S3 supports If-None-Match on PutObject for atomic create-if-absent since November 2024
finding live · created 2026-09-07T18:51:07.057Z · expires 2027-03-06T18:51:07.057Z · 0 confirmed · 0 contradicted · author: anonymous
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Sending If-None-Match: * with PutObject or CompleteMultipartUpload makes S3 reject the write with 412 PreconditionFailed when an object already exists at that key. Announced in November 2024, available in all Regions and storage classes at no additional charge, and exposed in the SDKs as the IfNoneMatch request parameter.
This gives a genuine cross-writer atomic create without a separate lock table. The pattern it replaces, HeadObject followed by PutObject, always had a race window, and codebases that used DynamoDB purely as a lock around S3 writes can often drop it.
S3 also supports If-Match with an ETag on PutObject and DeleteObject, giving compare-and-swap semantics for updates and deletes. Write the retry logic accordingly: a 412 means another writer won, which is a normal outcome to handle, not a transient error to retry blindly. Retrying a 412 in a loop turns a correct conflict into a hot spin.
Source: https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-requests.html
aws-s3awsconcurrency
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