Designing a RESTful Web Application | Hacker News Reader

Designing a RESTful Web Application

quandyfactory.com

68 pointsRyanMcGreal21 comments

"This collision between the inclination to regard PUT as creating and POST as updating is a significant source of confusion about how to well-design a RESTful system, and deserves more attention."

I like this text for explicitly bringing this problem up.

Roy would probably replace occurrences of the word RESTful with RPC in the article:

http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hyperte...

A nit: "XML is a framework". format != framework

"For example, if you want to add the ability to create an article, it might be tempting to create a URI called /create_article/. This is wrong, because it conflates the object (the resource) and the action (creation)."

So what URL do I use if I want to search articles? What if I want to approve articles? The simple CRUD perspective has fallen apart for me pretty quickly in practice- I need more than 6 verbs (adding HTTP OPTIONS and HEAD in there). I think HATEOAS is a pretty good approach, but there still seems to be a lot of noun-ification going on in RESTful apps just to stick with the style. (e.g, PUT //article/1234/approval used when disapproving an article)

I've always thought that the URL to use when searching articles is the same you would use when retrieving a collection of articles, just with the search terms or filters included as parameters, ie, /articles?q=terms

Approving articles could be as simple as posting TRUE to the "approved" attribute of an article, ie, /articles/1234/approved

You just need to make sure that every resource that can be modified or retrieved has a URL.

Or treat search as a first-class resource.

I wonder if it makes more sense to treat search as a first class method, i.e. issue an HTTP SEARCH request against a resource.

That could be done; WebDAV introduced several new methods to HTTP. The drawback is that your clients lose the ability to use standard HTTP libraries to talk to your service because you're using a non-standard set of methods. The 'Uniform' in URI and URL doesn't just apply to the resource identifiers; in a RESTful system the methods and representations both need to be uniform as well. That allows general-purpose clients to be able to interact with the service, and that was what made the web take off when earlier FTP and Gopher systems did not.

To search use the search resource. Then you have, resource: "search results containing the word 'tuesday'"; url: /search?q=tuesday

To approve you create a new approval resource: POST to /approval with the id of the resource to approve. This creates a new resource at /approval/1

Then you can revoke the approval by DELETE /approval/1

A search resource generally isn't going to be a good idea in a RESTful design. What does it search? Your service will typically contain more than just one set of resources, so you'd end up either creating a separate search resource for each set or passing set identifiers of some sort as a query parameter to the search resource.

The typical model is to have 'container' resources. For example, if /articles/9001 is a resource, /articles is the container. You can POST to the container to create a new resource, and you can GET the container to find out some information about it, such as the number of resources it contains and a list of them. For this part of the design I'll typically return a list of links to the first N resources in the container and a navigation link to the next N resources. That navigation link uses the same URL for the container resource, but with a query parameter that tells it which resource to start on. I'll typically also implement search/filter query parameters to allow further control over the list of resources to include in the container's representation.

> search articles?

GET /search?q=terms

> What if I want to approve articles?

POST /articles/articleID?approved=1

It's actually got potential to be very clean.

It's fascinating how we're just starting to use HTTP as it was designed, nearly twenty years later... :)

Regarding response data formats, xhtml has the advantage of being both html (a format browsers know how to handle) and xml (a format that is well structured and parsable.)

Semantically, xhtml can represent all of the same data structures/types as JSON: lists are UL elements, dictionaries are OL elements, numbers, strings, and booleans are really just strings in both, objects are really just dictionaries, and nulls in xhtml can be a special string as in JSON or an empty element.

In both cases, the client has to either already know about the data structure and how to navigate it, or can treat it generically. XHTML has an advantage here; every data item can be labeled with an id or class attribute to make it easier to find. You can't do that in JSON unless everything is a dictionary value.

For methods, browsers support GET and POST, and they can simulate PUT with upload controls. For DELETE, the common approach is to use POST with _method=DELETE as a parameter. That's easy to adjust in your service as the request comes in so most of your service doesn't have to know about it.

Browsers are also pretty good about sending other headers, particularly the ones that have to do with caching. That'll force you to deal with them, which is a good thing.

Ideally, you'll want to just generate a data structure internally, and use the Accept request header to determine whether to send a text/json or text/html response. That way clients that want JSON can ask for and get it, while generic clients (eg: browsers) will get something they can handle. You'll also be generating the xhtml programatically, which will help to keep it uniform and predictably structured.

  >xhtml has the advantage of being both html
  >(a format browsers know how to handle) and
  >xml (a format that is well structured and parsable.)

