ETag header

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

The HTTPETag (entity tag)response header is an identifier for a specific version of a resource.It letscaches 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 to prevent simultaneous updates of a resource from overwriting each other ("mid-air collisions").

If the resource at a given URL changes, a newETag valuemust be generated.A comparison of them can determine whether two representations of a resource are the same.

Header type	Response header,Representation header
Forbidden request header	No

Syntax

http

ETag: W/"<etag_value>"ETag: "<etag_value>"

Directives

W/Optional: W/ (case-sensitive) indicates that aweak 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.WeakETag values of two representations of the same resources might be semantically equivalent, but not byte-for-byte identical.This means weak ETags prevent caching whenbyte 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 ofASCII characters placed between double quotes, like"675af34563dc-tr34".The method by whichETag 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.

Examples

http

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

Avoiding mid-air collisions

With the help of theETag and theIf-Match headers, you can detect mid-air edit collisions (conflicts).

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

http

ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"

When saving changes to a wiki page (posting data), thePOST requestwill contain theIf-Match header containing theETagvalues 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 a412 Precondition Failed error is thrown.

Caching of unchanged resources

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

http

If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"

The server compares the client'sETag (sent withIf-None-Match) with theETag for its current version of the resource, and if both values match (that is, the resource has not changed), the server sends back a304 Not Modified status, without a body, which tells the client that the cached version of the response is still good to use (fresh).