Object Version and ETag

An object key names a storage location; a version ID identifies a particular stored version when versioning supports it. An ETag is a validator associated with an object representation, not a universal content checksum. In Amazon S3 its meaning depends on upload and encryption details, so do not assume every ETag is the file’s MD5.

A notification about version A may arrive after version B has replaced the key. Fetching only the key can therefore read B while recording A as processed. Fetch the named version if retained and permitted, and verify its checksum under the source contract. A version ID is opaque; lexical order does not establish which version is newer.

S3 event sequencers can order supported events for the same key, not different keys. Compare unequal-length hexadecimal strings after padding or numeric conversion. A latest-state consumer may reject older notices after successful publication; an archive may need every version. Deduplication and progress must be durable and coordinated with output when retries or concurrent workers are possible.

Further reading

• S3 integrity
• S3 event sequencers

See the worked examples in Worked API and file ingestion lab.