What is the main difference between PATCH and PUT request?

Question

I am using a PUT request in my Rails application. Now, a new HTTP verb, PATCH has been implemented by browsers. So, I want to know what the main difference between PATCH and PUT requests are, and when we should use one or the other.

Answer 1

HTTP verbs are probably one of the most cryptic things about the HTTP protocol. They exist, and there are many of them, but why do they exist?

Rails seems to want to support many verbs and add some verbs that aren't supported by web browsers natively.

Here's an exhaustive list of http verbs: http://annevankesteren.nl/2007/10/http-methods

There the HTTP patch from the official RFC: https://datatracker.ietf.org/doc/rfc5789/?include_text=1

The PATCH method requests that a set of changes described in the request entity be applied to the resource identified by the Request- URI. The set of changes is represented in a format called a "patch document" identified by a media type. If the Request-URI does not point to an existing resource, the server MAY create a new resource, depending on the patch document type (whether it can logically modify a null resource) and permissions, etc.

The difference between the PUT and PATCH requests is reflected in the way the server processes the enclosed entity to modify the resource identified by the Request-URI. In a PUT request, the enclosed entity is considered to be a modified version of the resource stored on the origin server, and the client is requesting that the stored version be replaced. With PATCH, however, the enclosed entity contains a set of instructions describing how a resource currently residing on the origin server should be modified to produce a new version. The PATCH method affects the resource identified by the Request-URI, and it also MAY have side effects on other resources; i.e., new resources may be created, or existing ones modified, by the application of a PATCH.

As far as I know, the PATCH verb is not used as it is in rails applications... As I understand this, the RFC patch verb should be used to send patch instructions like when you do a diff between two files. Instead of sending the whole entity again, you send a patch that could be much smaller than resending the whole entity.

Imagine you want to edit a huge file. You edit 3 lines. Instead of sending the file back, you just have to send the diff. On the plus side, sending a patch request could be used to merge files asynchronously. A version control system could potentially use the PATCH verb to update code remotely.

One other possible use case is somewhat related to NoSQL databases, it is possible to store documents. Let say we use a JSON structure to send back and forth data from the server to the client. If we wanted to delete a field, we could use a syntax similar to the one in mongodb for $unset. Actually, the method used in mongodb to update documents could be probably used to handle json patches.

Taking this example:

db.products.update(
   { sku: "unknown" },
   { $unset: { quantity: "", instock: "" } }
)

We could have something like this:

PATCH /products?sku=unknown
{ "$unset": { "quantity": "", "instock": "" } }

Last, but not least, people can say whatever they want about HTTP verbs. There is only one truth, and the truth is in the RFCs.