So far, we have discussed the fundamentals of the client-server/web services paradigm, and the way they communicate with standard protocols; however, we are yet to touch upon REST-based communication and after all, that's what this book is about. This section will cover the introduction of web APIs, and how a web API is a development model for web services. The communication between the devices is REST-based. RESTful APIs do not use/require XML- based web service protocols, such as SOAP or WSDL, to support their interfaces, but they use simplified representations instead.

The following diagram depicts the web API and their simplified representations as the client side and the server side are exposed to each other through high-level interfaces:

So the web API, as shown in the preceding diagram, is available on both the client side and the server side. The client-side interfaces are generally exposed as JavaScript or browser plugins, and the server-side interfaces are generally exposed through the web as JSON/XML. Some of the key terminologies that we will come across concerning web APIs are endpoint, uniform resource identifier (URI), and resources.

The web API is an application programming interface (API) for either a web server or for a web browser. So, Web API is a concept or methodology for accessing any API (available over the web) through the HTTP protocol. There are many categories of APIs, SOAP, XML-RPC, JSON-RPC, REST, and so on. APIs can be developed with any programming language such as Java, .NET, and many more.

So now you have got an idea of what is a Web API is and where REST API development fits into the Web API landscape, let's move on and see more details of SOA, ROA, REST, RESTful APIs, and their key constituents in the following sections.