A 12 MB artifact is split into three chunks. The client uploads chunk 2 twice, loses the response to finalize, and retries. A weak API creates two artifacts or assembles corrupted data.
Define the contract
type Chunk={sessionId:string,index:number,sha256:string,size:number};
type Finalize={sessionId:string,orderedHashes:string[],artifactSha256:string};
PUT /sessions/:id/chunks/:index should be idempotent for identical bytes and reject a different hash at an existing index. POST /sessions/:id/finalize should compare-and-set uploading → finalizing → complete, returning the same artifact ID after a retry.
created -> uploading -> finalizing -> complete
\-> expired
| Injection | Expected result |
|---|---|
| duplicate identical chunk | 200, no extra storage |
| duplicate changed chunk | 409 |
| missing index | 422 with missing list |
| finalize response lost | retry returns same artifact |
| expired session | 410 and cleanup scheduled |
Compute every chunk hash server-side and verify the final artifact hash after ordered assembly. Store temporary parts outside the public artifact namespace. A sweeper should remove expired sessions only when they are not finalizing.
End-to-end tests must cross UI selection, API authorization, temporary storage, assembly, download, and cleanup. Rollback disables new sessions while preserving completed downloads and garbage-collecting partial data.
A local filesystem fixture does not reproduce S3 consistency, multipart APIs, or production throughput. Which state transition in your upload flow currently has no idempotency key?
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