Call a Server-side Method on a Resource in a RESTful Way

Why aim for a RESTful design?

The RESTful principles bring the features that make web sites easy (for a random human user to "surf" them) to the web services API design, so they are easy for a programmer to use. REST isn't good because it's REST, it's good because it's good. And it is good mostly because it is simple.

The simplicity of plain HTTP (without SOAP envelopes and single-URI overloaded POST services), what some may call "lack of features", is actually its greatest strength. Right off the bat, HTTP asks you to have addressability and statelessness: the two basic design decisions that keep HTTP scalable up to today's mega-sites (and mega-services).

But REST is not the silver bulltet: Sometimes an RPC-style ("Remote Procedure Call" - such as SOAP) may be appropriate, and sometimes other needs take precedence over the virtues of the Web. This is fine. What we don't really like is needless complexity. Too often a programmer or a company brings in RPC-style Services for a job that plain old HTTP could handle just fine. The effect is that HTTP is reduced to a transport protocol for an enormous XML payload that explains what's "really" going on (not the URI or the HTTP method give a clue about it). The resulting service is far too complex, impossible to debug, and won't work unless your clients have the exact setup as the developer intended.

Same way a Java/C# code can be not object-oriented, just using HTTP does not make a design RESTful. One may be caught up in the rush of thinking about their services in terms of actions and remote methods that should be called. No wonder this will mostly end up in a RPC-Style service (or a REST-RPC-hybrid). The first step is to think differently. A RESTful design can be achieved in many ways, one way is to think of your application in terms of resources, not actions:

💡 Instead of thinking in terms of actions it can perform ("do a search for places on the map")...

...try to think in terms of the results of those actions ("the list of places on the map matching a search criteria").

I'll go for examples below. (Other key aspect of REST is the use of HATEOAS - I don't brush it here, but I talk about it quickly at another post.)

Issues of the first design

Let's take a look a the proposed design:

ACTION http://api.animals.com/v1/dogs/1/

First off, we should not consider creating a new HTTP verb (ACTION). Generally speaking, this is undesirable for several reasons:

• (1) Given only the service URI, how will a "random" programmer know the ACTION verb exists?
• (2) if the programmer knows it exists, how will he know its semantics? What does that verb mean?
• (3) what properties (safety, idempotence) should one expect that verb to have?
• (4) what if the programmer has a very simple client that only handles standard HTTP verbs?
• (5) ...

Now let's consider using POST (I'll discuss why below, just take my word for it now):

POST /v1/dogs/1/ HTTP/1.1
Host: api.animals.com

{"action":"bark"}

This could be OK... but only if:

• {"action":"bark"} was a document; and
• /v1/dogs/1/ was a "document processor" (factory-like) URI. _{A "document processor" is a URI that you'd just "throw things at" and "forget" about them - the processor may redirect you to a newly created resource after the "throwing". E.g. the URI for posting messages at a message broker service, which, after the posting would redirect you to a URI that shows the status of the message's processing.}

I don't know much about your system, but I'd already bet both aren't true:

• {"action":"bark"} is not a document, it actually is the method you are trying to ninja-sneak into the service; and
• the /v1/dogs/1/ URI represents a "dog" resource (probably the dog with id==1) and not a document processor.

So all we know now is that the design above is not so RESTful, but what is that exactly? What is so bad about it? Basically, it is bad because that is complex URI with complex meanings. You can't infer anything from it. How would a programmer know a dog have a bark action that can be secretly infused with a POST into it?

Designing your question's API calls

So let's cut to the chase and try to design those barks RESTfully by thinking in terms of resources. Allow me to quote the Restful Web Services book:

A POST request is an attempt to create a new resource from an existing one. The existing resource may be the parent of the new one in a data-structure sense, the way the root of a tree is the parent of all its leaf nodes. Or the existing resource may be a special "factory" resource whose only purpose is to generate other resources. The representation sent along with a POST request describes the initial state of the new resource. As with PUT, a POST request doesn’t need to include a representation at all.

