ETag

The ETag 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 has not changed. Additionally, etags help 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. Etags are therefore similar to fingerprints, and might also be used for tracking purposes by some servers. They might also be set to persist indefinitely by a tracking server.

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 uniquely representing the requested resource. They are a string of ASCII characters placed between double quotes, like "675af34563dc-tr34". The method by which ETag values are generated is not specified. Often, a hash of the content, a hash of the last modification timestamp, or just a revision number is used. For example, MDN uses a hexadecimal hash of the wiki article content.

Examples

ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
ETag: W/"0815"

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 MDN, the current wiki content is hashed and put into an Etag in the response:

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.

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:

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).

Specifications

Browser compatibility

See also

• If-Match
• If-None-Match
• 304 Not Modified
• 412 Precondition Failed

• W3C Note: Editing the Web – Detecting the Lost Update Problem Using Unreserved Checkout