public T Receive()
{
// for reading, we give the impression to the client that we provide a single message at a time
// which means we maintain a cursor and enumerator in the background and hide it from the caller
if (_enumerator == null)
{
_enumerator = InitializeCursor();
}
// there is no end when you need to sit and wait for messages to arrive
while (true)
{
try
{
// do we have a message waiting?
// this may block on the server for a few seconds but will return as soon as something is available
if (_enumerator.MoveNext())
{
// yes - record the current position and return it to the client
_startedReading = true;
_lastId = _enumerator.Current.Id;
_position.Update(_positionQuery, Update.Set("last", _lastId), UpdateFlags.Upsert, SafeMode.False);
var message = _enumerator.Current.Message;
var delay = DateTime.UtcNow - _enumerator.Current.Enqueued;
Log.Debug(m => m("Received {0} after {1}", _queueName, delay));
return(message);
}
if (!_startedReading)
{
// for an empty collection, we'll need to re-query to be notified of new records
Log.Debug("Cursor Empty");
Thread.Sleep(100);
_enumerator.Dispose();
_enumerator = InitializeCursor();
}
else
{
// if the cursor is dead then we need to re-query, otherwise we just go back to iterating over it
if (_enumerator.IsDead)
{
Log.Debug("Cursor Dead");
_enumerator.Dispose();
_enumerator = InitializeCursor();
}
}
}
catch (Exception ex)
{
// cursor died or was killed
_enumerator.Dispose();
_enumerator = InitializeCursor();
}
}
}
/// <summary>
/// Receives an event (or action) from queue. Does not change the message state. Used for PubSub (1 to Many - all gets the same messages) use cases
/// </summary>
/// <returns></returns>
private T ReceiveEvent()
{
if (_enumerator == null)
{
_enumerator = InitializeCursor();
}
// running untill we have something to return
while (true)
{
var t1 = DateTime.UtcNow;
try
{
// do we have a message waiting?
// this may block on the server for a few seconds but will return as soon as something is available
if (_enumerator.MoveNext())
{
// yes - record the current position and return it to the client
Interlocked.Increment(ref _totalReceived);
_lastId = _enumerator.Current.Id;
var current = _enumerator.Current;
var message = current.Message;
var delay = DateTime.UtcNow - _enumerator.Current.Enqueued;
Log.Debug(m => m("Received {0} after {1}", _queueName, delay));
return(message);
}
// if the cursor is dead then we need to re-query, otherwise we just go back to iterating over it
if (_enumerator.IsDead)
{
Log.Debug("Cursor Dead");
_enumerator.Dispose();
_enumerator = InitializeCursor();
}
// If we are here it means that the server returned without items, usually it sits in a 2 seconds block on the server, but if initial request is empty then return immedietly.
//This is a safety mechanism to protect the mongo server from our DOS...(due to this cursor behavior, and from unknown bugs / 'behaviors')
var iterationTime = DateTime.UtcNow - t1;
if (iterationTime.TotalMilliseconds < SHORT_SLEEP_INTERVAL) // minimal polling interval : TODO - move to Config
{
Thread.Sleep(SHORT_SLEEP_INTERVAL);
}
}
catch (Exception ex)
{
Log.Warn("[ReceiveEvents inside while true] Got exception. Will reinitialize and continue. Ex={0}", ex);
// cursor died or was killed
if (_enumerator != null)
{
_enumerator.Dispose();
}
_enumerator = InitializeCursor();
//This is a safety mechanism to protect the mongo server from our DOS...
var iterationTime = DateTime.UtcNow - t1;
if (iterationTime.TotalMilliseconds < SHORT_SLEEP_INTERVAL) // minimal polling interval : TODO - move to Config
{
Thread.Sleep(SHORT_SLEEP_INTERVAL);
}
}
}
}
