Strong entity tag

A strong entity tag is an entity tag that satisfies HTTP's strong-validator requirements. Its value must change whenever the HTTP representation data change in a way observable in a successful GET response. Its syntax has no weak W/ prefix, for example "revision-7".^[ref]

Suppose a resource serves the four bytes blue with tag "revision-7". Changing the first byte to produce Blue requires a different strong entity tag, even if the application regards the capitalization difference as unimportant. A revision counter can implement this example, but the client must treat the quoted tag as opaque: the syntax does not promise that a number can be parsed or incremented to predict the next tag.^[ref]

Strong comparison succeeds only when both entity tags are strong and their quoted opaque values match character for character. Thus "revision-7" matches "revision-7", but not W/"revision-7" and not "Revision-7". Removing the weak marker in a client would change the meaning of the validator rather than upgrade the server's guarantee.^[ref]

An HTTP conditional request using If-Match requires strong comparison. This is useful when an update should proceed only against the representation the client previously observed. The guarantee is scoped to the resource: identical tag text on two unrelated resources does not establish that their data are equal. Conversely, a strong validator may change for a relevant metadata change as well as a data change, so a changed tag alone is not a byte-by-byte difference report.^[ref]