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I'm new to web services world. I have created a rest api in ERP software that creates sales order. How will anyone outside the world know what parameters to send to this and in what format ? From all the videos I have watched and all materials read they talk about no api documentation is needed as it is REST service, People just know what request payload to send. I m not sure if I understand how that is possible. Its like I give somebody the url and tell them go figure. I tried sending orders with different parameter list and it is creating errors. But, if I send it in the way it accepts then it is working fine. Not sure if I understand the concepts well. Should I be creating documentation of this api telling what the request payload should look like ?
I completely disagree with the answer given by Joessel, which just propagates a typical RPC take on it, which is NOT REST at all!
In regards to how a service utilizing the REST architecture style should inform clients about what properties a resource supports and stuff like that, just look at traditional HTML pages. How are Web server able to tell your Browser what input it expects?
HTML is a media type that specifies the syntax to use as well as the semantics of each of the elements and attributes wich are admissible in a HTML document. I.e. HTML Forms enables a server to inform a client on the respective properties a resource support. In addition to that, a server also teaches a client on the respective target URI to send the request to, the HTTP operation to use upon sending the request as well as the media type to use for marshalling the request to a respective representation format. This is why you DON'T NEED any other documentation to interact with Web pages. Most arbitrary Web clients support HTML documents by default and therefore you don't need to reimplement the wheel to process such documents.
For non-HTML resources it is also just a matter of whether your client supports the respective media type or not. I.e. PNG files also follow a certain standard which allows arbitrary clients to show images instead of the actual bytes on your screen.
Most of those so called "REST APIs", which are truely RPC ones, just use custom JSON based message structures. JSON itself just defines the basic syntax but no semantics for any elements, attributes or other properties. It doesn't even add support for links. JSON Hyper-Schema is an extension which at least triest o add support for it, though it already requires to use an other media type than application/json. Though, if such formats are not well-defined and standardized, widespread adoption will not be possible on the long run. Hence creating a common media type is of importance to increase interoperability for such media types. I.e. for JSON based formats, HAL+JSON, HAL Forms, ION, and others provide definitions for basic JSON based message structures, with support for links and other features like form-support and other things.
So, if you take a closer look at the Web, you will find many concepts that you can reuse for a truely RESTful design. After all, REST just takes the ideas used on the Web for decades and attempts to offer the benefits resulting from these concepts to applications rather than humans alone. As such, it is always a good idea to first design the interaction flow as if one would interact with a traditional Web page and then take the concepts used in that design and apply it onto your application domain model.
As you don't need external documentation to interact with Web pages, so you don't need external documentation to interact with well defined message formats that follow a common media type as well. Through content-type negotiation both server and client will communicate with representation formats both support. So, the more (different) media types you support, the more likely you will be in the end to interact with different parties in that environment and if all of the supported media types are standardized you might not need any external documentation at all.
If you follow the academic definition of REST defined by Dr Fielding in his dissertation, it is true that such a service doesn't need any documentation. Hovewer, the chance are high that your service is not RESTful by his definition and that you need a documention to help your consumers. You can find an informative discussion about why a REST api doesn't need a documentation here.
That being said, you will probably need a documentation in most case. Depending on your needs and consumers of your api, a simple text file could be enough; maybe a markdown that you can easily share with other developers. Just remember to add all information needed to understand each route (verb, path, query, body, response...), and you can even throw some example code to make it easy to start.
And if you need something more robust, I can only advise you to follow the open api specification. One of the easiest way to get started is to use the swagger editor . You will even be able to publish your documentation in a nice way using one of the many tools out there (ex: redoc).
Writing a documention is never fun but nobody will use a product that they cannot understand, however great the product is.
Good luck,
Edit:
I rewrote the introduction as it was misleading, thanks to #Roman Vottner for pointing it out. THe previous intro was:
I don't know who told you that a documentation is not needed for a REST Api but I find this highly misleading for newcomers... A documentation is (super) important, especially if you are not the sole consumer of your API, and in that case I would say it's mandatory.
