REST stands for Representational State Transfer, a term coined by Roy Fielding in 2000. It is an architecture style for designing loosely coupled applications over HTTP, that is often used in the development of web services. REST does not enforce any rule regarding how it should be implemented at lower level, it just put high level design guidelines and leave you to think of your own implementation.
In my last employment, I designed RESTful APIs for telecom major company for 2 good years. In this post, I will be sharing my thoughts apart from normal design practices. You may not agree with me on a few points, and that’s perfectly OK. I will be happy to discuss anything from you with an open mind.
Let’s start with standard design specific stuff to clear what ‘Roy Fielding’ wants us to build. Then we will discuss my stuff which will be more towards finer points while you design your RESTful APIs.
REST defines 6 architectural constraints which make any web service – a true RESTful API.
As the constraint name itself applies, you MUST decide APIs interface for resources inside the system which are exposed to API consumers and follow religiously. A resource in the system should have only one logical URI and that should provide a way to fetch related or additional data. It’s always better to synonymise a resource with a web page.
Any single resource should not be too large and contain each and everything in its representation. Whenever relevant, a resource should contain links (HATEOAS) pointing to relative URIs to fetch related information.
Also, the resource representations across system should follow certain guidelines such as naming conventions, link formats or data format (xml or/and json).
All resources should be accessible through a common approach such as HTTP GET and similarly modified using a consistent approach.
This essentially means that client application and server application MUST be able to evolve separately without any dependency on each other. A client should know only resource URIs and that’s all. Today, this is normal practice in web development so nothing fancy is required from your side. Keep it simple.
Roy fielding got inspiration from HTTP, so it reflects in this constraint. Make all client-server interaction stateless. Server will not store anything about latest HTTP request client made. It will treat each and every request as new. No session, no history.
If client application needs to be a stateful application for the end user, where user logs in once and do other authorized operations thereafter, then each request from the client should contain all the information necessary to service the request – including authentication and authorization details.
In today’s world, caching of data and responses is of utmost important wherever they are applicable/possible. The webpage you are reading here is also a cached version of the HTML page. Caching brings performance improvement for client side, and better scope for scalability for a server because the load has reduced.
In REST, caching shall be applied to resources when applicable and then these resources MUST declare themselves cacheable. Caching can be implemented on the server or client side.
REST allows you to use a layered system architecture where you deploy the APIs on server A, and store data on server B and authenticate requests in Server C, for example. A client cannot ordinarily tell whether it is connected directly to the end server, or to an intermediary along the way.
Code on demand (optional)
Well, this constraint is optional. Most of the time you will be sending the static representations of resources in form of XML or JSON. But when you need to, you are free to
return executable code to support a part of your application e.g. clients may call your API to get a UI widget rendering code. It is permitted.
Notice that all the above constraints are most closely related to WWW (the web). Using RESTful APIs, you can do the same thing with your web services what you do to web pages.