<<*REpresentational State Transfer or REST*
REST is essentially a philosophy of web service architecture, not a
standard like SOAP. Emphasis is on strict interpretation of the roles of
the various actions; GET, PUT, POST and DELETE and the other features
defined in the HTTP 1.1 protocol. <http://tools.ietf.org/html/rfc2616> I
discussed the perceived virtues of the REST approach in this article
<http://searchsoa.techtarget.com/tip/0,289483,sid26_gci1227190,00.html>
more than two years ago. Since that time we have seen continued growth
in support for REST style services such as Amazon's S3 service and
Google's Data APIs.
As the standards associated with SOAP have gotten more and more complex,
developers have been attracted to REST. This enthusiasm for REST has led
to many services getting called RESTful when in fact they are a shallow
gloss over a RPC style. For example, strictly speaking, a GET operation
should only get a representation of a resource without modifying it. The
ActiveMQ <http://activemq.apache.org/>open source messaging server
allows GET operations to read a message which is then erased but claims
a RESTful interface. You can find lots of arguments in developer blogs
and forums as to whether or not a particular service is RESTful. To
simplify, I am trying to stick to Fielding's original idea and the
HTTP1.1 specification.
*The Essentials of RESTful Architecture*
The following table, table 5.1 in Fielding's dissertation, gives the
various data elements in RESTful architecture with modern examples. The
HTTP 1.1 protocol provides many features potentially useful in web
services. For example, services which are able to read and use the
If-Modified-Since HTTP header in a request can avoid duplicating output
of an unchanged resource. Services which return an image should be able
to interpret the Accept HTTP header to return the client's preferred
media type. It seems reasonable to expect that any formal descriptive
language for RESTful web services should cover as many of these "data
elements" as possible.
*Data Element*
*Modern Web Examples*
resource
the intended conceptual target of a hypertext reference
resource identifier
URL, URN
representation
HTML document, JPEG image
representation metadata
media type, last-modified time
resource metadata
source link, alternates, vary
control data
if-modified-since, cache-control
*How to Describe a REST Service*
At the present time, the big services such as Yahoo, Amazon, Google and
Flickr communicate their HTTP interfaces to potential developers
primarily with text descriptions and example code. This has proven
sufficient for lots of examples to be created but lacks the professional
air you get with formal standards. and prevents use of automated tools
and formal SOA relation definitions.
*The Web Services Description Language - WSDL*
WSDL version 1, was first published as a W3C "note" rather than a
standard. Version 1.1, published in 2001, before interest in RESTful
style web services really got started, has been used in a variety of
toolkits for coding SOAP services and clients but did not provide for
REST style service descriptions.
WSDL version 2, a W3C recommendation released in June 2007 makes some
attempt to accomodate REST but continues to emphasize SOAP. The
published standard does not even give a REST example but I found this
Wikipedia article
<http://en.wikipedia.org/wiki/Web_Services_Description_Language> which
gives a RESTful interface example in WSDL. The idea appears to be to map
various REST style HTTP requests onto SOAP. It makes no provision for
selecting the media type of the representation to be returned. I suspect
that this approach will only be used when you have to expose the same
service with both SOAP and REST. Initial reaction in the REST developer
community to WSDL 2.0 does not seem to be very enthusiastic.
*The Web Application Description Language - WADL*
WADL is an attempt to define an XML formatted description of HTTP based
services that is both machine and human readable and no more complex
than it needs to be. The latest draft revision
<http://wadl.dev.java.net/wadl20090202.pdf> of the WADL specification,
which was just released in February, contains several important changes.
Apparently, experience with the Java based Jersey RESTful service
toolkit <https://jersey.dev.java.net/> is providing feedback on useful
features so this is far from a standard set in concrete. The WADL
specification addresses how to document the following features of a
RESTful web service.
* The resources offered and how to build a URL to address each one.
* The HTTP methods (GET, POST, etc.) supported.
* The parameters and their schema types which can appear in requests
* The response representations (media types)
* Additional documentation in plain text or XHTML markup.
*Conclusion*
Neither WSDL or WADL address all aspects of RESTful services as
identified in Fielding's dissertation. Some important features REST
could provide are not mentioned at all. Right now a lot of people are
happily exploring RESTful services without any commitment to a standard
descriptive language. In my opinion, although WSDL may work for some
simplified cases, cramming all the parameters needed to describe all
REST style operations into the WSDL 2.0 format is probably impossible.
WADL is looking pretty good but the specification and tools to make use
of it are in early stages.
*Resources*
Fielding's Dissertation, a valuable resource for understanding REST and
evolution of HTTP specifications.
<http://www.ics.uci.edu/%7Efielding/pubs/dissertation/top.htm>
This Wikipedia article contains many references to REST design
discussions. <http://en.wikipedia.org/wiki/REST>
Wikipedia article on WSDL 2.0 including an attept to define a
"RESTfulInterface" with WSDL.
<http://en.wikipedia.org/wiki/Web_Services_Description_Language>
WADL project at Java.net. <https://wadl.dev.java.net/>
Home page for the open source Jersey RESTful Web Service implementation.
<https://jersey.dev.java.net/>
More on WADL including a Yahoo example
<http://searchsoa.techtarget.com/tip/0,289483,sid26_gci1265367,00.html>>>
You can read this at:
http://searchsoa.techtarget.com/tip/0,289483,sid26_gci1351078_mem1,00.html?track=NL-449&ad=692635&asrc=EM_NLT_6207042
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