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Addressing Doubts about REST By Stefan Tilkov

from http://www.infoq.com/articles/tilkov-rest-doubts

Invariably, learning about REST means that you’ll end up wondering just how applicable the concept really is for your specific scenario. And given that you’re probably used to entirely different architectural approaches, it’s only natural that you start doubting whether REST, or rather RESTful HTTP, really works in practice, or simply breaks down once you go beyond introductory, “Hello, World”-level stuff. In this article, I will try to address 10 of the most common doubts people have about REST when they start exploring it, especially if they have a strong background in the architectural approach behind SOAP/WSDL-based Web services.

1. REST may be usable for CRUD, but not for “real” business logic

This is the most common reaction I see among people who are skeptical about REST benefits. After all, if all you have is create/read/update/delete, how can you possibly express more complicated application semantics? I have tried to address some of these concerns in the introductory article of this series, but this point definitely merits closer discussion.

First of all, the HTTP verbs - GET, PUT, POST, and DELETE - do not have a 1:1 mapping to the CRUD database operations. For example, both POST and PUT can be used to create new resources: they differ in that with PUT, it’s the client that determines the resource’s URI (which is then updated or created), whereas a POST is issued to a “collection” or “factory” resource and it’s the server’s task to assign a URI. But anyway, back to the question: how do you handle more complex business logic?

Any computation calc(a, b) that returns a result c can be transformed into a URI that identifies its result — e.g. x = calc(2,3) might become http://example.com/calculation?a=2&b=3. At first, this seems like a gross misuse of RESTful HTTP — aren’t we supposed to use URIs to identify resources, not operations? Yes, but in fact this is what we do: http://example.com/sum?augend=2&addend=3 identifies a resource, namely the result of adding 2 and 3. And in this particular (obviously contrived) example, using a GET to retrieve the result might be a good idea — after all, this is cacheable, you can reference it, and computing it is probably safe and not very costly.

Of course in many, if not most cases, using a GET to compute something might be the wrong approach. Remember that GET is supposed to be a “safe” operation, i.e. the client does not accept any obligations (such as paying you for your services) or assume any responsibility, when all it does is follow a link by issuing a GET. In many other cases, it’s therefore more reasonable to provide input data to the server so that it can create a new resource via POST. In its response, the server can indicate the URI of the result (and possibly issue a redirect to take you there). The result is then re-usable, can be bookmarked, can be cached when it’s retrieved … you can basically extend this model to any operation that yields a result — which is likely to be every one you can think of.

2. There is no formal contract/no description language

From RPC to CORBA, from DCOM to Web Services we’re used to having an interface description that lists the operations, their names, and the types of their input and output parameters. How can REST possibly be usable without an interface description language?

There are three answers to this very frequently asked question.

First of all, if you decide to use RESTful HTTP together with XML — a very common choice — the whole world of XML schema languages, such as DTDs, XML Schema, RELAX NG or Schematron are still available to you. Arguably, 95% of what you usually describe using WSDL is not tied to WSDL at all, but rather concerned with the XML Schema complex types you define. The stuff WSDL adds on top is mostly concerned with operations and their names — and describing these becomes pretty boring with REST’s uniform interface: After all, GET, PUT, POST and DELETE are all the operations you have. With regards to the use of XML Schema, this means that you can use your favorite data binding tool (if you happen to have one) to generate data binding code for your language of choice, even if you rely on a RESTful interface. (This is not an entirely complete answer, see below.)

Secondly, ask yourself what you need a description for. The most common — albeit not the only — use case for having some description is to generate stubs and skeletons for the interface you’re describing. It is usually not documentation, since the description in e.g. WSDL format tells you nothing about the semantics of an operation — it just lists a name. You need some human-readable documentation anyway to know how to call it. In a typical REST approach, what you would provide is documentation in HTML format, possibly including direct links to your resources. Using the approach of having multiple representations, you might actually have self-documenting resources — just do an HTTP GET on a resource from your browser and get an HTML document containing data as well as a list of the operations (HTTP verbs) you can perform on it and the content types it accepts and delivers.

Finally, if you insist on using a description language for your RESTful service, you can either use the Web Application Description Language (WADL) or — within limitations — WSDL 2.0, which according to its authors is able to describe RESTful services, too. Neither WADL nor WSDL 2 are useful for describing hypermedia, though — and given that this is one of the core aspects of REST, I’m not at all sure they’re sufficiently useful.

3. Who would actually want to expose so much of their application’s implementation internals?

Another common concern is that resources are too low-level, i.e. an implementation detail one should not expose. After all, won’t this put the burden of using the resources to achieve something meaningful on the client (the consumer)?

The short answer is: No. The implementation of a GET, PUT or any of the other methods on a resource can be just as simple or complicated as the implementation of a “service” or RPC operation. Applying REST design principles does not mean you have to expose individual items from your underlying data model — it just means that instead of exposing your business logic in an operation-centric way, you do so in a data-centric way.

