Prevent clients from getting the same record from API

Question

I have a very simple .NET core API that serves clients tasks. When a client requests a task, API updates it's status and returns the task to client. However i have noticed that if multiple clients request a task at the same time, they end up getting the same task. Here's relevant controller code:

        [HttpPost("next",Name = "RequestJob")]
        public ActionResult<QueJobReadDto> RequestJob(Robot robot)
        {
            var job = _repository.RequestJob();
            var registeredJob = _repository.RegisterJob(robot.RobotName, job);
            _repository.SaveChanges();
            return Ok(_mapper.Map<QueJobReadDto>(registeredJob));
        }

As you can see there are two methods used, "RequestJob" and "RegisterJob". RequestJob finds a record with a status of "In Que" and RegisterJob updates the status of the record to "In Progress". Here are the relevant implementations:

        public QueJob RequestJob()
        {
            
            return _context.QueJobs
                .Where(queJob => queJob.Status == "In Que")
                .OrderBy(queJob => queJob.JobGuid)
                .ToList()
                .FirstOrDefault();
        }

        public QueJob RegisterJob(string worker, QueJob qj)
        {
            qj.Status = "In Progress";
            qj.Start = DateTime.UtcNow;
            qj.Worker = worker;
            qj.JobGuid = qj.JobGuid;
            _context.Update(qj);
            return _context.QueJobs.Find(qj.JobGuid);
        }

My question is: how can i couple these methods so the clients will never get the same task?

Answer 1

If you want to try out the economy version, add this to the top of your controller

private const int LockTimeoutInMilliseconds = 100;
private static readonly object LockObject = new object();

Then

[HttpPost("next",Name = "RequestJob")]
public ActionResult<QueJobReadDto> RequestJob(Robot robot)
{
  if (!Monitor.TryEnter(LockObject, LockTimeoutInMilliseconds))
  {
     // return an approriate error messsage
  }

  try
  {
    var job = _repository.RequestJob();
    var registeredJob = _repository.RegisterJob(robot.RobotName, job);
    _repository.SaveChanges();
    return Ok(_mapper.Map<QueJobReadDto>(registeredJob));
  }
  finally
  {
    Monitor.Exit(LockObject); // ensure that the lock is released.
  }
}