Following the description above we can see that bark can be modeled as a subresource of a dog (since a bark is contained within a dog, that is, a bark is "barked" by a dog).

From that reasoning we already got:

• The method is POST
• The resource is /barks, subresource of dog: /v1/dogs/1/barks, representing a bark "factory". That URI is unique for each dog (since it is under /v1/dogs/{id}).

Now each case of your list has a specific behavior.

##1. bark just sends an e-mail to dog.email and records nothing.

Firstly, is barking (sending an e-mail) a synchronous or an asynchronous task? Secondly does the bark request require any document (the e-mail, maybe) or is it empty?

1.1 bark sends an e-mail to dog.email and records nothing (as a synchronous task)

This case is simple. A call to the barks factory resource yields a bark (an e-mail sent) right away and the response (if OK or not) is given right away:

POST /v1/dogs/1/barks HTTP/1.1
Host: api.animals.com
Authorization: Basic mAUhhuE08u724bh249a2xaP=

(entity-body is empty - or, if you require a **document**, place it here)

200 OK

As it records (changes) nothing, 200 OK is enough. It shows that everything went as expected.

1.2 bark sends an e-mail to dog.email and records nothing (as an asynchronous task)

In this case, the client must have a way to track the bark task. The bark task then should be a resource with it's own URI.:

POST /v1/dogs/1/barks HTTP/1.1
Host: api.animals.com
Authorization: Basic mAUhhuE08u724bh249a2xaP=

{document body, if needed;
NOTE: when possible, the response SHOULD contain a short hypertext note with a hyperlink
to the newly created resource (bark) URI, the same returned in the Location header
(also notice that, for the 202 status code, the Location header meaning is not
standardized, thus the importance of a hipertext/hyperlink response)}

202 Accepted
Location: http://api.animals.com/v1/dogs/1/barks/a65h44

This way, each bark is traceable. The client can then issue a GET to the bark URI to know it's current state. Maybe even use a DELETE to cancel it.

2. bark sends an e-mail to dog.email and then increments dog.barkCount by 1

This one can be trickier, if you want to let the client know the dog resource gets changed:

POST /v1/dogs/1/barks HTTP/1.1
Host: api.animals.com
Authorization: Basic mAUhhuE08u724bh249a2xaP=

{document body, if needed; when possible, containing a hipertext/hyperlink with the address
in the Location header -- says the standard}

303 See Other
Location: http://api.animals.com/v1/dogs/1

In this case, the location header's intent is to let the client know he should take a look at dog. From the HTTP RFC about 303:

This method exists primarily to allow the output of a POST-activated script to redirect the user agent to a selected resource.

If the task is asynchronous, a bark subresource is needed just like the 1.2 situation and the 303 should be returned at a GET .../barks/Y when the task is complete.

3. bark creates a new "bark" record with bark.timestamp recording when the bark occured. It also increments dog.barkCount by 1.

POST /v1/dogs/1/barks HTTP/1.1
Host: api.animals.com
Authorization: Basic mAUhhuE08u724bh249a2xaP=

(document body, if needed)

201 Created
Location: http://api.animals.com/v1/dogs/1/barks/a65h44

In here, the bark is a created due to the request, so the status 201 Created is applied.

If the creation is asynchronous, a 202 Accepted is required (as the HTTP RFC says) instead.

The timestamp saved is a part of bark resource and can be retrieved with a GET to it. The updated dog can be "documented" in that GET dogs/X/barks/Y as well.

4. bark runs a system command to pull the latest version of the dog code down from Github. It then sends a text message to dog.owner telling them that the new dog code is in production.

The wording of this one is complicated, but it pretty much is a simple asynchronous task:

POST /v1/dogs/1/barks HTTP/1.1
Host: api.animals.com
Authorization: Basic mAUhhuE08u724bh249a2xaP=

(document body, if needed)

202 Accepted
Location: http://api.animals.com/v1/dogs/1/barks/a65h44

The client then would issue GETs to /v1/dogs/1/barks/a65h44 to know the current state (if the code was pulled, it the e-mail was sent to the owner and such). Whenever the dog changes, a 303 is appliable.

