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|
ETag: W/"<etag_value>" ETag: "<etag_value>"
'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
ETagvalues 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.
- 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
ETagvalues 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.
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:
If the hashes don't match, it means that the document has been edited in-between and a
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:
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
Not Modified status, without a body, which tells the client that the cached version of the response is still good to use (fresh).
|RFC 7232, section 2.3: ETag||Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests|
|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|
- Full support
- Full support