One of the goals of the REST API architecture is decoupling of the client and the server.
One of the questions I have run across in planning a REST API is: "how does the client know what is a valid payload for POST methods?"
Somehow the API needs to communicate to the UI what a valid payload for a given resource’s POST method. Otherwise here we are back at depending on out-of-band knowledge being necessary to work with an API and we are tightly coupled again.
So I’ve had this idea that the API response for a GET on a resource would provide a specification for constructing a valid payload for the POST method on that resource. This would include field names, data type, max length, etc.
This guy has a similar idea.
What's the correct way to handle this? Are most people just relying on out-of-band information? What are people doing in the real world with this problem?
EDIT
Something I have come up with to solve this problem is illustrated in the following sequence diagram:
The client and the api service are separate. The client knows:
Entry point
How to navigate the API via the hypermedia.
Here's what happens:
Someone (user) requests the registration page from the client
The client requests the entry point from the API and receives all hypermedia links with appropriate meta data on how to traverse them legally.
Client constructs the registration form based on the meta data associated with the registration hypermedia POST method.
User fills in the form and submits.
Client POSTs to the API with the correct data and all is well.
No magic /meta resouces, no need to use a method for the meta data. Everything is provided by the API.
Thoughts?
Most people are relying on out-of-band information. This is usually ok, though, because most clients aren't being built dynamically, but statically. They rely on known parts of the API rather than being HATEOAS-driven.
If you are developing or want to support a metadata-driven client, then yes, you're going to need to come up with a schema for providing that information. The implementation you linked to seems reasonable after a quick skim. Note that you've only moved the problem, though. Clients still need to know how to interpret the information in the metadata responses.
Your are right, the client should understand the semantics of the links in the response, and choose the right one from them to achieve its goal. The client is coupled to the semantics the API provides about this and not to the API itself. So for example a client should not retrieve information from the URI structure, since it is tightly coupled to the actual API.
I know of 2 current solution types about this:
by HAL+JSON you use IANA link relations to describe what the link does, and vendor specific MIME types to describe the schema of the fields
by JSON-LD (or any other RDF format) with Hydra vocab you send back RDF metadata according to the operation the link calls. This meta-data can contain the validation details of the fields (xsd vocab) and the semantics of the fields (microdata, microformats, etc...). This information is completely decoupled from the API implementation, so it might be a better option than using vendor specific MIME types, but Hydra is still under development and HAL is much simpler.
However your solution is valid as well, I think you should check both of these, since they are already standard solutions, and the uniform interface / self-descripting message constraint of REST encourages the usage of existing standards instead of custom solutions. But it is up to you if you want to create an own standard.
I think you are asking about, Rest API meta data handling. Unlike SOAP, Rest APIs doesn't use meta data normally, but sometimes it can be pretty useful, once your api size gets bigger.
I think you should look into swagger. It is the most elegant you can find out for rest apis. I have being using it for sometime and with the annotation support it is being rather easy to work with. It also has many examples found on github. Other advantage is, it contains nice configurable ui.
Apart from that you can find other ways of doing it like WADL and WSDL 2.0. Even-though I haven't being using them, you can read more about them here.
With RFC 6861, you can link to your form with create-form and edit-form Link Relations, instead of the client constructing the form by itself. The corresponding form should have the necessary schema to construct the POST request.
For a SaaS startup I'm involved in, I am building both a RESTful web API and a couple of client apps on different platforms that consume it. I think I've got the API figured out, but now I'm turning to the clients. As I've been reading about REST, I see that a key part of REST is discovery, but there seems to be a lot of debate between two different interpretations of what discovery really means:
Developer discovery: The developer hard-codes copious amounts of API details into the client, such as resource URI's, query parameters, supported HTTP methods, and other details that they've discovered through browsing the docs and experimenting with the API's responses. This type of discovery IMHO necessitates cool linkage and the API versioning question, and leads to hard coupling of the client code to the API. Not much better than if using a well-documented collection of RPC's it seems.
