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HTTP-based APIs integrate easily with RESTful web services, but they often seem illogical, overlapping and inefficient. There are many ways to use HTTP methods, plenty of which aren't compatible with RESTful principles. There's also a long-running debate over how important it is to strictly follow REST or HTTP principles when constructing APIs.
This often comes from misunderstanding and misuse, so it's important to review the HTTP API methods and understand how to use them appropriately on resources, as well as resource collections. Let's take a quick look at the main difference between resources and resource collections, and then examine the five fundamental HTTP methods those involved with RESTful API development should know.
HTTP resources vs. resource collections
An HTTP resource is comparable to a data file: Developers can read and update the resource's contents, and it is hosted on a server and addressable via a URL. A resource collection is a set of related resources that can be viewed as a set of related files. The methods we examine in this piece can operate on resources, resource collections or both.
The relationship between the resources within a collection hinges on the software that supports the collection -- specifically, that software's implementation. The files can be concatenated, arranged in a tree or ordered in any other number of ways. If the implementations don't assign some dictated order to client behavior, the resulting multiplicity can create mismatches and cause applications to break.
Method 1: POST
POST is the only RESTful API HTTP method that primarily operates on resource collections. When creating a subordinate resource in a collection, applying POST to the parent resource prompts it to create a new resource, associate it with the proper hierarchy and return a dedicated URL for later reference. However, keep in mind that POST is not idempotent; you can't use this method more than once and expect a consistent outcome or result.
A significant benefit of POST is that it enables developers to explicitly define resources. This feature helps prevent teams from accidentally creating subordinate resources that pollute code, muddy references and cause applications to experience problems.
Method 2: PUT
The single-resource equivalent of POST is PUT, which updates a resource by replacing its content entirely. As a RESTful API HTTP method, PUT is the most common way to update resource information.
It's possible to create a resource with a PUT method, but this approach carries the risk of creating resources by accident, as noted above. If PUT is applied to a collection of resources, the entire resource collection gets replaced, which usually isn't the intention.
Method 3: PATCH
PATCH is another HTTP method used to update resources. As opposed to replacing resources, like the PUT method does, PATCH only modifies resource contents. As a general rule, these modifications should be expressed in a standard format, like JSON or XML.
Much like in PUT, it's poor practice to specifically apply PATCH methods to a whole resource collection -- that is, unless you truly intend to update every resource it contains.
A note on method documentation
All HTTP methods supply a return code and can also return data. It's the responsibility of the resource software implementation to generate proper return codes, and the relationship among methods, resource states and return codes must be documented for client reference.
If you aren't going to follow standard conventions regarding return codes, you need to clearly document that decision and explicitly detail the customized codes you use. Otherwise, you'll only confuse those who must maintain the applications down the road -- and likely were not around when you invented your own return code system.
Method 4: GET
The most common HTTP method is GET, which returns a representational view of a resource's contents and data. GET should be used in read-only mode, which keeps the data safe and the resource idempotent. You should get the same results no matter how many times you use this method, unless it is modified by another client in the interim.
The GET method is sometimes used to change the contents of a resource, but this is a precarious use of the method. It's common to compromise a client's ability to PATCH a resource if the resource detects a change since the PATCH client last conducted a GET.
Method 5: DELETE
The last HTTP method to examine is DELETE. When a DELETE method targets a single resource, that resource is removed entirely.
Implementations of DELETE are typically somewhat inconsistent: The URL for the resource may remain available even if the actual resource is absent. In this type of scenario, it's possible the server or resource implementation still changes the state of the vanished resource using the URL and likely reacts differently to subsequent DELETE executions.
While it's certainly possible, you should generally avoid using the DELETE method in a resource collection since it deletes all the contents within. Remember, the method isn't idempotent and shouldn't be treated as such.