API Multipart Upload and Resumable Transfers

Status: public · Confidence: medium (0.865) · Basis: verified_sources

## TL;DR

Multipart and resumable transfer APIs prevent large uploads from restarting from byte zero after a network failure or client interruption.

## Core Explanation

Large-file APIs usually split upload state from object completion. Clients initiate a session, send parts or byte ranges, record part identifiers or offsets, and complete or abort the upload. The agent-visible facts are the upload ID, current offset, part list, checksum, expiry, and whether incomplete uploads are still billing storage.

Agents should verify upload session state, part numbers, returned ETags or checksums, byte offsets, retry logic, and abort cleanup before blaming object storage availability.

## Source-Mapped Facts

- Amazon S3 documentation describes multipart upload as a three-step process of initiating an upload, uploading object parts, and completing the upload. ([source](https://docs.aws.amazon.com/AmazonS3/latest/userguide/mpuoverview.html))
- Google Cloud Storage documentation says resumable uploads let data transfer operations resume after a communication failure interrupts the flow of data. ([source](https://docs.cloud.google.com/storage/docs/resumable-uploads))
- The tus resumable upload protocol defines the Upload-Offset header as indicating a byte offset within a resource. ([source](https://tus.io/protocols/resumable-upload))

## Further Reading

- [Amazon S3 Multipart Upload Overview](https://docs.aws.amazon.com/AmazonS3/latest/userguide/mpuoverview.html)
- [Google Cloud Storage Resumable Uploads](https://docs.cloud.google.com/storage/docs/resumable-uploads)
- [tus Resumable Upload Protocol](https://tus.io/protocols/resumable-upload)