Common REST Design Pattern

Based on the same architectural pattern of the web, "REST" has a growing dominance of the SOA (Service Oriented Architecture) implementation these days. In this article, we will discuss some basic design principles of REST.

SOAP : The Remote Procedure Call Model

Before the REST become a dominance, most of SOA architecture are built around WS* stack, which is fundamentally a RPC (Remote Procedure Call) model.

Under this model, "Service" is structured as some "Procedure" exposed by the system. For example, WSDL is used to define the procedure call syntax (such as the procedure name, the parameter and their structure). SOAP is used to define how to encode the procedure call into an XML string. And there are other WS* standards define higher level protocols such as how to pass security credentials around, how to do transactional procedure call, how to discover the service location ... etc.

Unfortunately, the WS* stack are getting so complicated that it takes a steep learning curve before it can be used. On the other hand, it is not achieving its original goal of inter-operability (probably deal to different interpretation of what the spec says). In the last 2 years, WS* technology development has been slowed down and the momentum has been shifted to another model; REST.

REST: The Resource Oriented Model

REST (REpresentation State Transfer) is introduced by Roy Fielding when he captured the basic architectural pattern that make the web so successful. Observing how the web pages are organized and how they are linked to each other, REST is modeled around a large number of "Resources" which "link" among each other. As a significant difference with WS*, REST raises the importance of "Resources" as well as its "Linkage", on the other hand, it push down the importance of "Procedures".

Under the WS* model, "Service" in the SOA is organized as large number of "Resources". Each resource will have a URI that make it globally identifiable. A resource is represented by some format of "Representation" which is typically extracted by an idempotent HTTP GET. The representation may embed other URI which refers to other resources. This emulates an HTML link between web pages and provide a powerful way for the client to discover other services by traversing its links. It also make building SOA search engine possible.

On the other hand, REST down play the "Procedure" aspect and define a small number of "action" based on existing HTTP Methods. As we discussed above, HTTP GET is used to get a representation of the resource. To modify a resource, REST use HTTP PUT with the new representation embedded inside the HTTP Body. To delete a resource, REST use HTTP DELETE. To get metadata of a resource, REST use HTTP HEAD. Notice that in all these cases, the HTTP Body doesn't carry any information about the "Procedure". This is quite different from WS* SOAP where the request is always made using HTTP POST.

At the first glance, it seems REST is quite limiting in terms of the number of procedures that it can supported. It turns out this is not the case, REST allows any "Procedure" (which has a side effect) to use HTTP POST. Effectively, REST categorize the operations by its nature and associate well-defined semantics with these categories (ie: GET for read-only, PUT for update, DELETE for remove, all above are idempotent) while provide an extension mechanism for application-specific operations (ie: POST for application procedures which may be non-idempotent).

URI Naming Convention

Since resource is usually mapped to some state in the system, analyzing its lifecycle is an important step when designing how a resource is created and how an URI should be structured.

Typically there are some eternal, singleton "Factory Resource" which create other resources. Factory resource typically represents the "type" of resources. Factory resource usually have a static, well-known URI, which is suffixed by a plural form of the resource type. Some examples are ...

"Resource Instance", which are created by the "Factory Resource" usually represents an instance of that resource type. "Resource instances" typically have a limited life span. Their URI typically contains some unique identifier so that the corresponding instance of the resource can be located. Some examples are ...

http://xyz.com/books/4545
http://xyz.com/users/123

"Dependent Resource" are typically created and owned by an existing resource during part of its life cycle. Therefore "dependent resource" has an implicit life-cycle dependency on its owning parent. When a parent resource is deleted, all the dependent resource it owns will be deleted automatically. Dependent resource use an URI which has prefix of its parent resource URI. Some examples are ...

http://xyz.com/books/4545/tableofcontent
http://xyz.com/users/123/shopping_cart

Creating Resource

To create a resource instance of a particular resource type, make an HTTP POST to the Factory Resource URI. If the creation is successful, the response will contain a URI of the resource that has been created.

To create a book ...

POST /books HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<book>
  <title>...</title>
  <author>Ricky Ho</author>
</book>

HTTP/1.1 201 Created
Content-Type: application/xml; charset=utf-8
Location: /books/4545

<ref>http://xyz.com/books/4545</ref>

To create a dependent resource, make an HTTP POST to its owning resource's URI

To upload the content of a book (using HTTP POST) ...

POST  /books/4545  HTTP/1.1
Host: example.org
Content-Type: application/pdf
Content-Length: nnnn

{pdf data}

HTTP/1.1 201 Created
Content-Type: application/pdf
Location: /books/4545/content

<ref>http://xyz.com/books/4545/tableofcontent</ref> 

HTTP POST is typically used to create a resource when its URI is unknown to the client before its creation. However, if the URI is known to the client, then an idempotent HTTP PUT should be used with the URI of the resource to be created.

To upload the content of a book (using HTTP PUT) ...

HTTP/1.1 201 Created
Content-Type: application/pdf
Location: /books/4545/content

<ref>http://xyz.com/books/4545/tableofcontent</ref>

HTTP/1.1 200 OK

Finding Resources

Make an HTTP GET to the factory resource URI, criteria pass in as parameters. (Note that it is up to the factory resource to interpret the query parameter).

