Understanding REST: Verbs, error codes, and authentication

Question

I am (as a follow-up to this question) looking for a way to wrap APIs around default functions in my PHP-based web applications, databases and CMSs.

I have looked around and found several "skeleton" frameworks. In addition to the answers in my question, there is Tonic, a REST framework I like because it is very lightweight.

I like REST the best for its simplicity, and would like to create an API architecture based on it. I'm trying to get my head around the basic principles and have not fully understood it yet. Therefore, a number of questions.

1. Am I understanding it right?

Say I have a resource "users". I could set up a number of URIs like so:

/api/users     when called with GET, lists users
/api/users     when called with POST, creates user record
/api/users/1   when called with GET, shows user record
               when called with PUT, updates user record
               when called with DELETE, deletes user record

is this a correct representation of a RESTful architecture so far?

2. I need more verbs

Create, Update and Delete may be enough in theory, but in practice I will have the need for a lot more verbs. I realize these are things that could be embedded in an update request, but they are specific actions that can have specific return codes and I wouldn't want to throw them all into one action.

Some that come to mind in the user example are:

activate_login
deactivate_login
change_password
add_credit

how would I express actions such as those in a RESTful URL architecture?

My instinct would be to do a GET call to a URL like

/api/users/1/activate_login 

and expect a status code back.

That deviates from the idea of using HTTP verbs, though. What do you think?

3. How to return error messages and codes

A great part of REST's beauty stems from its use of standard HTTP methods. On an error, I emit a header with a 3xx,4xx or 5xx error status code. For a detailed error description, I can use the body (right?). So far so good. But what would be the way to transmit a proprietary error code that is more detailed in describing what went wrong (e.g. "failed to connect to database", or "database login wrong")? If I put it into the body along with the message, I have to parse it out afterwards. Is there a standard header for this kind of thing?

4. How to do authentication

• What would a API key based authentication following REST principles look like?
• Are there strong points against using sessions when authenticating a REST client, other than that it's a blatant violation of the REST principle? :) (only half kidding here, session based authentication would play well with my existing infrastructure.)

EDIT: Thanks for all your great responses guys. I will be going through them tomorrow or the day after.

Answer 1

I would suggest (as a first pass) that PUT should only be used for updating existing entities. POST should be used for creating new ones. i.e.

/api/users     when called with PUT, creates user record

doesn't feel right to me. The rest of your first section (re. verb usage) looks logical, however.

Answer 2

1. Use post when you don't know how the new resource URI would look like (you create new user, application would assign the new user it's id), PUT for updating or creating resources that you know how are they going to be represented (example: PUT /myfiles/thisismynewfile.txt)
2. return the error description in message body
3. You can use HTTP authentication (if it's enough) Web services should be stateles

Answer 3

For the examples you stated I'd use the following:

activate_login

POST /users/1/activation

deactivate_login

DELETE /users/1/activation

change_password

PUT /passwords (this assumes the user is authenticated)

add_credit

POST /credits (this assumes the user is authenticated)

For errors you'd return the error in the body in the format that you got the request in, so if you receive:

DELETE /users/1.xml

You'd send the response back in XML, the same would be true for JSON etc...

For authentication you should use http authentication.

Answer 4

Verbose, but copied from the HTTP 1.1 method specification at http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html

9.3 GET

The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not the source text of the process, unless that text happens to be the output of the process.

The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field. A conditional GET method requests that the entity be transferred only under the circumstances described by the conditional header field(s). The conditional GET method is intended to reduce unnecessary network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already held by the client.

The semantics of the GET method change to a "partial GET" if the request message includes a Range header field. A partial GET requests that only part of the entity be transferred, as described in section 14.35. The partial GET method is intended to reduce unnecessary network usage by allowing partially-retrieved entities to be completed without transferring data already held by the client.

The response to a GET request is cacheable if and only if it meets the requirements for HTTP caching described in section 13.

See section 15.1.3 for security considerations when used for forms.

9.5 POST

The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions:

  - Annotation of existing resources;
  - Posting a message to a bulletin board, newsgroup, mailing list,
    or similar group of articles;
  - Providing a block of data, such as the result of submitting a
    form, to a data-handling process;
  - Extending a database through an append operation.

The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI. The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database.

The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status, depending on whether or not the response includes an entity that describes the result.

If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain an entity which describes the status of the request and refers to the new resource, and a Location header (see section 14.30).

