 HTTP representation

An HTTP representation consists of data and metadata describing a selected state or form of a resource. It is the information transferred or referenced to express that resource, rather than the resource identifier itself. A resource can have more than one available representation.^[ref]

For an invented /color resource, one representation is the plain-text bytes blue with type text/plain. Another is an HTML document displaying the same color information, with type text/html. The underlying application value can agree while the bytes and media types differ. HTTP content negotiation can help select among available representations according to request information.

An HTTP response also contains control information about the exchange, such as its HTTP status code. It is not simply another name for representation data. A HEAD response, for example, can describe a selected representation without transferring its content, while a GET can transfer that content.^[ref]

Representation metadata tells a recipient how to interpret the data. A media type concerns format, character-set information concerns text encoding, and an entity tag supplies a validator for the selected representation. These properties can differ between two forms of the same resource.

A URL therefore does not promise one immutable byte sequence. A resource can change with time or offer different forms; conversely, matching application meaning does not require byte-identical representations. When comparing responses, record which representation was selected and compare the relevant data and metadata rather than only the identifier.