What is the Race condition and its side effects in concurrency ?

Example

public class Unsafe {

   private int counter;

   public int increment() {

      return counter++;

   }

}

When above code is called by multiple threads for Read>Modify>Write operation, one can access or modify the variables that other threads might be using.

Thread 1.   counter=5  ----------> 5++ = 6 --------->  counter=6

Thread 2.   --------> counter=5 --------> 5++ = 6 --------->  counter=6

This concurrency problem called a RACE CONDITION.

The race condition requires a perfect timing and can lead to the serious side effects.

Solution

public class Safe {

   private int counter;

   public synchronized int increment() {

      return counter++;

   }

}

Side effects of race condition could be :

  1. Unreliable or incorrect result

  2. Serious Data integrity problem

  3. Getting right answer relies on lucky timing

Race condition don't always result in failure , Some unlucky timing is also required.

AVOIDING RACE CONDITION

To avoid race conditions, prevent other threads from using a variable while a thread is in middle of modifying it.

Ensure that other threads can modify the state only before one start or after finish, but not in the middle.