Runtime discovery - The client app itself is able to figure out everything it needs with little or no out-of-band information (presumably, only a knowledge of the media types the API deals with.) Links can be hot. But to make the API very efficient, a lot of link templating for query parameters seems to be needed, which makes out-of-band info creep back in. There are possibly other difficulties I haven't thought of yet since I haven't gotten to that point in development. But I do like the idea of loose coupling.
Runtime discovery seems to be the holy grail of REST, but I'm seeing precious little discussion about how to implement such a client. Almost all REST sources I've found seem to assume Developer discovery. Anyone know of some Runtime discovery resources? Best practices? Examples or libraries with real code? I'm working in PHP (Zend Framework) for one client. Objective-C (iOS) for the other.
Is Runtime discovery a realistic goal, given the present set of tools and knowledge in the developer community? I can write my client to treat all of the URI's in an opaque manner, but how to do this most efficiently is a question, especially over low-bandwidth connections. Anyway, URI's are only part of the equation. What about link templating in the Runtime context? How about communicating what methods are supported, aside from making a lot of OPTIONS requests?
This is definitely a tough nut to crack. At Google, we've implemented our Discovery Service that all our new APIs are built against. The TL;DR version is we generate a JSON Schema-like spec that our clients can parse - many of them dynamically.
That results means easier SDK upgrades for the developer and easy/better maintenance for us.
By no means the perfect solution, but many of our devs seem to like.
See link for more details (and make sure to watch the vid.)
Fascinating. What you are describing is basically the HATEOAS principle. What is HATEOAS you ask? Read this: http://en.wikipedia.org/wiki/HATEOAS
In layman's terms, HATEOAS means link following. This approach decouples your client from specific URL's and gives you the flexibility to change your API without breaking anyone.
You did your home work and you got to the heart of it: runtime discovery is holy grail. Don't chase it.
UDDI tells a poignant story of runtime discovery: http://en.wikipedia.org/wiki/Universal_Description_Discovery_and_Integration
One of the requirements that should be satisfied before you can call an API 'RESTful' is that it should be possible to write a generic client application on top of that API. With the generic client, a user should be able to access all the API's functionality. A generic client is a client application that does not assume that any resource has a specific structure beyond the structure that is defined by the media type. For example, a web browser is a generic client that knows how to interpret HTML, including HTML forms etc.
Now, suppose we have a HTTP/JSON API for a web shop and we want to build a HTML/CSS/JavaScript client that gives our customers an excellent user experience. Would it be a realistic option to let that client be a generic client application? No. We want to provide a specific look-and-feel for every specific data element and every specific application state. We don't want to include all knowledge about these presentation-specifics in the API, on the contrary, the client should define the look and feel and the API should only carry the data. This implies that the client has hard-coded coupling of specific resource elements to specific layouts and user interactions.
Is this the end of HATEOAS and thus the end of REST? Yes and no.
Yes, because if we hard-code knowledge about the API into the client, we loose the benefit of HATEOAS: server-side changes may break the client.
No, for two reasons:
Being "RESTful" is a property of the API, not of the client. As long as it is possible, in theory, to build a generic client that offers all capabilities of the API, the API can be called RESTful. The fact that clients don't obey the rules, is not the API's fault. The fact that a generic client would have a lousy user experience is not an issue. Why is it important to know that it is possible to have a generic client, if we don't actually have that generic client? This brings me to the second reason:
A RESTful API offers clients the option to choose how generic they want to be, i.e. how resilient to server-side changes they want to be. Clients which need to provide a great user experience may still be resilient to URI changes, to changes in default values and more. Clients doing batch jobs without user interaction may be resilient to other kinds of changes.
If you are interested in practical examples, checkout my JAREST paper. The last section is about HATEOAS. You will see that with JAREST, even highly interactive and visually attractive clients can be quite resilient to server-side changes, though not 100%.
