REST is not an architecture; rather, it is a set of constraints that creates a software architectural style, which can be used for building distributed applications. A major challenge to the distributed applications is attributed to the diversity of systems in an enterprise offering silos of business information, as depicted in the following diagram: 

Often, an enterprise demands simplified access and updates to data residing in different systems. Fielding arrived at REST by evaluating all the networking resources and technologies available for creating distributed applications. He observed that without any constraints, one may end up developing applications with no rules or limits that are hard to maintain and extend. After considerable research on building a better architecture for a distributed application, he ended up with the following constraints that define a RESTful system:

The following diagram illustrates a high-level architectural view of a RESTful system:

The preceding constraints do not dictate what kind of technology to use; they only define how the data is transferred between components and the benefits of the guidelines. Therefore, a RESTful system can be implemented in any available networking architecture. More importantly, there is no need for us to invent new technologies or networking protocols. We can very well use the existing networking infrastructures, such as the World Wide Web (WWW), to create RESTful architectures. Consequently, a RESTful architecture is one that is maintainable, extendable, and distributed.

Before all the REST constraints were formalized, we already had a working example of a RESTful system—the web. Now, you may ask why introduce these RESTful requirements to web application development when it is agreed that the web is already RESTful.

Here is the answer, we first need to qualify what it means for the web to be RESTful. On one hand, the static web is RESTful because static websites follow Fielding's definition of a RESTful architecture. For instance, the existing web infrastructure provides caching systems, stateless connections, and unique hyperlinks to resources, where resources include all the documents available on every website, and the representation of these documents is already set by files being browser-readable (the HTML files, for example). Therefore, the static web is a system built in the REST-like architectural style. In simple words, we can say that REST leverages these amazing features of the web with some constraints.

On the other hand, traditional dynamic web applications have not always been RESTful because they typically break some of the outlined constraints. For instance, most dynamic applications are not stateless, as servers require tracking users through the container sessions or client-side cookie schemes. Therefore, we conclude that the dynamic web is not normally built in the REST-like architectural style.

Now, you may be curious to learn more about a RESTful system. The rest of the chapter will definitely help you to know the internals. However, the topics on the RESTful system that we are going to discuss in the coming sections may need some basic knowledge of HTTP. So, let's take a crash course on HTTP to learn some basics and then proceed with our discussions thereafter. You can skip the next section if you are already familiar with HTTP.