Understanding open source architectures I

https://github.com/foursquare
https://code.google.com/p/foursquared/wiki/Design
https://code.google.com/p/foursquared/source/browse
https://sites.google.com/site/foursquareddev/ux/venueactivity

https://github.com/foursquare/Android-PullToRefresh
https://github.com/foursquare/SlidingMenu

Eclipse:
http://www.aosabook.org/en/eclipse.html

Graphite:
http://www.aosabook.org/en/graphite.html

http://www.tornadoweb.org/

http://blog.reddit.com/2008/06/reddit-goes-open-source.html

https://code.google.com/p/guava-libraries/wiki/GuavaExplained

Design a timer class

Design a timer class. The users of your class should be able to register to your class with the time in secs and their callbacks. These callbacks are called as the time expires.

• The timer runs in its own thread, and it sleeps for 1 sec (since, the interviewer had asked for 1 second as the granularity)
• It maintains a list of  'Task/Runnable' along with the remaining time to expire.
• Every time the timer wakes up after a sec, it reduces the expiry time for each task by 1 sec.
• And for all those tasks for which the expiry reaches zero, their callbacks are called.
• I suggested a faster way (where one doesn't need to iterate the list of tasks) if the timer was limited to a fixed number of seconds using an circular array of seconds.

http://www.java2s.com/Code/Java/Development-Class/AsimpleimplementationoftheJava13javautilTimerAPI.htm

http://www.artima.com/intv/blochP.html

How to design a web forum

This was asked to me as a design question in amazon telephonic round. The interviewer wanted to know the DB design and the web-app design for a forum which supports around 100K users with the following use-cases:


 -Sign up
- Posting
- Replying
- Searching [keywords + dates based]

I told him we could have following DB schemas:


Thread [tid, subject, date]
Post [pid, tid, uid, msg, datetime ]
User [uid, role, emailid, password_hash ...]
Role [rid, list flags(for permisssion etc.)...]

The plain DB solves the use cases for Sign-up, posting and replying but could become a bottle neck for search if we run a search query based on query in the Post table (with a where clause as 'like "%keyword%"). So, for improving the performance of this, i suggested running an independent background process which parses the msgs in the post table and stores the important keywords from this messsage in the  independent datastore/B-Tree structure to be able to index the keywords for faster searches. The corresponding value for the keywords is the 'pid' (post id) in the table. This should improve the performance of the query. the interviewer seemed to have liked the proposal. Let me know if you guys can think of a better strategy for faster searches.

After this he wanted me to move to the Java side and explain the design there. I told that we would make similar classes/objects like the tables above, define relationship between those classes and then define the operations on these classes.
For components, I told him i would use the MVC architecture, where the views would be JSPs and Controller would be the interacting servlets which would extract data from the views, call the business logic from the Model and then fill the results in a new view which is returned to the user. He asked me to delve a little deeper into the Model components. I told this would basically have two main layers one is DAO (which interacts with the DB) and other is the service layer (which is called by the controller and internally calls the DAO classes for DB interaction). The service classes would be using the above set of objects.
PS: I wasn't very comfortable with the component design part, so any comments are welcome.

How to design a good API

From  a talk by Joshua Bloch: http://www.youtube.com/watch?v=aAb7hSCtvGw
Slides: http://lcsd05.cs.tamu.edu/slides/keynote.pdf

• API design is very similar to Language and language feature design.
• Easy to evolve, hard to misuse
• Gather requirements with healthy degree of skepticism. Define use cases from the requirements. Often it is easier and more rewarding to solve a more general form of problem than solving a specific problem so keep your mind open.
• Keep the initial spec of the API short/1 page. And discuss with as many people and then flesh it out as confidence builds.
• Write to the API (means write clients using API interface) as you design the API, this is very important as it allows you to explore the use cases API is being designed to solve.
• API should do ONE thing good.

Reading: http://www.artima.com/intv/blochP.html

State Design Pattern

Read the chapter on State design patterns from Head First Design Patterns book.

They explain the GumBallMachine, which can be in different states and needs to perform each action in a different way depending on the current state.

To start with they declare the action() methods in the GumBallMachine class only and in each action method they use if-else ladder to check the current state and take action based on the current state. And show how this type of state machine coding makes the action methods cluttered.

And in the next approach they make a State interface which has list of common action methods. And then there are concrete state classes for each state, which implement this interface and perform the actions() depending on the current state(i.e. the class for the state). This allows you to store the current_state object in the GumBallMachine class and call the state.action() methods inside this class. Depending on the current state the apt methods are called for each action(). This decouples each state from other states and makes adding new states easier.