Wrapping up

Quoting Roy Fielding:

The only thing REST requires of methods is that they be uniformly defined for all resources (i.e., so that intermediaries don’t have to know the resource type in order to understand the meaning of the request).

In the above examples, POST is uniformly designed. It will make the dog "bark". That is not safe (meaning bark has effects on the resources), nor idempotent (each request yields a new bark), which fits the POST verb well.

A programmer would know: a POST to barks yields a bark. The response status codes (also with entity-body and headers when necessary) do the job of explaining what changed and how the client can and should proceed.

_{Note: The primary sources used were: "Restful Web Services" book, the HTTP RFC and Roy Fielding's blog.}

Edit:

The question and thus the answer have changed quite a bit since they were first created. The original question asked about the design of a URI like:

ACTION http://api.animals.com/v1/dogs/1/?action=bark

Below is the explanation of why it is not a good choice:

How clients tell the server WHAT TO DO with the data is the method information.

• RESTful web services convey method information in the HTTP method.
• Typical RPC-Style and SOAP services keep theirs in the entity-body and HTTP header.

WHICH PART of the data [the client wants the server] to operate on is the scoping information.

• RESTful services use the URI. SOAP/RPC-Style services once again use the entity-body and HTTP headers.

As an example, take Google's URI http://www.google.com/search?q=DOG. There, the method information is GET and the scoping information is /search?q=DOG.

Long story short:

• In RESTful architectures, the method information goes into the HTTP method.
• In Resource-Oriented Architectures, the scoping information goes into the URI.

And the rule of thumb:

If the HTTP method doesn’t match the method information, the service isn’t RESTful. If the scoping information isn’t in the URI, the service isn’t resource-oriented.

You can put the "bark" "action" in the URL (or in the entity-body) and use POST. No problem there, it works, and may be the simplest way to do it, but this isn't RESTful.

To keep your service really RESTful, you may have to take a step back and think about what you really want to do here (what effects will it have on the resources).

I can't talk about your specific business needs, but let me give you an example: Consider a RESTful ordering service where orders are at URIs like example.com/order/123.

Now say we want to cancel an order, how are we gonna do it? One may be tempted to think that is a "cancellation" "action" and design it as POST example.com/order/123?do=cancel.

That is not RESTful, as we talked above. Instead, we might PUT a new representation of the order with a canceled element sent to true:

PUT /order/123 HTTP/1.1
Content-Type: application/xml

<order id="123">
    <customer id="89987">...</customer>
    <canceled>true</canceled>
    ...
</order>

And that's it. If the order can't be canceled, a specific status code can be returned. ^{(A subresource design, like POST /order/123/canceled with the entity-body true may, for simplicity, also be available.)}

In your specific scenario, you may try something similar. That way, while a dog is barking, for example, a GET at /v1/dogs/1/ could include that information ^{(e.g. <barking>true</barking>)}. Or... if that's too complicated, loosen up your RESTful requirement and stick with POST.

Update:

I don't want to make the answer too big, but it takes a while to get the hang of exposing an algorithm (an action) as a set of resources. Instead of thinking in terms of actions ("do a search for places on the map"), one needs to think in terms of the results of that action ("the list of places on the map matching a search criteria").

You may find yourself coming back to this step if you find that your design doesn't fit HTTP's uniform interface.

Query variables are scoping information, but do not denote new resources (/post?lang=en is clearly the same resource as /post?lang=jp, just a different representation). Rather, they are used to convey client state (like ?page=10, so that state is not kept in the server; ?lang=en is also an example here) or input parameters to algorithmic resources (/search?q=dogs, /dogs?code=1). Again, not distinct resources.

HTTP verbs' (methods) properties:

Another clear point that shows ?action=something in the URI is not RESTful, are the properties of HTTP verbs:

• GET and HEAD are safe (and idempotent);
• PUT and DELETE are idempotent only;
• POST is neither.

Safety: A GET or HEAD request is a request to read some data, not a request to change any server state. The client can make a GET or HEAD request 10 times and it's the same as making it once, or never making it at all.

