Web service designers have tried for some time now to correlate CRUD (Create, Retrieve, Update and Delete) semantics with the Representational State Transfer (REST) verbs defined by the HTTP specification–GET, PUT, POST, DELETE, HEAD, etc.
So often, developers will try to correlate these two concepts–CRUD and REST–using a one-to-one mapping of verbs from the two spaces, like this:
- Create = PUT
- Retrieve = GET
- Update = POST
- Delete = DELETE
“How to Create a REST Protocol” is an example of a very well-written article about REST, but which makes this faulty assumption. (In fairness to the author, he may well have merely “simplified REST for the masses”, as his article doesn’t specifically state that this mapping is the ONLY valid mapping. And indeed, he makes the statement that the reader should not assume the mapping indicates a direct mapping to SQL operations.)
In the article, “I don’t get PUT versus POST” the author clearly understands the semantic differences between PUT and POST, but fails to understand the benefits (derived from the HTTP protocol) of the proper REST semantics. Ultimately, he promotes the simplified CRUD to REST mapping as layed out above.
But such a trivial mapping is inaccurate at best. The semantics of these two verb spaces have no direct correlation. This is not to say you can’t create a CRUD client that can talk to a REST service. Rather, you need to add some additional higher-level logic to the mapping to complete the transformation from one space to the other.
While Retrieve really does map to an HTTP GET request, and likewise Delete really does map to an HTTP DELETE operation, the same cannot be said of Create and PUT or Update and POST. In some cases, Create means PUT, but in other cases it means POST. Likewise, in some cases Update means POST, while in others it means PUT.
The crux of the issue comes down to a concept known as idempotency. An operation is idempotent if a sequence of two or more of the same operation results in the same resource state as would a single instance of that operation. According to the HTTP 1.1 specification, GET, HEAD, PUT and DELETE are idempotent, while POST is not. That is, a sequence of multiple attempts to PUT data to a URL will result in the same resource state as a single attempt to PUT data to that URL, but the same cannot be said of a POST request. This is why a browser always pops up a warning dialog when you back up over a POSTed form. “Are you sure you want to purchase that item again!?” (Would that the warning was always this clear!)
After that discussion, a more realistic mapping would seem to be:
- Create = PUT iff you are sending the full content of the specified resource (URL).
- Create = POST if you are sending a command to the server to create a subordinate of the specified resource, using some server-side algorithm.
- Retrieve = GET.
- Update = PUT iff you are updating the full content of the specified resource.
- Update = POST if you are requesting the server to update one or more subordinates of the specified resource.
- Delete = DELETE.
NOTE: “iff” means “if and only if”.
Create can be implemented using an HTTP PUT, if (and only if) the payload of the request contains the full content of the exactly specified URL. For instance, assume a client issues the following Create OR Update request:
HTTP/1.1 PUT /GrafPak/Pictures/1000.jpg ... <full content of 1000.jpg ... >
This command is idempotent because sending the same command once or five times in a row will have exactly the same effect; namely that the payload of the request will end up becoming the full content of the resource specified by the URL, "/GrafPak/Pictures/1000.jpg".
On the other hand, the following request is NOT idempotent because the results of sending it either once or several times are different:
HTTP/1.1 POST /GrafPak/Pictures ... <?xml version="1.0" encoding="UTF-8"?> <GrafPak operation="add" type="jpeg"> <[CDATA[ <full content of some picture ... > ]]> </GrafPak>
Specifically, sending this command twice will result in two "new" pictures being added to the Pictures container on the server. According to the HTTP 1.1 specification, the server's response should be something like "201 Created" with Location headers for each response containing the resource (URL) references to the newly created resources--something like "/GrafPak/Pictures/1001.jpg" and "/GrafPak/Pictures/1002.jpg".
The value of the Location response header allows the client application to directly address these new picture objects on the server in subsequent operations. In fact, the client application could even use PUT to directly update these new pictures in an idempotent fashion.
What it comes down to is that PUT must create or update a specified resource by sending the full content of that same resource. POST operations, on the other hand, tell a web service exactly how to modify the contents of a resource that may be considered a container of other resources. POST operations may or may not result in additional directly accessible resources.