ASP.NET Core Web API - PUT vs PATCH - rest

Question

I've been learning how to build an ASP.NET Core Web API using the official documentation. However, under the PUT section it says:
According to the HTTP specification, a PUT request requires the client to send the entire updated entity, not just the changes. To support partial updates, use HTTP PATCH.
I know that POST is used to create a new resource and PUT is used to update a particular resource. But I cannot understand what is meant by "entire updated entity, not just the changes".

Answer 1

I cannot understand what is meant by "entire updated entity, not just the changes".
You should review the HTTP specification, which describes the semantics of PUT.
A successful PUT of a given representation would suggest that a subsequent GET on that same target resource will result in an equivalent representation being sent in a 200 (OK) response.
Representation here refers to the bytes that would appear as the payload of the GET request. For a web page, we're talking about the HTML. For a resouce that responds with application/json, we're talking about the entire json document.
In other words, if you want to create/edit a json document, and save it on the web server, then you use HTTP PUT, and the payload of the request should be the entire JSON document.
For cases where the JSON document is very big (much bigger than the HTTP headers) and the changes you are making are small, then you might instead choose to send the changes in a patch request, where the payload of the request is a patch document (probably JSON Patch or JSON Merge Patch
Now, here's the magic trick -- everybody on the web understands HTTP requests the same way. PUT always means PUT, PATCH always means PATCH. So you can use PUT and PATCH to make changes to things on the web that aren't really documents, but might instead be something else (like an entity).
That's pretty much the point of an HTTP API - it's a facade that (when seen from the outside) makes our service look like just another web server.
The fact that your resource is really rows in a database instead of a document on a file system is an implementation detail that is hidden behind the Web API.

Answer 2

Supposed you have a json Data like below
{
"Id":123,
"FirstName":"John",
"MiddleName":"Random",
"LastName":"Doe"
}
Suppose, you want to change middle name here. you want to remove it. You can use both put and patch to do so.
PUT
Here you've to send the whole new entity (the changed state of the json model) in your request. Though you're only changing the Middle name part, the full json data is being sent in the request payload
PUT /user?id=123
{
"Id":123,
"FirstName":"John",
"MiddleName":"", //Only this is being changed.
"LastName":"Doe"
}
PATCH
In this case, you can only send the diff between the old model and the new model, and that will be it. So, here, in this case, only the changes are being sent, not the whole updated entity
PATCH /user?id=123
{
"MiddleName":"", //Only this is being changed.
}
Using PATCH can be handy when you want to edit only a few properties of a huge object.