Idempotence: An idempotent operation in one that has the same effect whether you apply it once or more than once (in math, multiplying by zero is idempotent). If you DELETE a resource once, deleting again will have the same effect (the resource is GONE already).

^{_{POST is neither safe nor idempotent. Making two identical POST requests to a 'factory' resource will probably result in two subordinate resources containing the same information. With overloaded (method in URI or entity-body) POST, all bets are off.}}

Both these properties were important to the success of the HTTP protocol (over unreliable networks!): how many times have you updated (GET) the page without waiting until it is fully loaded?

Creating an action and placing it in the URL clearly breaks the HTTP methods' contract. Once again, the technology allows you, you can do it, but that is not RESTful design.

I answered earlier, but this answer contradicts my old answer and follows a much different strategy for coming to a solution. It shows how the HTTP request is built from the concepts that define REST and HTTP. It also uses PATCH instead of POST or PUT.

It goes through the REST constraints, then the components of HTTP, then a possible solution.

REST

REST is a set of constraints intended to be applied to a distributed hypermedia system in order to make it scalable. Even to make sense of it in the context of remotely controlling an action, you have to think of remotely controlling an action as a part of a distributed hypermedia system -- a part of a system for discovering, viewing, and modifying interconnected information. If that's more trouble than it's worth, then it's probably no good to try to make it RESTful. If you just want a "control panel" type GUI on the client that can trigger actions on the server via port 80, then you probably want a simple RPC interface like JSON-RPC via HTTP requests/responses or a WebSocket.

But REST is a fascinating way of thinking and the example in the question happens to be easy to model with a RESTful interface, so let's take on the challenge for fun and for education.

REST is defined by four interface constraints:

identification of resources; manipulation of resources through representations; self-descriptive messages; and, hypermedia as the engine of application state.

You ask how you can define an interface, meeting these constraints, via which one computer tells another computer to make a dog bark. Specifically, you want your interface to be HTTP, and you don't want to discard the features that make HTTP RESTful when used as intended.

Let's begin with the first constraint: resource identification.

Any information that can be named can be a resource: a document or image, a temporal service (e.g. "today's weather in Los Angeles"), a collection of other resources, a non-virtual object (e.g. a person), and so on.

So a dog is a resource. It needs to be identified.

More precisely, a resource R is a temporally varying membership function M_R(t), which for time t maps to a set of entities, or values, which are equivalent. The values in the set may be resource representations and/or resource identifiers.

You model a dog by taking a set of identifiers and representations and saying they are all associated with each other at a given time. For now, let's use the identifier "dog #1". That brings us to the second and third constraints: resource representation and self-description.

REST components perform actions on a resource by using a representation to capture the current or intended state of that resource and transferring that representation between components. A representation is a sequence of bytes, plus representation metadata to describe those bytes.

Following is a sequence of bytes capturing the intended state of the dog, i.e. the representation we wish to be associated with the identifier "dog #1" (note that it only represents part of the state as it does not regard the dog's name, health, or even past barks):

It has been barking every 10 minutes since the time this state change was effected, and will continue indefinitely.

It is supposed to be attached to metadata that describes it. This metadata might be useful:

It is an English statement. It describes part of the intended state. If it is received multiple times, only allow the first to have an effect.

Finally, let's look at the fourth constraint: HATEOAS.

REST ... views an application as a cohesive structure of information and control alternatives through which a user can perform a desired task. For example, looking-up a word in an on-line dictionary is one application, as is touring through a virtual museum, or reviewing a set of class notes to study for an exam. ... The next control state of an application resides in the representation of the first requested resource, so obtaining that first representation is a priority. ... The model application is therefore an engine that moves from one state to the next by examining and choosing from among the alternative state transitions in the current set of representations.

In a RESTful interface, the client receives a resource representation in order to figure out how it should receive or send a representation. There must be a representation somewhere in the application from which the client can figure out how to receive or send all representations it should be able to receive or send, even if it follows a chain of representations to arrive at that information. This seems simple enough:

The client asks for a representation of a resource identified as the homepage; in response, it gets a representation that contains an identifier of every dog the client might want. The client extracts an identifier from it and asks the service how it can interact with the identified dog, and the service says the client can send an English statement describing part of the intended state of the dog. Then the client sends such a statement and receives a success message or an error message.