To search for books with a certain author ...

GET /books?author=Ricky HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8

HTTP/1.1 200 OK
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<books>
  <book>
    <ref>http://xyz.com/books/4545</ref>
    <title>...</title>
    <author>Ricky</author>
  </book>
  <book>
    <ref>http://xyz.com/books/4546</ref>
    <title>...</title>
    <author>Ricky</author>
  </book>
</books>

Another school of thoughts is to embed the criteria in the URI path, such as ... http://xyz.com/books/author/Ricky

I personally prefers the query parameters mechanism because it doesn't imply any order of search criteria.

Lookup a particular resource

Make an HTTP GET to the resource object URI

Lookup a particular book...

GET /books/4545 HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8

HTTP/1.1 200 OK
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<book>
  <title>...</title>
  <author>Ricky Ho</author>
</book>

In case the resource have multiple representation format. The client should specify within the HTTP header "Accept" of its request what format she is expecting.

Lookup a dependent resource

Make an HTTP GET to the dependent resource object URI

Download the table of content of a particular book...

GET /books/4545/tableofcontent HTTP/1.1
Host: xyz.com
Content-Type: application/pdf

HTTP/1.1 200 OK
Content-Type: application/pdf
Content-Length: nnn

{pdf data}

Modify a resource

Make an HTTP PUT to the resource object URI, pass in the new object representation in the HTTP body

Change the book title ...

PUT /books/4545 HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<book>
  <title>Changed title</title>
  <author>Ricky Ho</author>
</book>

HTTP/1.1 200 OK

Delete a resource

Make an HTTP DELETE to the resource object URI

Delete a book ...

DELETE /books/4545 HTTP/1.1
Host: xyz.com

HTTP/1.1 200 OK

Resource Reference

In some cases, we do not want to create a new resource, but we want to add a "reference" to an existing resource. e.g. consider a book is added into a shopping cart, which is another resource.

Add a book into the shopping cart ...

POST  /users/123/shopping_cart  HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<?xml version="1.0" ?>
<add>
  <ref>http://xyz.com/books/4545</ref>
</add>

HTTP/1.1 200 OK

Show all items of the shopping cart ...

GET  /users/123/shopping_cart  HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8

HTTP/1.1 200 OK
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<?xml version="1.0" ?>
<shopping_cart>
  <ref>http://xyz.com/books/4545</ref>
  ...
<shopping_cart> 

Note that the shopping cart resource contains "resource reference" which acts as links to other resources (which is the books). Such linkages create a resource web so that client can discovery and navigate across different resources.

Remove a book from the shopping cart ...

POST  /users/123/shopping_cart  HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<?xml version="1.0" ?>
<remove>
  <ref>http://xyz.com/books/4545</ref>
</remove>

HTTP/1.1 200 OK

Note that we are using HTTP POST rather than HTTP DELETE to remove a resource reference. This is because we are remove a link but not the actual resource itself. In this case, the book still exist after it is taken out from the shopping cart.

Checkout the shopping cart ...

POST  /orders  HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<?xml version="1.0" ?>
<ref>http://xyz.com/users/123/shopping_cart</ref>

HTTP/1.1 201 Created
Content-Type: application/xml; charset=utf-8
Location: /orders/2008/04/10/1001

<?xml version="1.0" ?>
<ref>http://xyz.com/orders/2008/04/10/1001</ref>

Note that here the checkout is implemented by creating another resource "Order" which is used to keep track of the fulfillment of the purchase.

Transaction Resource

One of the common criticism of REST is because it is so tied in to HTTP (which doesn't support a client callback mechanism), doing asynchronous service or notification on REST is hard. So how do we implement long running transactions (which typically require asynchronicity and callback support) in REST ?

The basic idea is to immediately create a "Transaction Resource" to return back to the client. While the actual processing happens asynchronously in the background, the client at any time, can poll the "Transaction Resource" for the latest processing status. Lets look at an example to request for printing a book, which may take a long time to complete

Print a book

POST  /books/123  HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

?xml version="1.0" ?>
<print>http://xyz.com/printers/abc</print>

HTTP/1.1 200 OK
Content-Type: application/xml; charset=utf-8
Location: /transactions/1234

<?xml version="1.0" ?>
<ref>http://xyz.com/transactions/1234</ref> 

Note that a response is created immediately which contains the URI of a transaction resource, even before the print job is started. Client can poll the transaction resource to obtain the latest status of the print job.

Check the status of the print Job ...

GET /transactions/1234 HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8

HTTP/1.1 200 OK
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<transaction>
  <type>PrintJob</type>
  <status>In Progress</status>
</transaction>

It is also possible to cancel the transaction if it is not already completed.

Cancel the print job

POST  /transactions/1234  HTTP/1.1
Host: xyz.com
Content-Type: application/xml; charset=utf-8
Content-Length: nnn

<?xml version="1.0" ?>
<cancel/>

HTTP/1.1 200 OK

Conclusion

The Resource Oriented Model that REST advocates provides a more natural fit for our service web. Therefore, I suggest that SOA implementation should take the REST model as a default approach.