Responses to this method are not cacheable, unless the response includes appropriate Cache-Control or Expires header fields. However, the 303 (See Other) response can be used to direct the user agent to retrieve a cacheable resource.

POST requests MUST obey the message transmission requirements set out in section 8.2.

See section 15.1.3 for security considerations.

9.6 PUT

The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server. If the Request-URI does not point to an existing resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create the resource with that URI. If a new resource is created, the origin server MUST inform the user agent via the 201 (Created) response. If an existing resource is modified, either the 200 (OK) or 204 (No Content) response codes SHOULD be sent to indicate successful completion of the request. If the resource could not be created or modified with the Request-URI, an appropriate error response SHOULD be given that reflects the nature of the problem. The recipient of the entity MUST NOT ignore any Content-* (e.g. Content-Range) headers that it does not understand or implement and MUST return a 501 (Not Implemented) response in such cases.

If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries SHOULD be treated as stale. Responses to this method are not cacheable.

The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The URI in a POST request identifies the resource that will handle the enclosed entity. That resource might be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the URI in a PUT request identifies the entity enclosed with the request -- the user agent knows what URI is intended and the server MUST NOT attempt to apply the request to some other resource. If the server desires that the request be applied to a different URI,

it MUST send a 301 (Moved Permanently) response; the user agent MAY then make its own decision regarding whether or not to redirect the request.

A single resource MAY be identified by many different URIs. For example, an article might have a URI for identifying "the current version" which is separate from the URI identifying each particular version. In this case, a PUT request on a general URI might result in several other URIs being defined by the origin server.

HTTP/1.1 does not define how a PUT method affects the state of an origin server.

PUT requests MUST obey the message transmission requirements set out in section 8.2.

Unless otherwise specified for a particular entity-header, the entity-headers in the PUT request SHOULD be applied to the resource created or modified by the PUT.

9.7 DELETE

The DELETE method requests that the origin server delete the resource identified by the Request-URI. This method MAY be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation has been carried out, even if the status code returned from the origin server indicates that the action has been completed successfully. However, the server SHOULD NOT indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible location.

A successful response SHOULD be 200 (OK) if the response includes an entity describing the status, 202 (Accepted) if the action has not yet been enacted, or 204 (No Content) if the action has been enacted but the response does not include an entity.

If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries SHOULD be treated as stale. Responses to this method are not cacheable.

Answer 5

re 1: This looks fine so far. Remember to return the URI of the newly created user in a "Location:" header as part of the response to POST, along with a "201 Created" status code.

re 2: Activation via GET is a bad idea, and including the verb in the URI is a design smell. You might want to consider returning a form on a GET. In a Web app, this would be an HTMl form with a submit button; in the API use case, you might want to return a representation that contains a URI to PUT to to activate the account. Of course you can include this URI in the response on POST to /users, too. Using PUT will ensure your request is idempotent, i.e. it can safely be sent again if the client isn't sure about success. In general, think about what resources you can turn your verbs into (sort of "nounification of verbs"). Ask yourself what method your specific action is most closely aligned with. E.g. change_password -> PUT; deactivate -> probably DELETE; add_credit -> possibly POST or PUT. Point the client to the appropriate URIs by including them in your representations.

re 3. Don't invent new status codes, unless you believe they're so generic they merit being standardized globally. Try hard to use the most appropriate status code available (read about all of them in RFC 2616). Include additional information in the response body. If you really, really are sure you want to invent a new status code, think again; if you still believe so, make sure to at least pick the right category (1xx -> OK, 2xx -> informational, 3xx -> redirection; 4xx-> client error, 5xx -> server error). Did I mention inventing new status codes is a bad idea?

re 4. If in any way possible, use the authentication framework built into HTTP. Check out the way Google does authentication in GData; in general, don't put API keys in your URIs. Try to avoid sessions to enhance scalability and support caching - if the response to a request differs because of something that has happened before, you've usually tied yourself to a specific server process instance. It's much better to turn session state into either client state (e.g. make it part of subsequent requests) or make it explicit by turning it into (server) resource state, i.e. give it its own URI.

Answer 6

1. You've got the right idea about how to design your resources, IMHO. I wouldn't change a thing.

2. Rather than trying to extend HTTP with more verbs, consider what your proposed verbs can be reduced to in terms of the basic HTTP methods and resources. For example, instead of an activate_login verb, you could set up resources like: /api/users/1/login/active which is a simple boolean. To activate a login, just PUT a document there that says 'true' or 1 or whatever. To deactivate, PUT a document there that is empty or says 0 or false.

