In a world increasingly governed by data and interactivity between applications, REST APIs have taken a prominent place in IT development. Nowadays, designing, developing, and maintaining a REST API are among the most common business-as-usual tasks for a back-end or full-stack developer. Therefore, understanding APIs and associated standards, as well as the ability to effectively and build a secure and functioning API, is essential for a developer. In this article, we explain everything about REST APIs, their usefulness, their difference from other standards, and their operating principles.
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What Is a REST API?
APIs are the basis of all interactions between different applications. Many companies and organizations today offer APIs to interact with their applications. This allows third-party application developers to perform operations such as transmitting or accessing data from one application to another via this API. This is called a public API. This means that this API is intended for interaction with third-party apps. It can be completely open or restricted to certain clients via authentication (for example, OAuth).
An API can also allow bricks of the same application to communicate with each other, which is very useful for microservices architectures. It can also be reserved solely for interaction between the various apps of a single company, as with Uber and Airbnb for example. Here, it is a private API that is not accessible by third-party applications.
Very often, documentation is made available and serves as a user guide to the API. It lists all possible operations: Each action is accessible via a specific URL called an endpoint, expects specific parameters (mandatory or optional), and returns a structured response in a defined format, for example in JSON.
What Is the REST API Standard?
REST (for representational state transfer) is a type of API architecture that provides several standards and conventions that must be followed to facilitate communication between applications. APIs that respect the REST standard are called REST APIs or RESTful APIs.
The Principles of a REST Architecture
The REST standard imposes six architectural constraints that must all be respected by a system in order for it to qualify as a RESTful system. Strict adherence to these six constraints ensures optimal reliability, scalability, and extensibility. The six principles of REST architecture are discussed
Separation of client and server
Server-side and client-side responsibilities are separated so that each side can be implemented independently of the other. The server-side code (the API) and the client-side code can each be changed without affecting the other, as long as both continue to communicate in the same format. In a REST architecture, different clients send requests to the same endpoints, perform the same actions, and get the same responses.
Statelessness
The communication between client and server does not keep track of the state of sessions from one request to the next. The state of a session is included in each request, so neither the client nor the server needs to know the state of the other to communicate. Each request is complete and self-sufficient. There is no need to maintain a continuous connection between client and server, which implies a greater tolerance to failure. In addition, this allows REST APIs to respond to requests from several clients without saturating the server’s ports. The exception to this rule is authentication so that the client does not have to specify its authentication information on every request.
Uniformity of the interface
The different actions and/or resources available with their specific endpoints and parameters must be decided and respected religiously, uniformly by the client and the server. Each response should contain enough information to be interpreted without the client needing any other information beforehand. Responses should not be too long and should contain links to other endpoints.
Caching
Responses can be cached to avoid overloading the server unnecessarily. Caching must be well managed: The REST API must specify whether and for how long a response can be cached to avoid the client receiving outdated information.
Layered architecture
A client connected to a REST API usually cannot distinguish whether it is communicating with the end server or an intermediate server. A REST architecture allows the API, for example, to receive requests on server A, store its data on server B, and manage authentications on server C.
Code on demand
This constraint is optional. It means that an API can return executable code instead of a response in JSON or XML, for example. This means that a RESTful API can extend the client’s code while simplifying it by providing executable code such as a JavaScript script or a Java applet.
REST API vs. SOAP API: What Is the Difference?
Created in 1998, SOAP has long been the most widely used protocol for interfacing different systems via the web. Its specifications are maintained by the World Wide Web Consortium (W3C). It is often wrongly compared to REST. Indeed, SOAP is a protocol, while REST is an architecture style. However, the two are not compatible because REST was created hoping to solve some problems associated with SOAP, which made it too inflexible a protocol.
The major drawback of SOAP that led to the majority adoption of REST is the use of XML. Using this language for requests and responses can be complex and verbose, especially since it needs to be interpreted, which represents an additional burden on both the client and server sides. It is intolerant to errors, which is a colossal risk for the continuity of modern apps. The client code is thus very dependent on the server code and vice versa, so much that one cannot be changed without modifying the other. Using SOAP thus requires a heavier configuration and maintenance, less adapted to the agile and open aspect of current web development than REST APIs.
Nevertheless, SOAP still has some advantages over REST in specific settings. It is compatible with all programming languages and many protocols such as HTTP, TCP, SMTP, JMS, or UDP. It also supports various extensions such as WS-Security, WS-Federation, WS-Coordination, etc. These extensions may or may not be used, for example, an API intended for public and open use will not need WS-Security.
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