I think the important point about HATEOAS is not that it is some holy grail client-side, but that it isolates the client from URI changes - it is assumed you are using known (or developer discovered custom) Link Relations that will allow the system to know which link for an object is the editable form. The important point is to use a media type that is hypermedia aware (e.g. HTML, XHTML, etc).
You write:
To make the API very efficient, a lot of link templating for query parameters seems to be needed, which makes out-of-band info creep back in.
If that link template is supplied in the previous request, then there is no out-of-band information. For example a HTML search form uses link templating (/search?q=%#) to generate a URL (/search?q=hateoas), but nothing is known by the client (the web browser) other than how to use HTML forms and GET.
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I've implemented two REST services: Twitter and Netflix. Both times, I struggled to find the use and logic involved in the decision to expose these services as REST instead of SOAP. I hope somebody can clue me in to what I'm missing and explain why REST was used as the service implementation for services such as these.
Implementing a REST service takes infinitely longer than implementing a SOAP service. Tools exist for all modern languages/frameworks/platforms to read in a WSDL and output proxy classes and clients. Implementing a REST service is done by hand and - get this - by reading documentation. Furthermore, while implementing these two services, you have to make "guesses" as to what will come back across the pipe as there is no real schema or reference document.
Why write a REST service that returns XML anyway? The only difference is that with REST you don't know the types each element/attribute represents - you are on your own to implement it and hope that one day a string doesn't come across in a field you thought was always an int. SOAP defines the data structure using the WSDL so this is a no-brainer.
I've heard the complaint that with SOAP you have the "overhead" of the SOAP Envelope. In this day and age, do we really need to worry about a handful of bytes?
I've heard the argument that with REST you can just pop the URL into the browser and see the data. Sure, if your REST service is using simple or no authentication. The Netflix service, for instance, uses OAuth which requires you to sign things and encode things before you can even submit your request.
Why do we need a "readable" URL for each resource? If we were using a tool to implement the service, do we really care about the actual URL?
A canary in a coal mine.
I have been waiting for a question like this for close to a year now. It was inevitable that this day would come and I am sure we are going to see many more questions like this in the coming months.
The warning signs
You are absolutely correct, it does take longer to build RESTful clients than SOAP clients. The SOAP toolkits take away lots of boilerplate code and make client proxy objects available with almost no effort. With a tool like Visual Studio and a server URL I can be accessing remote objects of arbitrary complexity, locally in under five minutes.
Services that return application/xml and application/json are so annoying for client developers. What are we supposed to do with that blob of data?
Fortunately, lots of sites that provide REST services also provide a bunch of client libraries so that we can use those libraries to get access to a bunch of strongly typed objects. Seems kind of dumb though. If they had used SOAP we could have code-gen’d those proxy classes ourselves.
SOAP overhead, ha. It’s latency that kills. If people are really concerned about the number of excess bytes going across the wire then maybe HTTP is not the right choice. Have you seen how many bytes are used by the user-agent header?
Yeah, have you ever tried using a web browser as debugging tool for anything other than HTML and javascript. Trust me it sucks. You can only use two of the verbs, the caching is constantly getting in the way, the error handling swallows so much information, it’s constantly looking for a goddamn favicon.ico. Just shoot me.
Readable URL. Only nouns, no verbs. Yeah, that’s easy as long as we are only doing CRUD operations and we only need to access a hierarchy of objects in one way. Unfortunately most applications need a wee bit more functionality than that.
The impending disaster
There are a metric boatload of developers currently developing applications that integrate with REST services who are in the process of coming to the same set of conclusions that you have. They were promised simplicity, flexibility, scalability, evolvabilty and the holy grail of serendipitous reuse. The characteristics of the web itself, how can things go wrong.
However, they are finding that versioning is just as much of a problem, but the compiler doesn’t help detect issues. The hand written client code is a pain to maintain as the data structures evolve and URLs get refactored. Designing APIs around just nouns and four verbs can be really hard, especially with RESTful Url zealots telling you when you can and cannot use query strings.
