Programming Web Services with Perl

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Chapter 11: REST: Representational State Transfer

This much space has been devoted to URIs because it’s extremely important to prop-

erly understand and implement resource names.

Methods

The HTTP specification provides a number of generic methods (the HEAD, GET, and

POST methods were discussed in Chapter 2), but only five are relevant to this discus-

sion:

GET, HEAD, POST, PUT, and DELETE. Table 11-3 recalls the operations from

Chapter 2 and defines the new ones.

Unifying the method vocabulary provides tremendous opportunities for simplifying

interactions. It is precisely because HTTP has few methods that HTTP clients and

servers can grow and be extended independently without confusing each other.

Essentially, what are needed are methods that correspond to the “CRUD” concept:

Create, Retrieve, Update, and Delete. In HTTP they are called

GET, POST, DELETE, and

PUT.

GET method

To get a representation from a resource, a client uses the HTTP method GET. This

operation is idempotent, which means that a client may use the result of a previous

operation instead of repeating it; the state of the server shouldn’t be changed in ways

that are visible to the client.

GET is restricted to a single URL line, which enforces one

of the design principles: everything interesting should be URL-addressable. Wanting

to create a system in which resources aren’t URL-addressable means needing to jus-

tify that decision.

POST method

Modifying a resource uses the POST operation. This meaning of POST can be ambigu-

ous. It can mean append, create, remove, or modify a portion of resource, or some-

thing else entirely. This ambiguity leads to a number of misuses. For example, in

violation of one of the REST principles, it can be used for data queries without side

effects.

Table 11-3. REST methods

Method Meaning

GET Retrieves a resource

HEAD Retrieves representation and resource metadata

POST Inserts, updates, or extends a resource; may change the state of other resources

PUT Creates, updates, or replaces a resource

DELETE Deletes a resource