 Entity tag

An entity tag is an opaque validator associated with a selected HTTP representation and commonly supplied in an ETag HTTP header field. It allows requests to refer to a previously observed version or equivalence class without interpreting the representation's contents. The tag's spelling need not reveal how the server generated it.^[ref]

A strong entity tag has no weak marker; a weak entity tag has the prefix W/. Strong comparison succeeds only when both tags are non-weak and their opaque values match character by character. Weak comparison checks the opaque values while ignoring whether either tag is marked weak.^[ref]

For example, "v1" and W/"v1" match weakly but not strongly. Two occurrences of "v1" match under either comparison. Tags "AbC" and "abc" match under neither: field-name case insensitivity does not make the opaque value case-insensitive.

A useful HTTP conditional request illustrates the purpose. Suppose a client retains representation data tagged "v1", then sends GET with If-None-Match: "v1". If the selected current tag matches, this precondition fails and the server returns 304 without transferring that response content. The retained data can then be used according to the applicable caching rules. If-None-Match uses weak comparison, while If-Match uses strong comparison.^[ref]

An entity tag is therefore a protocol validator, not an instruction to lowercase or decode its opaque string. Different representations of one resource can require different validators. Compare the tag for the selected representation under the comparison rule required by the field, rather than assuming that equal resource URLs alone establish equal data.