Developers are going to start asking why are we wasting our effort on support both Json formats and Xml formats, why not just focus our efforts on one and do it well?
How did things go so wrong
I’ll tell you what went wrong. We as developers let the marketing departments take advantage of our primary weakness. Our eternal search for the silver bullet blinded us to the reality of what REST really is. On the surface REST seems so easy and simple. Name your resources with Urls and use GET, PUT, POST and DELETE. Hell, us devs already know how to do that, we have been dealing with databases for years that have tables and columns and SQL statements that have SELECT, INSERT, UPDATE and DELETE. It should have been a piece of cake.
There are other parts of REST that some people discuss, such as self-descriptiveness, and the hypermedia constraint, but these constraints are not so simple as resource identification and the uniform interface. The seem to add complexity where the desired goal is simplicity.
This watered down version of REST became validated in developer culture in many ways. Server frameworks were created that encouraged Resource Identification and the uniform interface, but did nothing to support the other constraints. Terms started to float around differentiating the approaches, (HI-REST vs LO-REST, Corporate REST vs Academic REST, REST vs RESTful).
A few people scream out that if you don’t apply all of the constraints it’s not REST. You will not get the benefits. There is no half REST. But those voices were labelled as religious zealots who were upset that their precious term had been stolen from obscurity and made mainstream. Jealous people who try to make REST sound more difficult than it is.
REST, the term, has definitely become mainstream. Almost every major web property that has an API supports "REST". Twitter and Netflix are two very high profile ones. The scary thing is that I can only think of one public API that is self-descriptive and there are a handful that truly implement the hypermedia constraint. Sure some sites like StackOverflow and Gowalla support links in their responses, but there are huge gaping holes in their links. The StackOverflow API has no root page. Imagine how successful the web site would have been if there was no home page for the web site!
You were misled I’m afraid
If you have made it this far, the short answer to your question is those APIs (Netflix and Twitter) do not conform to all of the constraints and therefore you will not get the benefits that REST apis are supposed to bring.
REST clients do take longer to build than SOAP clients but they are not tied to one specific service, so you should be able to re-use them across services. Take the classic example, of a web browser. How many services can a web browser access? What about a Feed Reader? Now how many different services can the average Twitter client access? Yes, just one.
REST clients are not supposed to be built to interface with a single service, they are supposed to be built to handle specific media types that could be served by any service. The obvious question to that is, how can you build a REST client for a service that delivers application/json or application/xml. Well you can’t. That’s because those formats are completely useless to a REST client. You said it yourself,
you have to make "guesses" as to what
will come back across the pipe as
there is no real schema or reference
document
You are absolutely correct for services like Twitter. However, the self-descriptive constraint in REST says that the HTTP content type header should describe exactly the content that is being transmitted across the wire. Delivering application/json and application/xml tells you nothing about the content.
When it comes to considering the performance of REST based systems it is necessary look at the bigger picture. Talking about envelope bytes is like talking about loop unwinding when comparing a quick-sort to a shell-sort. There are scenarios where SOAP can perform better, and there are scenarios where REST can perform better. Context is everything.
REST gains much of its performance advantage by being very flexible about what media types it supports and by having sophisticated support for caching. For caching to work well though nearly all of the constraints must be adhered to.
Your last point about readable urls is by far the most ironic. If you truly commit to the hypermedia constraint, then every URL could be a GUID and the client developer would lose nothing in readability.
The fact that URIs should be opaque to the client is one of the most key things when developing REST systems. Readable URLs are convenient for the server developer and well structured URLs make it easier for the server framework to dispatch requests, but those are implementation details that should have no impact on the developers consuming the API.
The Twitter API is not even close to being RESTful and that is why you are unable to see any benefit to using it over SOAP. The Netflix API is much closer but it’s use of generic media types demonstrates that failing to adhere to even a single constraint can have a profound impact on the benefits derived from the service.
