/// <summary>
/// Actually does the work of processing a single message.
/// </summary>
/// <param name="queueId"></param>
/// <returns></returns>
public async Task <bool> DoWork()
{
const string savepointName = "BeforeMessageHandler";
// get a message.
var messageContext = await _queueReader.GetNext(QueueName);
// there are no messages, so we are done. Return false so the transaction will roll back, will sleep for a while.
if (messageContext == null)
{
return(false);
}
// we found a message to process.
_log.Debug($"Processing {messageContext.MessageData.MessageId}");
var started = DateTime.UtcNow;
try
{
_counters.StartMessage();
// creat a save point. If anything goes wrong we can roll back to here,
// increment the retry count and try again later.
_dataAccess.CreateSavepoint(savepointName);
// determine what type of message it is.
var messageType = Type.GetType(messageContext.Headers.MessageClass);
if (messageType == null)
{
throw new ApplicationException($"Message {messageContext.MessageData.MessageId} as a message class of {messageContext.Headers.MessageClass} which was not a recognized type.");
}
// Get the message handlers for this message type from the Dependency Injection container.
// the list will contain both regular handlers and sagas.
// if a message has mulitple handlers, we'll get multiple handlers.
var method = typeof(IFindQueueHandlers).GetMethod("FindHandlers");
var genericMethod = method.MakeGenericMethod(messageType);
var handlers = genericMethod.Invoke(_findHandlers, null);
var castHandlers = (handlers as IEnumerable <object>).ToArray();
// sanity check that the Depenency Injection container found at least one handler.
// we shouldn't process a message that has no handlers.
if (castHandlers.Length < 1)
{
throw new ApplicationException($"There were no message handlers for {messageContext.Headers.MessageClass}.");
}
// invoke each of the handlers.
foreach (var handler in castHandlers)
{
// determine if this handler is a saga.
var handlerType = handler.GetType();
var handlerIsSaga = handlerType.IsSubclassOfSaga();
if (handlerIsSaga)
{
messageContext.SagaKey = _sagaMessageMapManager.GetKey(handler, messageContext.Message);
_log.Debug($"Active Saga {messageContext.SagaKey}");
await _queueReader.LoadSaga(handler, messageContext);
if (messageContext.SagaData != null && messageContext.SagaData.Blocked)
{
// the saga is blocked. delay the message and try again later.
_log.Info($"The saga {handlerType} for key {messageContext.SagaKey} is blocked. The current message will be delayed and retried.");
_dataAccess.RollbackToSavepoint(savepointName);
await _queueReader.DelayMessage(messageContext, 250);
_counters.SagaBlocked();
return(true);
}
if (messageContext.SagaData == null && !handlerType.IsSagaStartHandler(messageType))
{
// the saga was not locked, and it doesn't exist, and this message doesn't start a saga.
// we are processing a saga message but it is not a saga start message and we didnt read previous
// saga data from the DB. This means we are processing a non-start messge before the saga is started.
// we could continute but that might be bad. Its probably better to stop and draw attention to a probable bug in the saga or message order.
throw new ApplicationException($"A Message of Type {messageType} is being processed, but the saga {handlerType} has not been started for key {messageContext.SagaKey}. An IHandleSagaStartMessage<> handler on the saga must be processed first to start the saga.");
}
}
// find the right method on the handler.
var parameterTypes = new[] { typeof(QueueContext), messageType };
var handleMethod = handler.GetType().GetMethod("Handle", parameterTypes);
// Invoke and await the method.
// should it have a seperate try-catch around this and treat it differently?
// that would allow us to tell the difference between a problem in a handler, or if the problem was in the bus code.
// does that mater for the retry?
{
var taskObject = handleMethod.Invoke(handler, new object[] { messageContext, messageContext.Message });
var castTask = taskObject as Task;
await castTask;
}
if (handlerIsSaga)
{
await _queueReader.SaveSaga(handler, messageContext);
_log.Debug($"Inactive Saga {messageContext.SagaKey}");
}
}
// if nothing threw an exception, we can mark the message as processed.
await _queueReader.Complete(messageContext);
// return true so the transaction commits and the main loop looks for another mesage right away.
return(true);
}
catch (Exception ex)
{
// there was an exception, Rollback to the save point to undo
// any db changes done by the handlers.
_log.Warn($"There was an execption processing the message. {ex}");
_dataAccess.RollbackToSavepoint(savepointName);
// increment the retry count, (or maybe even fail the message)
await _queueReader.Fail(messageContext, ex);
// return true so the transaction commits and the main loop looks for another mesage right away.
return(true);
}
finally
{
_counters.FinishMessage(started);
}
}