• You have a single node system hosting a web site
• Every time a page is visited, the onPageVisit(page) method is called
• Your task is to count the number of visits for each page
• No need to persist data

Please choose to implement a pageview counter by:

• using any inbuilt java data structure
• allow multiple threads to read and write at the same time

Please use the below skeleton for the implementation of the logic

 public static class PageVisitCounter 
 {
      public void onPageVisit(String page) 
      {
          //implement
      }
      
      public long getPageVisits(String page)
      {
          //implement
      }
 }

I have tried to implement the PageViewCounter in two three ways:

• One with both the Write and Read are protected using ReentrantReadWriteLock.
• Another one with the Write protected and non-protected read using ReentrantLock.

Update - 02March2022

• I have implemented a new one with No explicit locking by using a Long Adder

The code is extensively commented to give an idea on my thinking while developing this code.

To compile and run the project:

• checkout the project first using git clone

• run mvn clean install in the parent directory

• you can also run mvn test to run the tests

• you will see the below lines which shows all the tests passed

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running PageVisitCounterWriteSafeReadUnsafeTest
Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 3.603 sec
Running PageVisitCounterLockFreeTest
Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 3.448 sec
Running PageVisitCounterReadWriteSafeTest
Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 3.485 sec

Results :

Tests run: 9, Failures: 0, Errors: 0, Skipped: 0

I have used Java 11 SDK to compile and run the program.

• Optimize the code using lambdas available with the ConcurrentHashMap functions.
• Try to implement a lockfree pagevieewcounter module.

This repository is released under the MIT license. In short, this means you are free to use this software in any personal, open-source or commercial projects. Attribution is optional but appreciated.