And where is the advantage in that over JSON? It's just a Javascript object and browsers know very well how to handle it. That's the point.

  >Semantically, xhtml can represent all of
  >the same data structures/types as JSON

XHTML has HTML semantics, i.e. it's intended to be used to mark the structure of the text. It has no advantage for a general purpose data exchange.

  >dictionaries are OL elements

Nope. Or did you think about DL/DT/DD? Still nope.

  >numbers, strings, and booleans are really just strings in both

Nope. In JSON only strings are strings. Numbers, booleans and null are Javascript numbers, booleans and null.

  >objects are really just dictionaries

Objects are objects. Entire JSON response is an object, you can assign it to variable and use straight away.

  >and nulls in xhtml can be a special string as in JSON or an empty element.

As I said, nulls in JSON are Javascript nulls, not "some special string".

  >In both cases, the client has to either already know about
  >the data structure and how to navigate it, or can treat it
  >generically. XHTML has an advantage here; every data item
  >can be labeled with an id or class attribute to make it
  >easier to find. You can't do that in JSON unless everything
  >is a dictionary value.

Uhm. If we are talking about data in key-value then JSON has a huge advantage there. Not only you can do it in JSON, but you do exactly that and with very little overhead—that's exactly what makes JSON so attractive and popular.

var article = {"title":"Adventures in JSON land", "authors":[{"name":"Jane Doe"}], "published":false, "tags":["JSON", "Javascript", "data format"]}.

That's it. Want to access your data? It's right there:

  article.title -> "Adventures in JSON land"
  article.authors[0].name -> "Jane Doe"
  article.tags.length -> 3

Now do the same in XHTML and you will have much better understanding what advantages of JSON are.

The advantage of XHTML over JSON is that you don't need a specialized client to work with it. You can use the browser, which is a general-purpose client, and that will benefit you hugely during development and testing. There's a whole world of tools for testing web pages, both browser-based and library-based, and all of that can be used with your service if you use xhtml representations.

  >Nope. In JSON only strings are strings. Numbers, booleans and null are Javascript numbers, booleans and null.

I assume you've looked at a JSON message. On the wire, it's all strings. Null is represented by the four characters n, u, l, and l. It's the message parser that turns that into a real null, and an XML parser can do that too.

I'm not saying that JSON doesn't have an advantage when your client language is javascript, but even in javascript the best-practice is to use a real parser for JSON rather than just eval. If you've got to use a real parser anyway, you can also use the xml parser that's built into the browser. Combined with a library like jQuery which gives you xpath-like retrieval it's pretty easy:

    <div id="article">
        <dl>
            <dt>Title: </dt><dd id="title">Adventures in JSON land</dd>
            <dt>Authors: </dt><dd id="authors">
                <ul>
                    <li class="name">Jane Doe</li>
                </ul>
            </dd>
            <dt>Published:</dt><dd id="published">false</dd>
            <dt>Tags:</dt><dd id="tags">
                <ul>
                    <li class="tag">JSON</li>
                    <li class="tag">Javascript</li>
                    <li class="tag">data format</li>
                </ul>
            </dd>
       </dl>
    </div>

Retrieve that into msg, and then:

    $("#title", msg) -> "Adventures in JSON land"
    $("#authors:first .name", msg) -> "Jane Doe"
    $("#tags li", msg).length -> 3

To be fair, an extra step is needed for turning true, false, and null into real boolean and null values. That's a solvable problem, and so is turning the xhtml into a javascript object just like the JSON parser does if that's what you want.

Also, notice that I recommended the service be able to produce BOTH json and xhtml. Special-purpose clients can use the json representation, and general-purpose clients can use the xhtml representation. That gives you the best of both worlds, and the code that produces either/or from an internal data structure is easy and small so it's not likely to introduce inconsistencies between the two representations.

I'm thinking of forwarding this link to my boss, so that he can get a quick tour of REST :) Thanks!

another nit: GET requests are idempotent, but that's a necessary, not sufficient, condition. You should just say they have no side effects.

And another: PUT is conceptually closer to 'replace' than 'update'. To me, 'update' implies a delta, as in SQL.

Thanks to danielharan, jsharpe and _sh for helpfully picking nits with the entry. I've updated it to reflect your observations.

The author seems to get confused himself about PUT vs. POST. At one point he says POST is used to create resources, and at another point he says PUT matches up roughly with SQL's INSERT (ignoring idempotence).

> It's tempting to assume that the HTTP verbs line up nicely with the SQL CRUD verbs: [...] They're superficially similar but they're not identical, and treating them as such leads to this kind of gotcha.

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