Why do we need RESTful Web Services?

Question

REST should be used if it is very important for you to minimize the coupling between client and server components in a distributed application.

This may be the case if your server is going to be used by many different clients that you do not have control over. It may also be the case if you want to be able to update the server regularly without needing to update the client software.

I can assure you that achieving this low level of coupling is not easy to do. It is critical to follow all of the constraints of REST to succeed. Maintaining a purely stateless connection is difficult. Picking the right media-types and squeezing your data into the formats is tricky. Creating your own media types can be even harder.

Adapting rich server behaviour into the uniform HTTP interface can be confusing and at times appears pedantic in comparison to the relatively straightforward RPC approach.

Despite the difficulties, the benefits are that you have a service that a client developer should be able to easily understand due to the consistent use of the HTTP protocol. The service should be easily discoverable due to hypermedia and the client should be extremely resilient to changes on the server.

The benefits of hypermedia and the avoidance of session state makes load balancing simple and service partitioning feasible. The strict conformance to HTTP rules make the availability of tools like debuggers and caching proxies wonderful thing.

Update

It seems to me that REST is another 'last word of fashion' (or I can be totally wrong because I haven't ever seen REST in practice).

I think REST has become fashionable because people attempting to do SOA type projects have found that using the SOAP stack they are not realizing the benefits that were promised. People keep turning back to the web as an example of simple integration methodologies. Unfortunately, I think people underestimate the amount of planning and foresight that went into creating the web and they oversimplify what needs to be done to allow the kind of serendipitous reuse that does occur on the web.

You say that you have never seen REST in practice, but that cannot possibly be true if you ever use a web browser. The web browser is a REST client.

• Why do you not need to do a browser update when someone changes some html on a web site?
• Why can I add a complete new set of pages to a web site and the "client" can still access those new pages without an update?
• Why do I not need to provide a "service-description-language" to the web browser to tell it when it goes to http://example.org/images/cat that the return type will be a jpeg image and when you go to http://example.org/description/cat the return type will be text/html?
• Why can I use a web browser to visit sites that did not exist when the browser was released? How can the client know about these sites?

These may sound like inane questions, but if you know the answer, then you can start to see what REST is all about. Look at StackOverflow for more benefits of REST. When I am looking at a question, I can bookmark that page or send the url to a friend and he can see the same information. He doesn't have to navigate through the site to find that question.

StackOverflow uses a variety of OpenId services for authentication, gravatar.com for avatar images, google-analytics and Quantserve for analytical information. This kind of multi-company integration is the type of thing the SOAP world only dreams of. One of the best examples is the fact that the jQuery libraries that are used to drive the StackOverflow UI are retrieved from Google's Content Delivery Network. The fact that SO could direct the client (i.e. your web browser) to download code from a third-party site to improve performance is testament to the low coupling between web client and server.

These are examples of a REST architecture at work.

Now some web sites / applications do break the rules of REST and then the browser does not work as expected.

• The infamous back button problem is caused by using server side session state.
• Load balancing can become a pain when you have server side session state.
• Flash applications often prevent the URL from specifically identifying a representation.
• The other problem that breaks web browsers is poor conformance to media-type standards. We hear all of the time about how IE6 needs to be killed. The problem there is that standards were not properly followed, or were ignored for whatever reason.
• The use of login sessions are the source of many security holes.

REST is everywhere. It is the part of the web that makes it work well. If you want to build distributed applications that can scale like the web, be resilient to change like the web and promote re-use as the web has done, then follow the same rules they did when building web browsers.

REST was kicked off, to my knowledge, by Roy Fielding's dissertation Architectural Styles and the Design of Network-based Software Architectures, which is worth a read if you haven't looked at it.

At the top of the dissertation is a quote:

Almost everybody feels at peace with nature: listening to the ocean waves against the shore, by a still lake, in a field of grass, on a windblown heath. One day, when we have learned the timeless way again, we shall feel the same about our towns, and we shall feel as much at peace in them, as we do today walking by the ocean, or stretched out in the long grass of a meadow.

— Christopher Alexander, The Timeless Way of Building (1979)

And that really does sum it up there. REST is in many ways more elegant.

