Thread liveness

Deadlock

t1 = Thread(some_unit)    // thread is not started
t1.start()                // thread is executing

// e.g. we are in thread t0
t1.join()                // it means thread t0 waits for thread t1 to complete 

A deadlock occurs when two threads wait for each other indefinitely, so that neither can make any progress.

Deadlock example

public class DeadlockExample {

    private Lock lock1 = new ReentrantLock(true);
    private Lock lock2 = new ReentrantLock(true);

    public static void main(String[] args) {
        DeadlockExample deadlock = new DeadlockExample();
        new Thread(deadlock::operation1, "T1").start();
        new Thread(deadlock::operation2, "T2").start();
    }

    public void operation1() {
        lock1.lock();
        print("lock1 acquired, waiting to acquire lock2.");
        sleep(50);

        lock2.lock();
        print("lock2 acquired");

        print("executing first operation.");

        lock2.unlock();
        lock1.unlock();
    }

    public void operation2() {
        lock2.lock();
        print("lock2 acquired, waiting to acquire lock1.");
        sleep(50);

        lock1.lock();
        print("lock1 acquired");

        print("executing second operation.");

        lock1.unlock();
        lock2.unlock();
    }

    // helper methods

}

Livelock

Livelock is a situation when there is a condition both both thread to finish the work, by as far as threads affect condition of each other they can not finish.

Livelock is another concurrency problem and is similar to deadlock. In livelock, two or more threads keep on transferring states between one another instead of waiting infinitely as we saw in the deadlock example. Consequently, the threads are not able to perform their respective tasks.

A great example of livelock is a messaging system where, when an exception occurs, the message consumer rolls back the transaction and puts the message back to the head of the queue. Then the same message is repeatedly read from the queue, only to cause another exception and be put back on the queue. The consumer will never pick up any other message from the queue.

Example of livelock

public class LivelockExample {

    private Lock lock1 = new ReentrantLock(true);
    private Lock lock2 = new ReentrantLock(true);

    public static void main(String[] args) {
        LivelockExample livelock = new LivelockExample();
        new Thread(livelock::operation1, "T1").start();
        new Thread(livelock::operation2, "T2").start();
    }

    public void operation1() {
        while (true) {
            tryLock(lock1, 50);
            print("lock1 acquired, trying to acquire lock2.");
            sleep(50);

            if (tryLock(lock2)) {
                print("lock2 acquired.");
            } else {
                print("cannot acquire lock2, releasing lock1.");
                lock1.unlock();
                continue;
            }

            print("executing first operation.");
            break;
        }
        lock2.unlock();
        lock1.unlock();
    }

    public void operation2() {
        while (true) {
            tryLock(lock2, 50);
            print("lock2 acquired, trying to acquire lock1.");
            sleep(50);

            if (tryLock(lock1)) {
                print("lock1 acquired.");
            } else {
                print("cannot acquire lock1, releasing lock2.");
                lock2.unlock();
                continue;
            }

            print("executing second operation.");
            break;
        }
        lock1.unlock();
        lock2.unlock();
    }

    // helper methods

}

Thread starvation

Starvation describes a situation where a thread is unable to gain regular access to shared resources and is unable to make progress. This happens when shared resources are made unavailable for long periods by "greedy" threads. For example, suppose an object provides a synchronized method that often takes a long time to return. If one thread invokes this method frequently, other threads that also need frequent synchronized access to the same object will often be blocked.

Not bad example of code for thread starvation