ETag

What is the ETag header?

The ETag (Entity Tag) response header is a unique string hash representing the current state of a resource. If the resource data changes, the ETag changes.

API Usage & Best Practices

• Saves bandwidth: Clients save the ETag and send it back on subsequent requests in the If-None-Match header. If the data is unchanged, the server responds with 304 Not Modified instead of rebuilding and sending the JSON.
• Optimistic concurrency: Can be used alongside If-Match to prevent clients from overwriting each other’s edits.
• Strong vs weak: A weak tag is written W/"...", with the W/ prefix outside the quotes, and claims only semantic equivalence. If-Match uses strong comparison, so a weak tag never matches there: if clients will send the tag back on writes, issue strong tags. Weak tags are fine for If-None-Match cache validation.

Examples

HTTP/1.1 200 OK
ETag: "3a84e27f01"

A weak tag, for a representation that is equivalent but not byte-for-byte identical:

HTTP/1.1 200 OK
ETag: W/"3a84e27f01"