SOAP is a protocol on top of HTTP, so it bypasses a lot of HTTP conventions to build new conventions in SOAP, and is in a number of ways redundant with HTTP. HTTP, however, is more than sufficient for retreiving, searching, writing, and deleting information via HTTP, and that's a lot of what REST is. Because REST is built with HTTP instead of on top of it, it also means that software that wants to integrate with it (such as a web browser) does not need to understand SOAP to do so, just HTTP, which has to be the most widely understood and integrated-with protocol in use at this point.

From here:

REST advantages:

• Lightweight - not a lot of extra xml markup
• Human Readable Results
• Easy to build - no toolkits required

Also check this out:

To be fair, REST isn't the best solution for every Web service. Data that needs to be secure should not be sent as parameters in URIs. And large amounts of data, like that in detailed purchase orders, can quickly become cumbersome or even out of bounds within a URI. In these cases, SOAP is indeed a solid solution. But it's important to try REST first and resort to SOAP only when necessary. This helps keep application development simple and accessible.

I can safely say I have spent a lot of time to understand this as a beginner but this is the best link to start with REST from scratch! http://www.codeproject.com/Articles/21174/Everything-About-REST-Web-Services-What-and-How-Pa

Just to pull you in,

Think of what a "traditional web service" is. It is an interface with exposed "methods." Clients know the methods' name, input and output and hence can call them.

Now imagine an interface that does not expose "methods". Instead, it exposes "objects". So when a client sees this interface, all it sees is one or more "objects". "An object" has no input and output – because "it does not do anything". It is a noun, not a verb. It is "a thing", not "an action".

For example, think of a traditional web service that provides the current weather conditions if you provide it with a city. It probably has a web method like GetWeatherInfo() which takes a city as input and provides weather data as output. It is easy for you so far to understand how a client will consume this web service.

Now imagine, in the place of the above web service, there is a new one that exposes cities as objects. So, when you look at it as a client, instead of GetWeatherInfo(), you see New York, Dallas, Los Angeles, London and so on. And these cities do not have any application specific methods hanging from them - they are apparently like inert gases - they themselves do not react.

You must be thinking – well, how does that help you, as a client, to get to the weather of Dallas? We will get to that in a few moments.

If all you get from a web service is a "set of objects", obviously you need a way to "act on them". The objects themselves have no methods for you to call, so you need a set of actions that you can apply onto these objects. In other words, you need to "apply a verb to the noun". If you see an object, say, an apple, which is "a noun", you can apply "a verb" like eat, to it. But not all verbs can be applied to all nouns. Like, you can drive a car, but cannot drive a television.

Thus, if a web service exposes only objects, and you are asked – well, let us now design a few standard actions or verbs that "all clients can apply to all objects they see", ...

Here are some ideas:

• REST constrains your service to use a uniform interface. You don't have to waste time daydreaming (or arguing) about all of the possibly ways your service could work - you get right to work identifying the resources in your system. Turns out to be a big job in itself, but fortunately the problems tend to be much-better defined.
• With resources, their associations, and their representations in hand, there's really very little to do in implementing your service because many decisions have been made for you.
• Your system will look very much like other RESTful systems; learning curves for teammates, partners, and clients will be reduced.
• You'll have a common vocabulary to discuss design problems with other developers, and even with those less technically minded (such as customers).
• As Darrel says, because you're using a hypertext-driven design, your service narrows the scope of coupling to just one thing - media types. This helps you as a developer because changes to your system are contained within a narrow band of contact. This helps your clients in that fewer of your changes will break their code.
• Almost every problem you might have in implementing REST can be solved by exposing a new resource or re-thinking your resource model. This focus is, IMO, a big productivity boost.

Bottom line, REST removes many of the most time-consuming and contentious design and implementation decisions from your team's workflow. It shifts your attention from implementing your service to designing it. And it does so without piling gobbledygook onto the HTTP protocol.

Most of the "pro" answers about REST seem to come from people who have never developed a SOAP web service or client using an environment which supplies appropriate tools for the task. They complain about issues that I've simply never encountered, using Visual Studio .NET and IBM's Rational Web Developer. I suppose that if you have to develop web services or clients in a scripting language, or some other language with little or no tool support, that these are valid complaints.

I also have to admit that several of the "pro" points sound like things that might actually be true - but that I've never seen an example that illustrates their value. In particular, I'd greatly appreciate it if someone would post a comment containing a link to a good example of a REST web service. This should be one that uses multiple levels of resource, possibly in a hierarchy, and which uses media types properly. Maybe if I look at a good example, I'll understand, in which case, I'll come back here and admit it.