HTTP

HTTP implements REST constraints as follows:

resource identification: URI

resource representation: entity-body

self-description: method or status code, headers, and possibly parts of the entity-body (e.g. the URI of an XML schema)

HATEOAS: hyperlinks

You've decided on http://api.animals.com/v1/dogs/1 as the URI. Let's assume the client got this from some page on the site.

Let's use this entity-body (the value of next is a timestamp; a value of 0 means 'when this request is received'):

{"barks": {"next": 0, "frequency": 10}}

Now we need a method. PATCH fits the "part of the intended state" description we decided on:

The PATCH method requests that a set of changes described in the request entity be applied to the resource identified by the Request-URI.

And some headers:

To indicate the language of the entity-body: Content-Type: application/json

To make sure it only happens once: If-Unmodified-Since: <date/time this was first sent>

And we have a request:

PATCH /v1/dogs/1/ HTTP/1.1
Host: api.animals.com
Content-Type: application/json
If-Unmodified-Since: <date/time this was first sent>
[other headers]

{"barks": {"next": 0, "frequency": 10}}

On success, the client should receive a 204 status code in response, or a 205 if the representation of /v1/dogs/1/ has changed to reflect the new barking schedule.

On failure, it should receive a 403 and a helpful message why.

It is not essential to REST for the service to reflect the bark schedule in a representation in response to GET /v1/dogs/1/, but it would make the most sense if a JSON representation included this:

"barks": {
    "previous": [x_1, x_2, ..., x_n],
    "next": x_n,
    "frequency": 10
}

Treat the cron job as an implementation detail that the server hides from the interface. That's the beauty of a generic interface. The client doesn't have to know what the server does behind the scenes; all it cares about is that the service understands and responds to requested state changes.

Most people use POST for this purpose. It is appropriate for performing "any unsafe or nonidempotent operation when no other HTTP method seems appropriate".

APIs such as XMLRPC use POST to trigger actions that can run arbitrary code. The "action" is included in the POST data:

POST /RPC2 HTTP/1.0
User-Agent: Frontier/5.1.2 (WinNT)
Host: betty.userland.com
Content-Type: text/xml
Content-length: 181

<?xml version="1.0"?>
<methodCall>
   <methodName>examples.getStateName</methodName>
   <params>
      <param>
         <value><i4>41</i4></value>
         </param>
      </params>
   </methodCall>

The RPC is example is given to show that POST is the conventional choice of HTTP verbs for server-side methods. Here's Roy Fielding thoughts on POST -- he pretty much says it is RESTful to use the HTTP methods as specified.

Note that RPC itself isn't very RESTful because it isn't resource oriented. But if you need statelessness, caching, or layering, it's not hard to make the appropriate transformations. See http://blog.perfectapi.com/2012/opinionated-rpc-apis-vs-restful-apis/ for an example.

POST is the HTTP method designed for

Providing a block of data...to a data-handling process

Server-side methods handling non-CRUD-mapped actions is what Roy Fielding intended with REST, so you're good there, and that's why POST was made to be non-idempotent. POST will handle most posting of data to server-side methods to process the information.

That said, in your dog-barking scenario, if you want a server-side bark to be performed every 10 minutes, but for some reason need the trigger to originate from a client, PUT would serve the purpose better, because of its idempotence. Well, strictly by this scenario there's no apparent risk of multiple POST requests causing your dog to meow instead, but anyway that's the purpose of the two similar methods. My answer to a similar SO question may be useful for you.

If we assume Barking is an inner / dependent / sub resource which the consumer can act on, then we could say:

POST http://api.animals.com/v1/dogs/1/bark

dog number 1 barks

GET http://api.animals.com/v1/dogs/1/bark

returns the last bark timestamp

DELETE http://api.animals.com/v1/dogs/1/bark

doesn't apply! so ignore it.