HTTP ETag

The ETag or entity tag is part of , the protocol for the World Wide Web. It is one of several mechanisms that HTTP provides for web cache validation, which allows a client to make conditional requests. This allows caches to be more efficient, and saves bandwidth, as a web server does not need to send a full response if the content has not changed. ETags can also be used for optimistic concurrency control, as a way to help prevent simultaneous updates of a resource from overwriting each other.

An ETag is an opaque identifier assigned by a web server to a specific version of a resource found at a URL. If the resource representation at that URL ever changes, a new and different ETag is assigned. Used in this manner ETags are similar to fingerprints, and they can be quickly compared to determine whether two representations of a resource are the same.

The use of ETags in the HTTP header is optional (not mandatory as with some other fields of the HTTP 1.1 header). The method by which ETags are generated has never been specified in the HTTP specification.

Common methods of ETag generation include using a collision-resistant hash function of the resource's content, a hash of the last modification timestamp, or even just a revision number.

In order to avoid the use of stale cache data, methods used to generate ETags should guarantee (as much as is practical) that each ETag is unique. However, an ETag-generation function could be judged to be "usable" if it can be proven (mathematically) that duplication of ETags would be "acceptably rare", even if it could or would occur.

Some earlier checksum functions, such as CRC32 and CRC64, are known to suffer from this hash collision problem. Because of this they are not good candidates for use in ETag generation.

The ETag mechanism supports both strong validation and weak validation. They are distinguished by the presence of an initial "W/" in the ETag identifier, as:

A strongly validating ETag match indicates that the content of the two resource representations is byte-for-byte identical and that all other entity fields (such as Content-Language) are also unchanged. Strong ETags permit the caching and reassembly of partial responses, as with byte-range requests.

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