Problem 4.9 — BookMyShow (Ticket Booking)

The seat's own state diagram — the entire concurrency story in one picture.

The atomic hold: exactly one of two concurrent users gets the seat, and holds expire automatically.

public enum SeatStatus { AVAILABLE, HELD, BOOKED, BLOCKED }

public final class Seat {
  private final String id;
  private volatile SeatStatus status = SeatStatus.AVAILABLE;
  private String heldBy;
  private Instant holdExpiresAt;

  public Seat(String id) { this.id = id; }

  // Atomic claim — the same shape as ParkingSpot.tryOccupy() and BookCopy.checkout().
  public synchronized boolean tryHold(String userId, Duration ttl, Instant now) {
    releaseIfExpired(now);
    if (status != SeatStatus.AVAILABLE) return false;
    status = SeatStatus.HELD;
    heldBy = userId;
    holdExpiresAt = now.plus(ttl);
    return true;
  }

  public synchronized boolean confirm(String userId, Instant now) {
    releaseIfExpired(now);
    if (status != SeatStatus.HELD || !heldBy.equals(userId)) return false;
    status = SeatStatus.BOOKED;
    return true;
  }

  private void releaseIfExpired(Instant now) {
    if (status == SeatStatus.HELD && now.isAfter(holdExpiresAt)) {
      status = SeatStatus.AVAILABLE;              // stale hold self-heals — no sweeper needed to be CORRECT,
      heldBy = null;                               // though a sweeper job frees memory/rows sooner
    }
  }
}

// Equivalent DB-level guarantee for a multi-instance deployment (no shared JVM memory):
// UPDATE seats SET status='HELD', held_by=?, hold_expires_at=?
//   WHERE id = ? AND (status = 'AVAILABLE' OR (status = 'HELD' AND hold_expires_at < NOW()));
// -- if rowsAffected == 0, someone else holds it or it's already booked.