Building Web Services the REST Way

Building Web Services the REST Way

I will first provide a brief introduction to REST and then describe how to build Web services in the REST style.

What is REST?

REST is a term coined by Roy Fielding in his Ph.D. dissertation [1] to describe an architecture style of networked systems. REST is an acronym standing for Representational State Transfer.

Why is it called Representational State Transfer?

The Web is comprised of resources. A resource is any item of interest. For example, the Boeing Aircraft Corp may define a 747 resource. Clients may access that resource with this URL:

http://www.boeing.com/aircraft/747

    

A representation of the resource is returned (e.g., Boeing747.html). The representation places the client application in a state. The result of the client traversing a hyperlink in Boeing747.html is another resource is accessed. The new representation places the client application into yet another state. Thus, the client application changes (transfers) state with each resource representation --> Representational State Transfer! Here is Roy Fielding's explanation of the meaning of Representational State Transfer: "Representational State Transfer is intended to evoke an image of how a well-designed Web application behaves: a network of web pages (a virtual state-machine), where the user progresses through an application by selecting links (state transitions), resulting in the next page (representing the next state of the application) being transferred to the user and rendered for their use."

Motivation for REST

The motivation for REST was to capture the characteristics of the Web which made the Web successful. Subsequently these characteristics are being used to guide the evolution of the Web.

REST - An Architectural Style, Not a Standard

REST is not a standard. You will not see the W3C putting out a REST specification. You will not see IBM or Microsoft or Sun selling a REST developer's toolkit. Why? Because REST is just an architectural style. You can't bottle up that style. You can only understand it, and design your Web services in that style. (Analogous to the client-server architectural style. There is no client-server standard.) While REST is not a standard, it does use standards:

• HTTP
• URL
• XML/HTML/GIF/JPEG/etc (Resource Representations)
• text/xml, text/html, image/gif, image/jpeg, etc (MIME Types)

The Classic REST System

The Web is a REST system! Many of those Web services that you have been using these many years - book-ordering services, search services, online dictionary services, etc - are REST-based Web services. Alas, you have been using REST, building REST services and you didn't even know it. REST is concerned with the "big picture" of the Web. It does not deal with implementation details (e.g., using Java servlets or CGI to implement a Web service). So let's look at an example of creating a Web service from the REST "big picture" perspective.

Parts Depot Web Services

Parts Depot, Inc (fictitious company) has deployed some web services to enable its customers to:

• get a list of parts
• get detailed information about a particular part
• submit a Purchase Order (PO)

Let's consider how each of these services are implemented in a RESTful fashion.

Get Parts List

The web service makes available a URL to a parts list resource. For example, a client would use this URL to get the parts list:

http://www.parts-depot.com/parts

    

Note that "how" the web service generates the parts list is completely transparent to the client. All the client knows is that if he/she submits the above URL then a document containing the list of parts is returned. Since the implementation is transparent to clients, Parts Depot is free to modify the underlying implementation of this resource without impacting clients. This is loose coupling.

Here's the document that the client receives:

<?xml version="1.0"?>

    <p:Parts xmlns:p="http://www.parts-depot.com" 

    xmlns:xlink="http://www.w3.org/1999/xlink">

    <Part id="00345" xlink:/>

    <Part id="00346" xlink:/>

    <Part id="00347" xlink:/>

    <Part id="00348" xlink:/>

    </p:Parts>

    

[Assume that through content negotiation the service determined that the client wants the representation as XML (for machine-to-machine processing).] Note that the parts list has links to get detailed info about each part. This is a key feature of REST. The client transfers from one state to the next by examining and choosing from among the alternative URLs in the response document.

Get Detailed Part Data

The web service makes available a URL to each part resource. Example, here's how a client requests part 00345:

http://www.parts-depot.com/parts/00345

    

Here's the document that the client receives:

<?xml version="1.0"?>

    <p:Part xmlns:p="http://www.parts-depot.com"   

    xmlns:xlink="http://www.w3.org/1999/xlink">

    <Part-ID>00345</Part-ID>

    <Name>Widget-A</Name>

    <Description>This part is used within the frap assembly</Description>

    <Specification xlink:/>

    <UnitCost currency="USD">0.10</UnitCost>

    <Quantity>10</Quantity>

    </p:Part>

    

Again observe how this data is linked to still more data - the specification for this part may be found by traversing the hyperlink. Each response document allows the client to drill down to get more detailed information.

Submit PO

The web service makes available a URL to submit a PO. The client creates a PO instance document which conforms to the PO schema that Parts Depot has designed (and publicized in a WSDL document). The client submits PO.xml as the payload of an HTTP POST.

Logical URLs versus Physical URLs

REST Web Services Characteristics

• Client-Server: a pull-based interaction style: consuming components pull representations.
• Stateless: each request from client to server must contain all the information necessary to understand the request, and cannot take advantage of any stored context on the server.
• Cache: to improve network efficiency responses must be capable of being labeled as cacheable or non-cacheable.
• Uniform interface: all resources are accessed with a generic interface (e.g., HTTP GET, POST, PUT, DELETE).
• Named resources - the system is comprised of resources which are named using a URL.
• Interconnected resource representations - the representations of the resources are interconnected using URLs, thereby enabling a client to progress from one state to another.
• Layered components - intermediaries, such as proxy servers, cache servers, gateways, etc, can be inserted between clients and resources to support performance, security, etc.

Principles of REST Web Service Design

2. Create a URL to each resource. The resources should be nouns, not verbs. For example, do not use this:

http://www.parts-depot.com/parts/getPart?id=00345

    

Note the verb, getPart. Instead, use a noun:

http://www.parts-depot.com/parts/00345

    

Summary

Acknowledgement

References

posted on 2008-09-11 14:04 gembin 閱讀(344) 評論(0)  編輯  收藏 所屬分類: Web Service