Similarly, to change or set passwords, just do PUTs to /api/users/1/password.

Whenever you need to add something (like a credit) think in terms of POSTs. For example, you could do a POST to a resource like /api/users/1/credits with a body containing the number of credits to add. A PUT on the same resource could be used to overwrite the value rather than add. A POST with a negative number in the body would subtract, and so on.

3. I'd strongly advise against extending the basic HTTP status codes. If you can't find one that matches your situation exactly, pick the closest one and put the error details in the response body. Also, remember that HTTP headers are extensible; your application can define all the custom headers that you like. One application that I worked on, for example, could return a 404 Not Found under multiple circumstances. Rather than making the client parse the response body for the reason, we just added a new header, X-Status-Extended, which contained our proprietary status code extensions. So you might see a response like:

HTTP/1.1 404 Not Found    
X-Status-Extended: 404.3 More Specific Error Here

That way a HTTP client like a web browser will still know what to do with the regular 404 code, and a more sophisticated HTTP client can choose to look at the X-Status-Extended header for more specific information.

4. For authentication, I recommend using HTTP authentication if you can. But IMHO there's nothing wrong with using cookie-based authentication if that's easier for you.

Answer 7

I noticed this question a couple of days late, but I feel that I can add some insight. I hope this can be helpful towards your RESTful venture.

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Point 1: Am I understanding it right?

You understood right. That is a correct representation of a RESTful architecture. You may find the following matrix from Wikipedia very helpful in defining your nouns and verbs:

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When dealing with a Collection URI like: http://example.com/resources/

• GET: List the members of the collection, complete with their member URIs for further navigation. For example, list all the cars for sale.

• PUT: Meaning defined as "replace the entire collection with another collection".

• POST: Create a new entry in the collection where the ID is assigned automatically by the collection. The ID created is usually included as part of the data returned by this operation.

• DELETE: Meaning defined as "delete the entire collection".

---

When dealing with a Member URI like: http://example.com/resources/7HOU57Y

• GET: Retrieve a representation of the addressed member of the collection expressed in an appropriate MIME type.

• PUT: Update the addressed member of the collection or create it with the specified ID.

• POST: Treats the addressed member as a collection in its own right and creates a new subordinate of it.

• DELETE: Delete the addressed member of the collection.

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Point 2: I need more verbs

In general, when you think you need more verbs, it may actually mean that your resources need to be re-identified. Remember that in REST you are always acting on a resource, or on a collection of resources. What you choose as the resource is quite important for your API definition.

Activate/Deactivate Login: If you are creating a new session, then you may want to consider "the session" as the resource. To create a new session, use POST to http://example.com/sessions/ with the credentials in the body. To expire it use PUT or a DELETE (maybe depending on whether you intend to keep a session history) to http://example.com/sessions/SESSION_ID.

Change Password: This time the resource is "the user". You would need a PUT to http://example.com/users/USER_ID with the old and new passwords in the body. You are acting on "the user" resource, and a change password is simply an update request. It's quite similar to the UPDATE statement in a relational database.

My instinct would be to do a GET call to a URL like /api/users/1/activate_login

This goes against a very core REST principle: The correct usage of HTTP verbs. Any GET request should never leave any side effect.

For example, a GET request should never create a session on the database, return a cookie with a new Session ID, or leave any residue on the server. The GET verb is like the SELECT statement in a database engine. Remember that the response to any request with the GET verb should be cache-able when requested with the same parameters, just like when you request a static web page.

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Point 3: How to return error messages and codes

Consider the 3xx, 4xx or 5xx HTTP error status codes as error categories. You can elaborate the error in the body.

Incorrect Database Login: In general you should use a 500 error for this. This is not a temporary error, so it indicates to the client that there is no need to retry until something is fixed from the server-side. Specify the details in the body, so that the client will be able to provide some information to us humans.

Failed to Connect to Database: This may be anything between 500 to 504, depending on what error you receive from your database. For example if the database is down for maintenance, you may use the 503 error, and specify the verbose reason in the body.

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Point 4: How to do authentication

As pointed out in point 1, instead of authenticating a user, you may want to think about creating a session. You will be returned a new "Session ID", along with the appropriate HTTP status code (200: Access Granted or 403: Access Denied).

