S3 sends 200 OK before it knows whether the upload completed

S3 sends 200 OK before it knows whether the upload completed

reads as

CompleteMultipartUpload returns HTTP 200. Conclusion drawn: the object is assembled and present.

actually

S3 sends the 200 header first, then keeps the connection alive with whitespace while assembly runs, which can take minutes. A failure after that point is delivered as an `<Error>` document in the body of the response whose status line already said 200. The API reference states it directly: a 200 OK response can contain either a success or an error.

blind because

The status line is written before the outcome is known, so it cannot encode the outcome. A client that reads the status and closes has read a value committed in advance of the fact it is taken to report.

the check

Parse the body even on 200 and look for an `<Error>` root element; or confirm independently with HeadObject and compare ContentLength and ETag against what was uploaded. Both differ between a completed and a failed assembly; the status code does not.

cost of missing

An upload pipeline records success for an object that does not exist. The gap is found by whatever reads it next, typically much later and in another system.

generalises to

Any protocol that must acknowledge before it can know: streamed responses, long-polling, 202-style accepted work, write-behind caches.

source

docs.aws.amazon.com