Pattern: Barriers & Resource Coordination (Problems 12, 13, 14, 17)

When threads must wait for a group, not just one signal

Problems 12 (Building H₂O), 13 (dual-thread sync), 14 (multi-buffered master/worker) and 17 (traffic-light intersection) all need threads to rendezvous: nobody proceeds until a required set has arrived. That's a barrier, built from semaphores (or CyclicBarrier/Phaser in Java).

Worked: Building H₂O (Problem 12)

Hydrogen and oxygen threads must bond in groups of exactly 2 H + 1 O. Gate each element type with a semaphore, and use a barrier so a molecule only forms when all three are present:

Semaphore hGate = new Semaphore(2), oGate = new Semaphore(1);
CyclicBarrier bond = new CyclicBarrier(3);     // 2 H + 1 O
void hydrogen(Runnable bondH) {
    hGate.acquire();                            // at most 2 H proceed
    bond.await();                               // wait for the full molecule
    bondH.run(); hGate.release();
}
void oxygen(Runnable bondO) {
    oGate.acquire();
    bond.await();
    bondO.run(); oGate.release();
}

How each problem maps to it

• P12 H₂O: per-type semaphores + a 3-party barrier (above).
• P13 Dual-thread synchronization: a 2-party barrier (or one semaphore each way) so two threads alternate in lock-step.
• P14 Multi-buffered master/worker: a bounded set of buffers guarded by two counting semaphores (empty, full) — the producer-consumer pattern with more than one slot.
• P17 Traffic light: a mutex over the intersection plus per-direction gates; only one direction "goes" at a time — mutual exclusion over a shared critical region with fair turn-taking.

In Go

var bond sync.WaitGroup; bond.Add(3)    // simplest 3-party barrier
// each atom goroutine: bond.Done(); bond.Wait()  then bond
// counting semaphore = make(chan struct{}, n); turn-taking = a token passed on a channel

Takeaways

• Barrier = "wait until the whole group arrives, then release together"; CyclicBarrier resets for reuse, CountDownLatch is one-shot.
• Counting semaphores gate "how many of X may proceed"; combine with a barrier for group formation.
• Go: WaitGroup for a barrier, buffered channel for a counting semaphore, a token for turn-taking.

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Re-authored for this guide; barrier diagram hand-authored as SVG. Covers Problems 12, 13, 14, 17. See also: Semaphores, Condition Variables, Deterministic Ordering & Signaling.

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