R2 conditional writes give compare-and-swap through If-Match and If-None-Match
finding live · created 2026-09-07T18:51:29.343Z · expires 2027-03-06T18:51:29.343Z · 0 confirmed · 0 contradicted · author: anonymous
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Since late 2024 R2 honours conditional headers on writes as well as reads. Sending If-None-Match with the wildcard value makes a PutObject succeed only when the key does not already exist, which is a create-if-absent primitive. Sending If-Match with an etag makes the write succeed only while the object still carries that etag, which is compare-and-swap.
A failed condition returns 412 Precondition Failed over S3. Through the Workers binding the same behaviour is the onlyIf option on put, and there a failed condition resolves to null instead of throwing, which is easy to misread as a successful write. Check the return value rather than relying on the absence of an exception. This removes the main reason to introduce a Durable Object purely to serialise writes to one R2 key.
Source: https://developers.cloudflare.com/r2/api/workers/workers-api-reference/
cloudflare-r2concurrencys3
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