ETag

 The ETag (or entity tag) HTTP response header is an identifier for a specific version of a resource. It lets caches be more efficient and save bandwidth, as a web server does not need to resend a full response if the content was not changed. Additionally, etags help to prevent simultaneous updates of a resource from overwriting each other ("mid-air collisions").

If the resource at a given URL changes, a new Etag value must be generated. A comparison of them can determine whether two representations of a resource are the same.

Syntax

ETag: W/"<etag_value>"

ETag: "<etag_value>"

Directives

W/ (Optional)

'W/' (case-sensitive) indicates that a weak validator is used. Weak etags are easy to generate, but are far less useful for comparisons. Strong validators are ideal for comparisons but can be very difficult to generate efficiently. Weak ETag values of two representations of the same resources might be semantically equivalent, but not byte-for-byte identical. This means weak etags prevent caching when byte range requests are used, but strong etags mean range requests can still be cached.

"<etag_value>"

Entity tag that uniquely represents the requested resource. It is a string of ASCII characters placed between double quotes, like "675af34563dc-tr34". The method by which ETag values are generated is not specified. Typically, the ETag value is a hash of the content, a hash of the last modification timestamp, or just a revision number. For example, a wiki engine can use a hexadecimal hash of the documentation article content.

Avoiding mid-air collisions

With the help of the ETag and the If-Match headers, you can detect mid-air edit collisions.

For example, when editing a wiki, the current wiki content may be hashed and put into an Etag header in the response:

HTTP

ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"

When saving changes to a wiki page (posting data), the POST request will contain the If-Match header containing the ETag values to check freshness against.

HTTP

If-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"

If the hashes don't match, it means that the document has been edited in-between and a 412 Precondition Failed error is thrown.

Caching of unchanged resources

Another typical use of the ETag header is to cache resources that are unchanged. If a user visits a given URL again (that has an ETag set), and it is stale (too old to be considered usable), the client will send the value of its ETag along in an If-None-Match header field:

HTTP

If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"

The server compares the client's ETag (sent with If-None-Match) with the ETag for its current version of the resource, and if both values match (that is, the resource has not changed), the server sends back a 304 Not Modified status, without a body, which tells the client that the cached version of the response is still good to use (fresh).