A related concern is that not enabling direct access to resources will increase security. This is based on an old fallacy known as “security by obscurity”, and one can argue that in fact it’s the other way round: By hiding which individual resources you access in your application-specific protocol, you can no longer easily use the infrastructure to protect them. By assigning individual URIs to meaningful resources, you can e.g. use Apache’s security rules (as well as rewriting logic, logging, statistics etc.) to work differently for different resources. By making these explicit, you don’t decrease, you increase your security.

4. REST works with HTTP only, it’s not transport protocol independent

First of all, HTTP is most emphatically not a transport protocol, but an application protocol. It uses TCP as the underlying transport, but it has semantics that go beyond it (otherwise it would be of little use). Using HTTP as a mere transport is abusing it.

Secondly, abstraction is not always a good idea. Web services take the approach of trying to hide many very different technologies under a single abstraction layer — but abstractions tend to leak. For example, there is a huge difference between sending a message via JMS or as an HTTP request. Trying to dumb widely different options down to their least common denominator serves no-one. An analogy would be to create a common abstraction that hides a relational database and a file system under a common API. Of course this is doable, but as soon as you address aspects such as querying, the abstraction turns into a problem.

Finally, as Mark Baker once coined: “Protocol independence is a bug, not a feature”. While this may seem strange at first, you need to consider that true protocol independence is impossible to achieve — you can only decide to depend on a different protocol that may or may not be on a different level. Depending on a widely accepted, officially standardized protocol such as HTTP is not really a problem. This is especially true if it is much more wide-spread and supported than the abstraction that tries to replace it.

5. There is no practical, clear & consistent guidance on how to design RESTful applications

There are many aspects of RESTful design where there are no “official” best practices, no standard way on how to solve a particular problem using HTTP in a way conforming to the REST principles. There is little doubt that things could be better. Still, REST embodies many more application concepts than WSDL/SOAP-based web services. In other words: while this criticism has a lot of value to it, it’s far more relevant for the alternatives (which basically offer you no guidance at all).

Occasionally, this doubt comes up in the form of “even the REST experts can’t agree how to do it”. In general, that’s not true — for example, I tend to believe that the core concepts I described here a few weeks ago haven’t been (nor will they be) disputed by any member of the REST community (if we can assume there is such a thing), not because it’s a particularly great article, but simply because there is a lot of common understanding once people have learned a little more than the basics. If you have any chance to try out an experiment, try whether it’s easier to get five SOA proponents to agree on anything than trying to get five REST proponents to do so. Based on past experience and long participation in several SOA and REST discussion groups, I’d tend to bet my money on the REST folks.

6. REST does not support transactions

The term “transaction” is quite overloaded, but in general, when people talk about transactions, they refer to the ACID variety found in databases. In an SOA environment — whether based on web services or HTTP only — each service (or system, or web app) implementation is still likely to interact with a database that supports transactions: no big change here, except you’re likely to create the transaction explicitly yourself (unless your service runs in an EJB container or another environment that handles the transaction creation for you). The same is true if you interact with more than one resource.

Things start to differ once you combine (or compose, if you prefer) transactions into a larger unit. In a Web services environment, there is at least an option to make things behave similarly to what people are used to from 2PC scenarios as supported e.g. in a Java EE environment: WS-Atomic Transaction (WS-AT), which is part of the WS-Coordination family of standards. Essentially, WS-AT implements something very similar or equal to the 2PC protocol specified by XA. This means that your transaction context will be propagated using SOAP headers, and your implementation will take care of ensuring the resource managers hook into an existing transaction. Essentially, the same model in EJB developer is used to — your distributed transaction behaves just as atomically as a local one.

There are lots of things to say about, or rather against, atomic transactions in an SOA environment:

  • Loose coupling and transactions, especially those of the ACID variety, simply don’t match. The very fact that you are co-ordinating a commit across multiple independent systems creates a pretty tight coupling between them.
  • Being able to do this co-ordination requires central control over all of the services — it’s very unlikely, probably impossible to run a 2PC transaction across company boundaries
  • The infrastructure required to support this is usually quite expensive and complicated.

For the most part, the need for ACID transactions in a SOA or REST environment is actually a design smell — you’ve likely modeled your services or resources the wrong way. Of course, atomic transactions are just one type of transaction — there are extended transaction models that might be a better match for loosely-coupled systems. They haven’t seen much adoption yet, though — not even in the Web services camp.

7. REST is unreliable

It’s often pointed out that there is no equivalent to WS-ReliableMessaging for RESTful HTTP, and many conclude that because of this, it can’t be applied where reliability is an issue (which translates to pretty much every system that has any relevance in business scenarios). But very often what you want is not necessarily some infrastructure component that handles message delivery; rather, you need to know whether a message has been delivered or not.

