Restful Resources

This past week, I've come across a couple really useful resources for REST style architectural development that I can recommend:

The first is the O'Reilly book "Restful Web Services". I'm about halfway through and it has already illuminated some dark corners of Restful web service design, such as:

• When to use POST vs. PUT. (Use POST when the server is responsible for generating the URI of the associated resource; use PUT when the client is responsible for generating the URI).
• Authentication.

The authors make a point of distinguishing between REST in general and REST when applied to web service design, for which they describe a set of concrete best practices they call "Resource Oriented Architecture". This is very nice, and reminds me of Effective Java, which provides a set of best practices for Java software development.

The second REST resource I would like to recommend is the "Poster" plugin for FireFox. Poster enables you to make GET, PUT, POST, and DELETE http calls from within FireFox and see the results. It is a nice way to obtain a sanity check on what your web service is doing when you don't quite understand why your unit tests are failing.

Sample Restlet Application for Hackystat

I decided to get my feet wet with Restlet by building a small little server with the following API:

http://localhost:9876/samplerestlet/file/{filename}

The idea is that it will retrieve and display {filename}, which is an instance of the "file" resource.

This was a nice way to wade into the Restlet framework; more than a Hello World app, lacking stuff we don't need (like Virtual Hosts), and requiring stuff we do (URL-based dispatching).

To see what I did, check out the following 'samplerestlet' module from SVN:

svn://www.hackystat.org/csdl/samplerestlet

To build and run it:

1. Download Restlet-1.0, unzip, and point a RESTLET_HOME env variable at it.
   
2. Build the system with "ant jar"
3. Run the result with "java -jar samplerestlet.jar"

This starts up an HTTP server that listens on port 9876.

Try retrieving the following in your browser: http://localhost:9876/samplerestlet/file/build.xml

Now look through the following three files, each only about 50 LOC:

1. build.xml, which shows what's needed to build the executable jar file.
2. Server.java, which creates the server and dispatches to a FileResource instance to handle URLs satisfying the file/{filename} pattern.
3. FileResource.java, which handles those GET requests by reading the file from disk and returning a string representation of it.
   

If this looks confusing, the Restlet Tutorial is a reasonable introduction. There's also a pretty good Powerpoint presentation that introduces both REST architectural design and the Restlet Framework at the Restlet Wiki Home Page. It comes with some decent sample code as well.

Next step is to add this kind of URL processing to SensorBase.