What is a Resumable Upload?
A resumable upload records accepted transfer progress so an interrupted client can continue from the last confirmed offset. It avoids sending again the portions of a file that the server has already received.
How Resumable Uploads work
The server maintains transfer state under an upload identifier, and the client queries or remembers the next byte position before resuming. Data is sent in ordered chunks whose offsets let the receiver detect gaps, overlaps, or stale client state. This differs from multipart upload as a concept: multipart can divide a file without necessarily defining cross-session recovery. It belongs in the ingest layer, ahead of validation, processing, and durable media storage.
Key facts
- Offset-based protocols typically require the client to send the exact server-confirmed position; a mismatch should be rejected instead of silently appending bytes.
- Resumption avoids retransmitting confirmed data, but it still needs expiration and cleanup policies so abandoned partial uploads do not consume storage indefinitely.
- Checksums can detect corruption within an uploaded chunk, while a final whole-file size or digest check verifies that correctly ordered chunks produced the intended object.
When Resumable Uploads matter
Choose resumable uploads for large files or networks where disconnections are likely, such as mobile connections. Client and server must agree on offsets, because stale progress data can corrupt or restart a transfer.