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## Chapter 18. REST principles

This chapter will attempt to define the concepts behind REST and explain what makes a service RESTful.

REST is often confused with performing a distinct operation depending on the HTTP method, while using more than the GET and POST methods. That’s highly misguided at best.

We will first attempt to define REST and will look at what it means in the context of HTTP and the Web. For a more in-depth explanation of REST, you can read Roy T. Fielding’s dissertation as it does a great job explaining where it comes from and what it achieves.

## 18.1. REST architecture

REST is a client-server architecture. The client and the server both have a different set of concerns. The server stores and/or manipulates information and makes it available to the user in an efficient manner. The client takes that information and displays it to the user and/or uses it to perform subsequent requests for information. This separation of concerns allows both the client and the server to evolve independently as it only requires that the interface stays the same.

REST is stateless. That means the communication between the client and the server always contains all the information needed to perform the request. There is no session state in the server, it is kept entirely on the client’s side. If access to a resource requires authentication, then the client needs to authenticate itself with every request.

REST is cacheable. The client, the server and any intermediary components can all cache resources in order to improve performance.

REST provides a uniform interface between components. This simplifies the architecture, as all components follow the same rules to speak to one another. It also makes it easier to understand the interactions between the different components of the system. A number of constraints are required to achieve this. They are covered in the rest of the chapter.

REST is a layered system. Individual components cannot see beyond the immediate layer with which they are interacting. This means that a client connecting to an intermediate component, like a proxy, has no knowledge of what lies beyond. This allows components to be independent and thus easily replaceable or extendable.

REST optionally provides code on demand. Code may be downloaded to extend client functionality. This is optional however because the client may not be able to download or run this code, and so a REST component cannot rely on it being executed.

## 18.2. Resources and resource identifiers

A resource is an abstract concept. In a REST system, any information that can be named may be a resource. This includes documents, images, a collection of resources and any other information. Any information that can be the target of an hypertext link can be a resource.

A resource is a conceptual mapping to a set of entities. The set of entities evolves over time; a resource doesn’t. For example, a resource can map to "users who have logged in this past month" and another to "all users". At some point in time they may map to the same set of entities, because all users logged in this past month. But they are still different resources. Similarly, if nobody logged in recently, then the first resource may map to the empty set. This resource exists regardless of the information it maps to.

Resources are identified by uniform resource identifiers, also known as URIs. Sometimes internationalized resource identifiers, or IRIs, may also be used, but these can be directly translated into a URI.

In practice we will identify two kinds of resources. Individual resources map to a set of one element, for example "user Joe". Collection of resources map to a set of 0 to N elements, for example "all users".

## 18.3. Resource representations

The representation of a resource is a sequence of bytes associated with metadata.

The metadata comes as a list of key-value pairs, where the name corresponds to a standard that defines the value’s structure and semantics. With HTTP, the metadata comes in the form of request or response headers. The headers' structure and semantics are well defined in the HTTP standard. Metadata includes representation metadata, resource metadata and control data.

The representation metadata gives information about the representation, such as its media type, the date of last modification, or even a checksum.

Resource metadata could be link to related resources or information about additional representations of the resource.

Control data allows parameterizing the request or response. For example, we may only want the representation returned if it is more recent than the one we have in cache. Similarly, we may want to instruct the client about how it should cache the representation. This isn’t restricted to caching. We may, for example, want to store a new representation of a resource only if it wasn’t modified since we first retrieved it.

The data format of a representation is also known as the media type. Some media types are intended for direct rendering to the user, while others are intended for automated processing. The media type is a key component of the REST architecture.

## 18.4. Self-descriptive messages

Messages must be self-descriptive. That means that the data format of a representation must always come with its media type (and similarly requesting a resource involves choosing the media type of the representation returned). If you are sending HTML, then you must say it is HTML by sending the media type with the representation. In HTTP this is done using the content-type header.

The media type is often an IANA registered media type, like `text/html` or `image/png`, but does not need to be. Exactly two things are important for respecting this constraint: that the media type is well specified, and that the sender and recipient agree about what the media type refers to.

This means that you can create your own media types, like `application/x-mine`, and that as long as you write the specifications for it and that both endpoints agree about it then the constraint is respected.

## 18.5. Hypermedia as the engine of application state

The last constraint is generally where services that claim to be RESTful fail. Interactions with a server must be entirely driven by hypermedia. The client does not need any prior knowledge of the service in order to use it, other than an entry point and of course basic understanding of the media type of the representations, at the very least enough to find and identify hyperlinks and link relations.

To give a simple example, if your service only works with the `application/json` media type then this constraint cannot be respected (as there are no concept of links in JSON) and thus your service isn’t RESTful. This is the case for the majority of self-proclaimed REST services.

On the other hand if you create a JSON based media type that has a concept of links and link relations, then your service might be RESTful.

Respecting this constraint means that the entirety of the service becomes self-discoverable, not only the resources in it, but also the operations you can perform on it. This makes clients very thin as there is no need to implement anything specific to the service to operate on it.

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