It may not be all their fault
I’ve done a whole lot of dumping on the service providers, but it takes two to dance RESTfully. A service may follow all of the constraints religiously and a client can still easily undo all of the benefits.
If a client hard codes urls to access certain types of resources then it is preventing the server from changing those urls. Any kind URL construction based on implicit knowledge of how the service structures its urls is a violation.
Making assumptions about what type of representation will be returned from a link can lead to problems. Making assumptions about the content of the representation based on knowledge that is not explicitly stated in the HTTP headers is definitely going to create coupling that will cause pain in the future.
Should they have used SOAP?
Personally, I don’t think so. REST done right allows a distributed system to evolve over the long term. If you are building distributed systems that have components that are developed by different people and need to last for many years, then REST is a pretty good option.
SOAP is an object-oriented, remote procedure call technology stack. It works by building a new abstraction on top of an existing protocol (HTTP).
REST is a document oriented approach, that simply uses the features of an existing protocol (HTTP). "REST" is just a buzzword -- the concept is this: Just use the web the way it was designed to work!
In response to edits to question:
"Implementing a REST service takes infinitely longer than implementing a SOAP service."
Um, no, it can't be infinitely longer. And in cases where what you are trying to retrieve is already a document or file, it's actually much faster. For example, the OGC spec for WMS (Web Mapping Service) defines both a SOAP and REST version of the protocol, and there's a reason why almost nobody implements the SOAP version -- it's because if you're trying to get a map, it's a lot easier to just build a URL and fetch image bytes from that URL than it is to bother with encapsulating it into a SOAP message. But yes, I will agree that if the point of the web service is to transfer some strongly-typed object in a domain object model, SOAP is better suited for that use.
"Why write a REST service that returns XML anyway?"
Well, yes, that can be silly. But it depends on what the XML is. If there's a clearly defined schema for it somewhere, then there's no ambiguity. For example, you can think of WSDL URLs as being a kind of RESTful web service for retrieving information about a web service. In this case, adding the overhead of another SOAP request would be pointless.
In general, REST wins when the content that is being transferred can be thought of as a file, as a single unit. SOAP wins when the content needs to be treated as an object with members.
"I've heard the complaint that with SOAP you have the "overhead" of the SOAP Envelope. In this day and age, do we really need to worry about a handful of bytes?"
Yes. Not in every circumstance, but there are sites with a great deal of traffic where it makes a difference. Is it enough of a difference to outweigh the semantic differences of using SOAP instead of REST? I doubt it. If you're doing an object remoting protocol and the number of bytes is making a difference, SOAP is probably not the tool for you anyway -- maybe you should be using CORBA or DCOM instead.
"I've heard the argument that with REST you can just pop the URL into the browser and see the data."
Yes, and this is a large argument in favor of REST if it makes sense to view the data in a browser. For example, with image data, it's an easy way to debug the service -- just paste the URL into your browser's address bar and see what the image looks like. Or if the data returned is in XML, and you have a referenced XML stylesheet that renders into readable HTML in the browser, then you get the benefit of semantic markup and easy visualization all in one package. But you are correct, this benefit mostly evaporates when working with more complex authentication schemes. If you can't encode all your authentication information into each HTTP request, then I would argue that it doesn't count as REST at all.
"Why do we need a "readable" URL for each resource? If we were using a tool to implement the service, do we really care about the actual URL?"
Well, it depends. Why do we need readable URLs for any resource on the web? You can read Tim Berners-Lee's essay Cool URIs Don't Change for the rationale, but basically, as long as the resource may still be useful in the future, the URI for that resource should stay the same.
Obviously, for transient resources (like the "today's Money" link in the essay) there is no need for it, since the need to reference the resource goes away if the corresponding resource goes away. But for more permanent resources (like StackOverflow questions, for example, or movies on IMDB), you want to have a URL that will work forever. When you're designing a web service, you need to decide if the resources themselves could outlive your service, and if so, then REST is probably the right way to go.
