// if work is available to do and another web server in the web farm isn't already doing,
// this returns a unit of work used to wrap the work.
public JobUnitOfWork ReserveWork(string workerId, IJob job)
{
long?workItemId = null;
// We do a double check here because this may be the first DB query that runs when starting
// WebBackgrounder. For those using EF Code First, this could trigger a database creation
// (typically on a dev box, hopefully not in production). A database can't be created inside
// a transaction scope so we run this query here first.
var lastWorkItem = _workItemRepository.GetLastWorkItem(job);
if (lastWorkItem.IsActive() && !lastWorkItem.IsTimedOut(job, () => DateTime.UtcNow))
{
return(null);
}
_workItemRepository.RunInTransaction(() =>
{
lastWorkItem = _workItemRepository.GetLastWorkItem(job);
if (lastWorkItem.IsTimedOut(job, () => DateTime.UtcNow))
{
lastWorkItem.Completed = DateTime.UtcNow;
_workItemRepository.SetWorkItemFailed(lastWorkItem.Id,
new TimeoutException("Workitem expired. Job timeout was " + job.TimeOut));
lastWorkItem = null;
}
if (lastWorkItem.IsActive())
{
workItemId = null;
return;
}
workItemId = _workItemRepository.CreateWorkItem(workerId, job);
}
);
if (workItemId == null)
{
return(null);
}
return(new JobUnitOfWork(_workItemRepository, workItemId.Value));
}