Distributed Systems Project

This is my project for the 4th year distributed systems module in college. For this project I was required to use the Servlet/JSP and Java RMI frameworks to develop a remote, asynchronous dictionary lookup service.

Project Overview

A JSP page should provide users with the ability to specify a string which will be checked against the dictionary. The HTML form information should be dispatched to a servlet that adds the client request to an in-queue and then returns a job ID to the web client. The web client should poll the web server periodically (every 10 seconds) and query if the request has been processed. Client requests in the inQueue should be periodically removed and processed (every 10 seconds). The processing of a client request will require a RMI method invocation to a remote object which implements an interface called DictionaryService. The remote object which implements DictionaryService should check if the string received exists in the dictionary, and return the dictionary definition of the string if it does exist in the dictionary, or "String not found" if it does not exist in the dictionary. Once the result of the dictionary lookup has been computed by the remote object, the returned response should be added to the outQueue on the Tomcat server and returned to the original web client when they next poll the server.

Project Requirements

The following are the minimum requirements for this project.

• A web client request should be placed in a message queue to await processing. Each request should be allocated a job number. The job number should be added to an inQueue (Map) along with the request string. The servlet handler should return the job number to the client which in turn should poll the server every 10 seconds for a response. When a response is received with a completed task, the result of the dictionary lookup should be displayed in the browser.

• An interface called DictionaryService should expose a remote method with the following signature:

   public String lookup(String s) throws RemoteException;
  

  Where s is the string to lookup in the dictionary, and the string returned is either the dictionary definition of s or the text "String not found". In the DictionaryServiceImpl, before looking up the query string in the dictionary the thread should be put to sleep for a time, i.e. Thread.sleep(1000), to slow the service down and simulate a real asynchronous service.

• A sample dictionary containing ~50 words.

Extra features

Extra features or enhancements are worth 20% of the project. The following are the list of extra features or enhancements included in this project.

• The JSP page communicates with the web application through a REST service. This REST service is implemented using the Jersey Java library.
• This project has multithreading functionality, meaning multiple RMI clients can make queries to the RMI Dictionary Service concurrently. This is achieved through the use of Java Thread pools.
• This project uses a slightly modified version of Webster’s Dictionary. This dictionary contains approximately 94000 words and definitions.
• The web application is a single page application (SPA). Rather than returning other pages from the REST service, it simply returns JSON which is used to update the SPA. This results in faster response times as all the HTML and CSS is loaded when the web application is started.
• Besides looking up words in the dictionary, this project also provides functionality for adding/modifying and removing words from the dictionary. Note that these operations only modify the in memory dictionary and changes are not persisted when the application is terminated. This functionality was implemented for demonstration purposes. All operations are case insensitive.

Running the application

This section will describe how to create and run both the JAR and WAR files.

Dictionary service

1. Open a terminal and navigate to the "bin" folder of the Eclipse project.
2. Create a JAR with the following command: jar –cf dictionary-service.jar *
3. Start the dictionary service with the following command: java –cp ./dictionary-service.jar ie.gmit.sw.ServiceSetup

Web application

1. Open a terminal and navigate to the "job-server" folder.
2. Next create a WAR. The easiest way to do this is by opening the project in Eclipse, right click on the project, go to Export -> WAR.
3. Download Tomcat if you haven't done so already.
4. Copy the WAR to the webapps folder under tomcat.
5. Start Tomcat by navigating to bin folder in Tomcat and running startup.
6. In a browser, navigate to http://localhost:8080/job-server/.

Conclusion

This project demonstrates how distributed systems can communicate and work together. Note that two methods of communication were used in this project. The first was Java RMI, which only works between homogeneous systems, or systems developed using the same technology, in this case Java. This was seen between the servlet and dictionary service. We were able to invoke methods on a remote Java object. The second form of communication was the passing of HTTP messages to the REST service from the JSP page. REST services allows communication between heterogeneous systems, meaning systems that consist of a different technologies. This project deviated slightly from the initial specification in order to incorporate more of the material covered throughout the module.