Typically, receiving a response message — such as a simple 200 OK in case of HTTP — means that you know your communication partner has received the request. Problems occur when you don’t receive a response: You don’t know whether your request has never reached the others side, or whether it has been received (resulting in some processing) and it’s the response message that got lost.

The simplest way to ensure the request message reaches the other side is to re-send it, which is of course only possible if the receiver can handle duplicates (e.g. by ignoring them). This capability is called idempotency. HTTP guarantees that GET, PUT and DELETE are idempotent — and if your application is implemented correctly, a client can simply re-issue any of those requests if it hasn’t received a response. A POST message is not idempotent, though — at least there are no guarantees in the HTTP spec that say it is. You are left with a number of options: You can either switch to using PUT (if your semantics can be mapped to it), use a common best practice described by Joe Gregorio, or adopt any of the existing proposals that aim to standardize this (such as Mark Nottingham’s POE, Yaron Goland’s SOA-Rity, or Bill de hÓra’s HTTPLR).

Personally, I prefer the best-practice approach — i.e., turn the reliability problem into an application design aspect, but opinions on this differ quite a bit.

While any of these solutions address a good part of the reliability challenge, there is nothing — or at least, nothing that I’m aware of — that would support delivery guarantees such as in-order delivery for a sequence of HTTP requests and responses. It might be worth pointing out, though, that many existing SOAP/WSDL scenarios get by without WS-Reliable Messaging or any of its numerous predecessors, too.

8. No pub/sub support

REST is fundamentally based on a client-server model, and HTTP always refers to a client and a server as the endpoints of communication. A client interacts with a server by sending requests and receiving responses. In a pub/sub model, an interested party subscribes to a particular category of information and gets notified each time something new appears. How could pub/sub be supported in a RESTful HTTP environment?

We don’t have to look far to see a perfect example of this: it’s called syndication, and RSS and Atom Syndication are examples of it. A client queries for new information by issuing an HTTP against a resource that represents the collection of changes, e.g. for a particular category or time interval. This would be extremely inefficient, but isn’t, because GET is the most optimized operation on the Web. In fact, you can easily imagine that a popular weblog server would have scale up much more if it had to actively notify each subscribed client individually about each change. Notification by polling scales extremely well.

You can extend the syndication model to your application resources — e.g., offer an Atom feed for changes to customer resources, or an audit trail of bookings. In addition to being able to satisfy a basically unlimited number of subscribing applications, you can also view these feeds in a feed reader, similarly to viewing a resource’s HTML representation in your browser.

Of course, this is not a suitable answer for some scenarios. For example, soft realtime requirements might rule this option out, and another technology might be more appropriate. But in many cases, the mixture of loose coupling, scalability and notification enabled by the syndication model is an excellent fit.

9. No asynchronous interactions

Given HTTP’s request/response model, how can one achieve asynchronous communication? Again, we have to be aware that there are multiple things people mean when they talk about asynchronicity. Some refer to the programming model, which can be blocking or non-blocking independently of the wire interactions. This is not our concern here. But how do you deliver a request from a client (consumer) to the server (provider) where the processing might take a few hours? How does the consumer get to know the processing is done?

HTTP has a specific response code, 202 (Accepted), the meaning of which is defined as “The request has been accepted for processing, but the processing has not been completed.” This is obviously exactly what we’re looking for. Regarding the result, there are multiple options: The server can return a URI of a resource which the client can GET to access the result (although if it has been created specifically due to this request, a 201 Created would probably be better). Or the client can include a URI that it expects the server to POST the result to once it’s done.

10. Lack of tools

Finally, people often complain about the lack of tools available to support RESTful HTTP development. As indicated in item #2, this is not really true for the data aspect — you can use all of the data binding and other data APIs you are used to, as this is a concern that’s orthogonal to the number of methods and the means of invoking them. Regarding plain HTTP and URI support, absolutely every programming language, framework and toolkit on the planet supports them out of the box. Finally, vendors are coming up with more and more (supposedly) easier and better support for RESTful HTTP development in their frameworks, e.g. Sun with JAX-RS (JSR 311) or Microsoft with the REST support in .NET 3.5 or the ADO.NET Data Services Framework.

Conclusion

So: Is REST, and its most common implementation, HTTP, perfect? Of course not. Nothing is perfect, definitely not for every scenario, and most of the time not even for a single scenario. I’ve completely ignored a number of very reasonable problem areas that require more complicated answers, for example message-based security, partial updates and batch processing, and I solemnly promise to address these in a future installment. I still hope I could address some of the doubts you have — and if I’ve missed the most important ones, you know what the comments are for.

Stefan Tilkov is the lead editor of InfoQ’s SOA community and co-founder, principal consultant and chief RESTafarian of Germany/Switzerland-based innoQ.



posted on 2009-07-10 17:36 gembin 阅读(522) 评论(0)  编辑  收藏


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