Typical Usage
In typical usage, when a URL is retrieved the web server will return the resource along with its corresponding ETag value, which is placed in an HTTP "ETag" field:
ETag: "686897696a7c876b7e"The client may then decide to cache the resource, along with its ETag. Later, if the client wants to retrieve the same URL again, it will send its previously saved copy of the ETag along with the request in a "If-None-Match" field.
If-None-Match: "686897696a7c876b7e"On this subsequent request, the server may now compare the client's ETag with the ETag for the current version of the resource. If the ETag values match, meaning that the resource has not changed, then the server may send back a very short response with an HTTP 304 Not Modified status. The 304 status tells the client that its cached version is still good and that it should use that.
However, if the ETag values do not match, meaning the resource has likely changed, then a full response including the resource's content is returned, just as if ETags were not being used. In this case the client may decide to replace its previously cached version with the newly returned resource and the new ETag.
ETag values can be used in web page monitoring systems. Efficient web page monitoring is hindered by the fact that most websites do not set the Etag headers for web pages. When a web monitor has no hints whether web content has been changed all content has to be retrieved, and analyzed, using computing resources for both the publisher and subscriber.
Read more about this topic: HTTP ETag
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