, to prevent lost updates.
- 304 Not Modified: The cached copy is still valid, so no body is sent. Answer to If-None-Match or If-Modified-Since.
Browse every code on the HTTP status codes page.
What Other Lanes Are in the Reference Section?
- HTTP status codes: Every IANA-registered status code, with a page per code.
- MIME types: File extensions to Content-Type, searchable both ways, plus a Content-Type parser.
- HTTP methods: GET, POST, PUT, PATCH, DELETE and the rest: safe, idempotent, cacheable.
Elsewhere in 2http: Header decoder, Cookie decoder, Cache-Control decoder, CORS checker, User-agent parser, and all eleven lanes on the 2http home page. Part of the 2suite.io privacy-first ecosystem.
Frequently Asked Questions
Why does If-Match fail with a W/ ETag?
If-Match requires the strong comparison, and a weak ETag never matches strongly. Servers must generate strong ETags for resources that support conditional updates.
Is HTTP 412 an error?
Yes. 412 is a client error response: the problem is in the request, so repeating it unchanged will usually fail again.
Is a 412 response cacheable by default?
No. Caches store a 412 only when Cache-Control or Expires explicitly allows it.
How Can You Verify That Nothing Leaves This Browser?
You do not have to take our word that your headers stay here. Developer tools show every request a page makes, so you can watch this page work and confirm that nothing is sent. The check takes under a minute.
- Open your browser developer tools with
F12, orCmd+Option+Ion a Mac, and select the Network tab. - Reload the page and let it finish. You will see the page's own files, its web fonts, and one anonymous page-view request. Nothing has been typed yet, so none of them can contain your headers.
- Click the clear icon in the Network tab so that the log is empty.
- Paste real headers into any lane and read the result.
- Look at the Network tab again. It is still empty. For extra proof, turn on offline mode after the page loads and every lane keeps working.
Only two things can add a line to that log, and neither carries your headers. Submitting the feedback form sends the vote and comment you typed. Typing characters in a script the page has not displayed yet, such as Cyrillic or Greek, can make the browser fetch the matching font file, and that request names only the font.
2http never sends a request on your behalf: the request builder writes code for you to run, and the meta inspector reads source you paste instead of fetching a URL. The production site also sends a Content-Security-Policy header that tells your browser to refuse connections to anywhere except this site and the anonymous analytics endpoint.