Ticket Agents And Selling Airline Tickets

• Multiple threads are often spawned so they can subdivide and collectively solve a larger problem.
  • Consider the case where 10 ticket agents answer telephones at United Airlines to jointly sell 1000 airline tickets (looking for full program? click here):

  static const unsigned int kBaseIDNumber = 101;
  static const unsigned int kNumAgents = 10;
  static const unsigned int kNumTickets = 1000;
  static mutex ticketsLock;
  static unsigned int remainingTickets = kNumTickets;
  
  static void ticketAgent(size_t id) {
    while (true) {
      ticketsLock.lock();
      if (remainingTickets == 0) break;
      remainingTickets--;
      cout << oslock << "Agent #" << id << " sold a ticket! (" << remainingTickets 
           << " more to be sold)." << endl << osunlock;
      ticketsLock.unlock();
      if (shouldTakeBreak()) 
        takeBreak();
    }
    ticketsLock.unlock();
    cout << oslock << "Agent #" << id << " notices all tickets are sold, and goes home!" 
         << endl << osunlock;
  }
  
  int main(int argc, const char *argv[]) {
    thread agents[kNumAgents];
    for (size_t i = 0; i < kNumAgents; i++)
      agents[i] = thread(ticketAgent, kBaseIDNumber + i);
    for (thread& agent: agents)
      agent.join();
    cout << "End of Business Day!" << endl;
  }

  • Yes, we use global variables, but our defense here is that multiple threads all need access to a shared resource. static global variables are not at all uncommon in multithreaded programs (although larger programs would probably package them in a dedicated library or module).
  • There's a significant critical region in this program, and we use a mutex object to mark the beginning and end of it.
    • We'll first write this program without the mutex to illustrate what the problems are.
    • We'll then insert the mutex to understand exactly how it solves it.