IfNoneMatch

IfNoneMatch is an HTTP request header used to make a request conditional on the current state of a resource, as identified by its ETag (entity tag). Clients send one or more ETag values, or the special value "*", to indicate that the request should proceed only if the resource’s current ETag does not match any of the listed values or, in the case of "*", that the resource does not exist. This enables cache validation and optimistic concurrency control.

Syntax and values

The header’s value consists of a comma-separated list of ETag values. Each value is an ETag, which may be strong (quoted) or weak (prefixed with W/). The asterisk "*" is a special value meaning “any current entity.” When using "*", the condition applies to the existence of the resource rather than to a specific ETag.

Behavior by method

- GET and HEAD: If the resource’s current ETag matches any value in the list, the server may respond with 304 Not Modified (no body). If there is no match, the server returns the full representation with 200 OK. In practice, a match typically indicates the client already has the latest version.

- Methods that modify state (such as PUT, POST, DELETE, PATCH): If the condition is not met (a matching ETag exists), the server commonly responds with 412 Precondition Failed. The special value "*" is often used with PUT to indicate creation only if the resource does not already exist; if the resource exists, the request fails with 412.

Relation to ETag

IfNoneMatch relies on ETag values provided by the server as validators of resource state. It is often used in conjunction with caching mechanisms and with concurrent updates to prevent unnecessary data transfer or conflicting changes.

See also

ETag, If-Match, Cache-Control, Last-Modified, RFC 7232.