A: The problem

You have been tasked to design Whisper.com -- a Micro-blogging service where users can post short text messages ("whisps") on a topic of interest. In this start-up phase, Whisper proposes to employ a non-replicated architecture and restricts itself to whisps on three topics:
1. Surviving Project 1 (topic ID: #proj1)
2. Swine flu rumors (topic ID: #swine)
3. Fall foliage (topic ID: #foliage)

Since Whisper hopes to one day make whisps more popular than tweets, they would like to use sound design principles to design their online site in order to allow for future growth. The site will employ a two tier design - a front-end and a back-end. The front-end tier will accept user posts and perform initial processing. The backend consists of a data management tier that stores new posts in a "database" and retrieves whisps for clients that have subscribed to them.

The database can be a simple file on disk; each whisp entry should contain at least the following fields: timestamp, posterID, whispID, whisp comment.The posterID is of the form @username, and each client is assumed to have a unique username. The whispID is of the form #topic and is restricted to the three topics listed above. The whisp comment is a free text comment that is restricted to 50 characters or less. You may optionally maintain an index on the database file for faster searching (if no index is maintained, queries on the database will need to be done as a linear search).

The front end server supports four operations:

1. post(@username, #topic, blog-comment) - which allows the user to post the blog-comment on the specified topic.
2. follow(@username,#topic) which allows a user to subscribe to all posts on the specified topic.
3. unsubscribe(@username,#topic) which allows a user to stop subscribing to the topic.
4. retrieve(@username,timestamp) which returns all whisps on all subscribed topics since the specified timestamp.

The database server supports two operations: query and store. The post operation allows the datbase to store a new whisp to the database file and has the form store(timestamp,@username,#topic, blog-comment) The query operation should allow a #topic and a time-stamp to be specified and return all posts on that topic since the specified time-stamp.

Assume that each client process follows (subscribes to) a subset of the available topics. The client should periodically post a comment on those topics and also retrieve all subscribed whips periodically.

It is important to handle errors gracefully without crashing the process. For example, if the comment exceeds 50 characters or an incorrect timestamp or an invalid topic is specified, the front-end tier should return appropriate errors.

• Other requirements:
  Each component can use RPCs or plain sockets.
  Each component should be able to accept multiple requests at the same time. This could be easily done using threads. Be aware of the thread synchronizing issues to avoid inconsistency or deadlock in your system. For instance, a database should be able to process multiple concurrent query requests; updates to the datbase should be done using synchronization.
  No GUIs are required. Simple command line interfaces are fine.

B. Evaluation and Measurement

1. Compute the average response time for each whisp by measuring the response time seen by a client for , say, 1000 sequential post requests. Also, measure the response times when multiple clients are concurrently making requests to the system, for instance, you can vary the number of clients and observe how the average response time changes. Some of these request can have incorrect arguments, in which case an error message should be printed and an error returned to the client process.
2. Following the idea shown above, compute the average response time for retrieve requests.
3. Design a test to show how the number of concurrent retrievals requests affects the query retrieval times at the database.

Make necessary plots to support your conclusions.

C. What you will submit

1. A copy of the output generated by running your program. When it receives a whisp, have your program print a message "whisp post @username, #topic, comment ". When a client issues a follow, unsubscribe or retrieve requests, having your program print the returned results in a nicely formatted manner.
2. A seperate (typed) document of approximately two pages describing the overall program design, a description of "how it works", and design tradeoffs considered and made. Also describe possible improvements and extensions to your program (and sketch how they might be made).
3. A program listing containing in-line documentation.
4. A seperate description of the tests you ran on your program to convince yourself that it is indeed correct. Also describe any cases for which your program is known not to work correctly.
5. Performance results.
6. A readme file about how to run your code on EDLAB machines.
7. After you turn in all the stuff mentioned above, please also send TA an email telling the location of your files.

D. Grading policy for all programming assignments

1. Program Listing
   works correctly ------------- 50%
   in-line documentation -------- 15%
2. Design Document
   quality of design ------------ 15%
   understandability of doc ------- 10%
3. Thoroughness of test cases ---------- 10%
4. Grades for late programs will be lowered 12 points per day late.