For the record, yes, I've been developing web pages since well before NetFlix or Twitter existed. And no, I've not yet had any need or opportunity to implement a client to either NetFlix or Twitter's services. But even if their services are atrociously difficult to work with, that doesn't mean the technology they implemented their services on top of is bad -- only that those two implementations are bad.
To make a long story short: REST and SOAP are just tools. They each have strengths and weaknesses. If the only tool you have is a hammer, then every problem looks like a nail. So get to know both tools, and learn how to use them correctly, and then choose the right tool for each job.
An honest question deserves an honest answer. But first, why did you use the text of this question as an answer to another question if you did not think it was rhetorical in nature?
Anyway:
"Tools exist for all modern languages/frameworks/platforms to read in a WSDL and output proxy classes and clients. Implementing a REST service is done by hand by reading documentation."
Just like browser vendors have read and re-read the HTML 4.01 specification up and down to try to implement a consistent browsing experience. Have you reflected on the fact that browsers were invented long before internet banking and stackoverflow, and yet, you can use a browser to do just those things. This is made possible because of the sole reason that everybody agrees to use HTML (and related formats like CSS, JS, JPEG etc).
Blogging is actually not that new, and someone came up with AtomPub, which allows any blogging software to access and update posts in a blog, much like any web browser can access any web page. That's pretty neat, and works because of the RESTful constraints imposed by the protocol.
But for Twitter and Netflix, there is no universal agreement that "all microblogs in existence shall use the media type application/tweet", mainly because microblogging is so new. Maybe in a few years time a few microblogging services settle on the same API so that Twitter, Facebook, Identica and can interoperate. None of their existing APIs are anywhere near RESTful, however much they claim, so I don't expect it to happen real soon.
"Furthermore, while implementing these two services, you have to make "guesses" as to what will come back across the pipe as there is no real schema or reference document."
You've hit the nail on the head. REST is all about distributed and hypermedia, and that pretty much sums it up. A browser looks at what it gets from a request and shows it to the user. A HTML page usually spawns a lot more GET requests, for example CSS, scripts and images. An image is typically only rendered to the screen, JavaScript is executed, and so on. Each time, the browser does what it does because it found the link in an <img> or <style> tag and the response media type was image/jpeg or text/css.
If Twitter makes a hypermedia based API, it will probably always return an application/tweet every time you follow a link to a tweet, but the client should never assume it, and always check what it gets before acting on it.
"Why write a REST service that returns XML anyway?"
This all boils down to media types. Like HTML, if you see an element that you've no idea what actually means, the HTML spec instructs you to ignore them, and process the "body" of the tag if it has one. Likewise, the atom spec instructs you to ignore unknown elements and foreign markup (from different namespaces) and not process the body (IIRC).
Designing media types for generic problem domains (as in the HTML media type for the rich text problem domain) is very hard. Making media types for very narrow problem domains is probably a lot easier (like a tweet). But it's always a good idea to design for extensibility and specify how clients (and servers) are supposed to react when they see elements or data items that don't match the spec. JPEG, for example has an Application-specific record type (e.g. APP1) which is used to contain all sorts of meta data.
"I've heard the complaint that with SOAP you have the "overhead" of the SOAP Envelope. In this day and age, do we really need to worry about a handful of bytes?"
No, we don't. REST is absolutely not about being efficient over the wire, it's actually trading wire efficiency in. REST's efficiency comes from the possibilities of caching enabled by all the other constraints: Fielding's dissertation notes: The trade-off, though, is that a uniform interface degrades efficiency, since information is transferred in a standardized form rather than one which is specific to an application's needs. The REST interface is designed to be efficient for large-grain hypermedia data transfer, optimizing for the common case of the Web, but resulting in an interface that is not optimal for other forms of architectural interaction. I don't think that the SOAP Envelope byte count overhead is a valid concern.
"I've heard the argument that with REST you can just pop the URL into the browser and see the data."
