PATCH applies partial modifications to a resource.
PATCH is somewhat analogous to the "update" concept found in CRUD (in general, HTTP is different than CRUD, and the two should not be confused).
PATCH request is considered a set of instructions on how to modify a resource. Contrast this with
PUT; which is a complete representation of a resource.
PATCH is not necessarily idempotent, although it can be. Contrast this with
PUT; which is always idempotent. The word "idempotent" means that any number of repeated, identical requests will leave the resource in the same state. For example if an auto-incrementing counter field is an integral part of the resource, then a
PUT will naturally overwrite it (since it overwrites everything), but not necessarily so for
POST) may have side-effects on other resources.
To find out whether a server supports
PATCH, a server can advertise its support by adding it to the list in the
Access-Control-Allow-Methods (for CORS) response headers.
Another (implicit) indication that
PATCH is allowed, is the presence of the
Accept-Patch header, which specifies the patch document formats accepted by the server.
|Request has body||Yes|
|Successful response has body||Yes|
|Allowed in HTML forms||No|