📞 Call Center Queue Concurrency Problem

🎯 Objective:

Simulate a call center where:

• There are a limited number of agents (say 3).

• Many customers (calls) are coming in at the same time.

• If all agents are busy, new calls must wait in a queue until an agent becomes free.

Only available agents can take calls.
The system should not lose any calls and must not assign 2 agents to the same call.

Solution Approaches

Basic Thread Pool Approach (Fixed Agents)

1. We'll use a fixed thread pool where:

   • Each thread represents an agent

   • Incoming calls are tasks submitted to the executor

   • The executor automatically handles queuing when all agents are busy

    import java.util.concurrent.*;

    public class BasicCallCenter {
        // 1. Configuration
        private static final int AGENT_COUNT = 3; 
        private static ExecutorService agents = Executors.newFixedThreadPool(AGENT_COUNT);

        // 2. Call Task Representation
        static class Call implements Runnable {
            private final int callId;

            public Call(int id) { 
                this.callId = id; 
            }

            public void run() {
                // 3. Call Handling Logic
                System.out.println("📞 Call " + callId + " started by " + 
                                  Thread.currentThread().getName());

                try {
                    // Simulate variable call duration (2-7 seconds)
                    Thread.sleep(2000 + (long)(Math.random() * 5000)); 
                } catch (InterruptedException e) {
                    System.out.println("❌ Call " + callId + " interrupted");
                }

                System.out.println("✅ Call " + callId + " completed");
            }
        }

        // 4. Simulation Driver
        public static void main(String[] args) {
            // Simulate 10 incoming calls
            for (int i = 1; i <= 10; i++) {
                agents.execute(new Call(i)); // 5. Dispatch calls
                System.out.println("🔔 Received call #" + i);

                // Random delay between calls (0-1s)
                try { Thread.sleep((long)(Math.random() * 1000)); 
                } catch (InterruptedException e) {}
            }

            // 6. Clean shutdown
            agents.shutdown();
        }
    }

❌ Non-Working Test Cases

1. Sustained Overload (Memory Crash)

Continuous call flood.

10,000 calls with 3 agents

    for (int i = 1; i <= 10_000; i++) {
        agents.execute(new Call(i));
    }

OutOfMemoryError: Unable to create new native thread

Unbounded queue grows indefinitely, consuming all memory.

2. Call Rejection Needed

Queue at capacity (business requirement), Attempt to queue 6th call when max capacity is 5
Silently accepts all calls
Should reject calls when overloaded
Solution: Use ThreadPoolExecutor with bounded queue:

ExecutorService agents = new ThreadPoolExecutor(
    3, 3, 0L, TimeUnit.MILLISECONDS,
    new ArrayBlockingQueue<>(5), // Bounded queue
    new ThreadPoolExecutor.AbortPolicy() // Rejection handler
);

More Cases and Other Approaches are covered in our course.