You will then be asking your RESTful server: "Can you GET me the resource for this Session ID?".

There is no authenticated mode - REST is stateless: You create a session, you ask the server to give you resources using this Session ID as a parameter, and on logout you drop or expire the session.

Answer 8

Simply put, you are doing this completely backward.

You should not be approaching this from what URLs you should be using. The URLs will effectively come "for free" once you've decided upon what resources are necessary for your system AND how you will represent those resources, and the interactions between the resources and application state.

To quote Roy Fielding

A REST API should spend almost all of its descriptive effort in defining the media type(s) used for representing resources and driving application state, or in defining extended relation names and/or hypertext-enabled mark-up for existing standard media types. Any effort spent describing what methods to use on what URIs of interest should be entirely defined within the scope of the processing rules for a media type (and, in most cases, already defined by existing media types). [Failure here implies that out-of-band information is driving interaction instead of hypertext.]

Folks always start with the URIs and think this is the solution, and then they tend to miss a key concept in REST architecture, notably, as quoted above, "Failure here implies that out-of-band information is driving interaction instead of hypertext."

To be honest, many see a bunch of URIs and some GETs and PUTs and POSTs and think REST is easy. REST is not easy. RPC over HTTP is easy, moving blobs of data back and forth proxied through HTTP payloads is easy. REST, however, goes beyond that. REST is protocol agnostic. HTTP is just very popular and apt for REST systems.

REST lives in the media types, their definitions, and how the application drives the actions available to those resources via hypertext (links, effectively).

There are different view about media types in REST systems. Some favor application specific payloads, while others like uplifting existing media types in to roles that are appropriate for the application. For example, on the one hand you have specific XML schemas designed suited to your application versus using something like XHTML as your representation, perhaps through microformats and other mechanisms.

Both tacts have their place, I think, the XHTML working very well in scenarios that overlap both the human driven and machine driven web, whereas the former, more specific data types I feel better facilitate machine to machine interactions. I find the uplifting of commodity formats can make content negotiation potentially difficult. "application/xml+yourresource" is much more specific as a media type than "application/xhtml+xml", as the latter can apply to many payloads which may or may not be something a machine client is actually interested in, nor can it determine without introspection.

However, XHTML works very well (obviously) in the human web where web browsers and rendering is very important.

You application will guide you in those kinds of decisions.

Part of the process of designing a REST system is discovering the first class resources in your system, along with the derivative, support resources necessary to support the operations on the primary resources. Once the resources are discovered, then the representation of those resources, as well as the state diagrams showing resource flow via hypertext within the representations because the next challenge.

Recall that each representation of a resource, in a hypertext system, combines both the actual resource representation along with the state transitions available to the resource. Consider each resource a node in a graph, with the links being the lines leaving that node to other states. These links inform clients not only what can be done, but what is required for them to be done (as a good link combines the URI and the media type required).

For example, you may have:

<link href="http://example.com/users" rel="users" type="application/xml+usercollection"/>
<link href="http://example.com/users?search" rel="search" type="application/xml+usersearchcriteria"/>

Your documentation will talk about the rel field named "users", and the media type of "application/xml+youruser".

These links may seem redundant, they're all talking to the same URI, pretty much. But they're not.

This is because for the "users" relation, that link is talking about the collection of users, and you can use the uniform interface to work with the collection (GET to retrieve all of them, DELETE to delete all of them, etc.)

If you POST to this URL, you will need to pass a "application/xml+usercollection" document, which will probably only contain a single user instance within the document so you can add the user, or not, perhaps, to add several at once. Perhaps your documentation will suggest that you can simply pass a single user type, instead of the collection.

You can see what the application requires in order to perform a search, as defined by the "search" link and it's mediatype. The documentation for the search media type will tell you how this behaves, and what to expect as results.

The takeaway here, though, is the URIs themselves are basically unimportant. The application is in control of the URIs, not the clients. Beyond a few 'entry points', your clients should rely on the URIs provided by the application for its work.

The client needs to know how to manipulate and interpret the media types, but doesn't much need to care where it goes.

These two links are semantically identical in a clients eyes:

<link href="http://example.com/users?search" rel="search" type="application/xml+usersearchcriteria"/>
<link href="http://example.com/AW163FH87SGV" rel="search" type="application/xml+usersearchcriteria"/>

So, focus on your resources. Focus on their state transitions in the application and how that's best achieved.