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.
Header type | Response header |
---|---|
Forbidden header name | no |
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. 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 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 whichETag
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
Specification | Title |
---|---|
RFC 7232, section 2.3: ETag | Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests |
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
ETag | Chrome Full support Yes | Edge Full support 12 | Firefox Full support Yes | IE Full support Yes | Opera Full support Yes | Safari Full support Yes | WebView Android Full support Yes | Chrome Android Full support Yes | Firefox Android Full support Yes | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support Yes |
Legend
- Full support
- Full support
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