Describing RESTful applications

The other day I came across this interesting article written by Subbu Allamaraju, where he discusses some of the aspects that drive the design of a pure and self-describing RESTful interface for a service (Or service contract in other words).

He points out three main pieces that every service contract should have:

1. An uniform interface. An uniform interface is probably one the aspects that we best know about REST, or we may heard often when a REST service is described. This refers to the fact that each service exposes the same interface, which is a good thing for eliminating ad hoc messages and focusing primarily on an standard API that defines pieces of information that can be retrieved and manipulated. Instead of adding special function calls or interfaces to the architecture, new services add new pieces of information can be manipulated using standard requests. For instance, in Http RESTful services, this point involves using standard http verbs, headers and status codes.

2. Resource representations based on media types. This is quite interesting, what he proposes is to use a more specialized "content-type" for any of the xml schemas that a service may return. He confirms that generic media types such as "application/xml" are not enough to distinguish resource representations. We might want to know if the resource represents an account or a customer without making any assumptions about the resource's URI. In those cases, content types like "application/vnd.bank.org.accounts+xml" or "application/vnd.bank.org.customers+xml" would be more helpful and still being valid according to the spec  "rfc3023, XML media types".

200 OK
Content-Type: application/vnd.bank.org.account+xml

<accounts xmlns="urn:org:bank:accounts">
    <account>
        <id>AZA12093</id>       
        <balance currency="USD">993.95</balance>
    </account>
</accounts>

3. Contextual links to resources. In addition to the resource representation, the client should also receive links representing what it can do next (Taking the form of a workflow). As Subbu concludes, this aspect decouples the client from the actual URIs of resources, and the client does not need to know the rest of the URIs until run-time.

200 OK
Content-Type: application/vnd.bank.org.account+xml;charset=UTF-8

<accounts xmlns="urn:org:bank:accounts">
    <account>
        <id>AZA12093</id>
        <link href="http://bank.org/account/AZA12093" rel="self"/>
        <link rel="
http://bank.org/rel/transfer edit"
              type="application/vnd.bank.org.transfer+xml"
              href="
http://bank.org/transfers"/>
        <link rel="
http://bank.org/rel/customer"
              type="application/vnd.bank.org.customer+xml"
              href="
http://bank.org/customer/7t676323a"/>
        <balance currency="USD">993.95</balance>
    </account>
    .....

As you can see in the example above, the account representation contains links to itself (Self link) and other resources. The operations that can be performed over those resources are expressed by the "rel" attribute.

Altough there is not built-in support in WCF to enforce these aspects automatically, it can be easily done in the service implementation using specific data contracts and the WebOperationContext. 

Setting up the content type before returning the service response:

public Account GetAccount(string id)

{

  WebOperationContext.Current.OutgoingResponse.ContentType = "application/vnd.bank.org.account+xml";

  return repository.Accounts.Where(a => a.Id == id).FirstOrDefault();

}

A link can be modeled as a regular data contract:

[DataContract(Name="link")]

public class Link

{

   [DataMember(Name="href")]

   public string Href { get; set; }

 

   [DataMember(Name = "rel")]

   public string Rel { get; set; }

 

   [DataMember(Name = "type")]

   public string Type { get; set; }

}

Read the article for more information.

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