Write the exact ETag exchange that stops two edits colliding

Two people save the same record minutes apart, and whoever saves second wins, with no warning that the first change vanished.

Intermediate to advancedRead 6 minDoing: about 30 minutes7 steps

▶ Start

What you'll have at the end

A written trace for two editors: the first save succeeds, the second is rejected with a 412 and a stale ETag

Before you start

You need

You need one endpoint that reads a record and a matching endpoint that writes to it, in any language, plus enough control over both to add and read plain HTTP headers.

Not covered

This does not cover merging two people's edits into one automatically, or reusing the same tag for cache validation on read-heavy endpoints.

Leans on

HTTP Cheat Sheetthe conditional-request headers you're about to wire together, all in one table
API Design Principles and Best Practices Cheat Sheetthe same exchange explained at the API-design level, plus how strong and weak tags differ

7 steps

Where to go next

Decode a failed status code

useful first if status codes in general still confuse you, before tackling this specific 412 exchange

Checked 18 Aug 2026

Part of the HTTP cookbook