Yes, that's also an invalid argument. It doesn't work that way. Even if it did work, most narrow REST APIs out there use media types that browsers have no idea about and it still won't work.
But there are a lot more possibilities than a browser to test a HTTP based API, like command line utilities or browser extensions that allow you to control almost any aspect of a HTTP request, inspect response headers and discover links for you to follow. But even so, this is nowhere near as easy as generating WSDL stubs and making a three line program to call the function anyway.
"Why do we need a "readable" URL for each resource? If we were using a tool to implement the service, do we really care about the actual URL?"
If you look at how the web works, I'm pretty sure that humans are by and large glad that the URI for a wikipedia page looks like this, http://en.wikipedia.org/wiki/Stack_overflow instead of http://en.wikipedia.org/wiki/?oldid=376349090. But it actually is not important to REST. The important thing to try to get right is to choose to place relevant data in the URI that is not likely to change. You might think that the database ID will never change, but what happens when two data sets need to be merged? All your primary keys change. The page title (Stack_overflow) will not change.
Sorry for the long response, but I believe this question is valid, and hasn't been addressed before here on SO. I'm sure Darrel Miller will add his answer once he's back too.
Edit: formatting
Martin Fowler has a post on the Richardson Maturity Model which does a great job explaining the difference between SOAP and REST.
WSDL and other document level protocols are redundant. The HTTP protocol supports a much richer set of operations besides just serving documents and submitting forms.
Supporters of REST are uncomfortable with that redundancy.
This is a multi-part question. I just watched a very interesting presentation on YQL by the lead developer (a graduate of my MS program). While it was very compelling, and I am looking forward to trying it out, I am wondering if anyone knows of alternative frameworks for querying multiple web service APIs to make them appear seamless, the apparent purpose of YQL?
Yahoo's strategy has been to create XML schema definitions that bind a given web service's parameters into their YQL Open Table query parameters, which I think is very clever. Is there any tool that attempts (perhaps I am naive here) to automate the discovery of parameters in say a REST API? I am aware that with SOAP APIs, because there is a published WSDL, it makes automation easier, but is there yet no way to do this with REST? Is anyone trying?
Yes people are trying to produce description languages for REST. The most popular effort is WADL. There are lots of questions about WADL here on SO. Is it a good idea? In my opinion no.
REST does not need a discovery model beyond what it already has with hypermedia, because is trying to solve a problem at a different architectural layer than web services. Web services deliver data to an application's business logic/domain model. REST is about delivering content and behaviour to a presentation layer.
How about an analogy? Think of the different between an object and struct in C++. A struct is just simple data that some client process is going to manipulate. That's what a web service does, it returns a chunk of data, a struct. Sure maybe it did a bunch of server side processing to produce the result, but the end result is a lump of data. A REST interface delivers an object. i.e. It contains both data and the methods that can be used to manipulate that object. By definition, if you understand the uniform interface and you understand the returned media type, you already know what you can do with the response. Discovery mechanisms are redundant.
If you find this hard to believe, the think about the web. How does a web browser discover web pages? The web has no formalized discovery mechanism, and yet there is a world of information out there that we can discover with a web browser.
There is this little website http://zachgrav.es/yql/tablesaw/ which indeed auto-discovers parameters in a REST api and turns it into a YQL compatible table.
There are two ways to find information. Either you use a 100% unambiguous language or you use a natural language. Anything in between like YQL is doomed to fail because it delivers neither and works well only with the examples its authors tout.
I blogged about this at http://zscraper.wordpress.com/2012/05/30/enough-with-crawling-2. My personal stance is that you'll always get the most accurate results if you do your homework first, i.e. study the target domain and figure out how to query it unambiguously.
To answer your question and give you an alternative -- try Bobik. This is a cloud-backed scraping service that you control via REST API. Compose your "queries" in traditional syntax (Bobik supports Javascript, JQuery, XPATH and CSS) and call Bobik to run them from any client-side environment (webpages, mobile apps, or your server).
Hope this helps.