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NServiceBus batch processing with Sagas.
Mauro Servienti
We have already seen how to consume, or simulate that, messages in batches. We might have a different use case, from the one outlined in the previous post, money transactions processing.
NServiceBus
batch processing
sagas

We have already seen how to consume, or simulate that, messages in batches. We might have a different use case from the one outlined in the previous post:

The system processes money transactions, due to government regulations, on a daily basis, all the processed transactions needs to be delivered to an external authority.

In the above sample use case On a daily basis is the batch definition. Or, in lack of a better term, the "business batch" definition I should say.

Can we model that with NServiceBus Sagas?

Why not? If we think about it what we just described is a long running business process cumulating money transactions details, that lasts a day, and that ends when transactions are delivered to the external authority. Something like:

class TransactionMonitorSaga : Saga<TransactionMonitorSaga.SagaState>,
  IHandleMessages<IMoneyTransactionProcessed>,
  IHandleMessages<DeliverProcessedTransactions>
{
  public class SagaState : ContainsSagaData
  {
    public List<String> TransactionIds { get; set; }
  }
  
  public void Handle( IMoneyTransactionProcessed message )
  {
    //cumulate
    this.Data.TransactionIds.Add( message.TransactionId );
  }
  
  public void Handle( DeliverProcessedTransactions message )
  {
    //deliver to third party
    //mark as complete
  }
}

The above is a "pseudo-near-to-complete" NServiceBus Saga sample.

Is that enough?

Absolutely no. The above sample is missing a key aspect, it misses the message that starts the Saga. We have an interesting issue now, how can we know when a new Saga needs to be started? Do we need to schedule somewhere a daily job that on a daily basis kicks off a new saga? What if that daily job fails and we miss one?

All good questions, that don't need an answer, because the solution is much simpler than expected:

class TransactionMonitorSaga : Saga<TransactionMonitorSaga.SagaState>,
  IAmStartedByMessages<IMoneyTransactionProcessed>,
  IHandleMessages<DeliverProcessedTransactions>
{
  public class SagaState : ContainsSagaData
  {
    [Unique]
    public String BatchId { get; set; }
    public List<String> TransactionIds { get; set; }
  }
  
  protected override void ConfigureHowToFindSaga(SagaPropertyMapper<TransactionMonitorSaga.SagaState> mapper)
  {
  	mapper.ConfigureMapping<IMoneyTransactionProcessed>(message => message.BatchDate)
                .ToSaga(sagaData => sagaData.BatchId);
  }
  
  public void Handle( IMoneyTransactionProcessed message )
  {
    //ensure correlation
    this.Data.BatchId = message.BatchDate;
    //cumulate
    this.Data.TransactionIds.Add( message.TransactionId );
  }
  
  public void Handle( DeliverProcessedTransactions message )
  {
    //deliver to third party
    //mark as complete
  }
}

There is a lot going on, let's go through the changes step by step:

  • We said that we need to find a way to start a new Saga instance, the simplest thing is to use the already defined IMoneyTransactionProcessed message and mark it as IAmStartedByMessages<TMessage> that indicates to NServiceBus that the incoming message type can start a new Saga.

At this stage what happens is that for each incoming IMoneyTransactionProcessed a new Saga will be started, and it is not really what we want:

  • Via the ConfigureHowToFindSaga method we explain to NServiceBus how to find and correlate existing Sagas to incoming messages

What happens now is interesting, what NServiceBus does for each incoming message is that if the message is defined as IAmStartedByMessages<TMessage> and a Saga correlation cannot be established a new Saga will be created, otherwise an existing one will be loaded.

  • The last step is as simple as defining a correlation id that can be stable on a daily basis, is there anything better that the money transaction date? e.g. a string like "20160907" will identify all the transactions processed on September the 7th.

The Saga infrastructure concurrency management finally guarantees that 2 Sagas with the same unique correlation id cannot be created.

Is that all folks?

No, we still miss one last step: transactions delivery to the authority. Again we don't want to setup something external to trigger the delivery process. One easy way to do is via Saga Timeouts.

class TransactionMonitorSaga : Saga<TransactionMonitorSaga.SagaState>,
  IAmStartedByMessages<IMoneyTransactionProcessed>,
  IHandleTimeouts<DeliverProcessedTransactions>
{
  public class SagaState : ContainsSagaData
  {
    [Unique]
    public String BatchId { get; set; }
    public List<String> TransactionIds { get; set; }
  }
  
  protected override void ConfigureHowToFindSaga(SagaPropertyMapper<TransactionMonitorSaga.SagaState> mapper)
  {
  	mapper.ConfigureMapping<IMoneyTransactionProcessed>(message => message.BatchDate)
                .ToSaga(sagaData => sagaData.BatchId);
  }
  
  public void Handle( IMoneyTransactionProcessed message )
  {
    //ensure correlation
    if( String.IsNullOrWhiteSpace(this.Data.BatchId))
    {
      this.Data.BatchId = message.BatchDate;
      this.RequestTimeout<DeliverProcessedTransactions>( /* define when, e.g. tomorrow */ );
    }
    //cumulate
    this.Data.TransactionIds.Add( message.TransactionId );
  }
  
  public void Timeout( DeliverProcessedTransactions message )
  {
    //deliver to third party
    //mark as complete
  }
}

We simply changed the DeliverProcessedTransactions to be a Timeout message, that will trigger the Saga instance later, where "later" is defined by us.

Conclusions

We've seen how simple is to leverage NServiceBus Sagas to implement some business batch processing logic. In the sample we had the opportunity to use a simple property on the incoming message as the correlation mechanism, if in your scenario that's not possible you can always inject your own complex Saga finding logic to satisfy whatever correlation need you might have.

That's all folks! By the way, take a look at what's new for Sagas in NServiceBus V6.