NServiceBus - How to control message handler ordering when Bus.Send() occurs on different threads / processes? - nservicebus

Question

Scenario:
I have a scenario where audit messages are sent via NServiceBus. The handlers insert and update a row on a preexisting database table, which we have no remit to change. The requirement is that we have control over the order that messages are handled, so that the Audit data reflects the correct system state. Messages processed out of order may cause the audit data to reflect an incorrect state.
Some of the Audit data is expected in a specific order, however some can be received at any time after the initial message, such as a status update which will be sent several times during the process.
In my test project I have been testing using a server, (specifically the ISpecifyMessageHandlerOrdering functionality) with the end point configured as follows:
public class MyServer : IConfigureThisEndpoint, AsA_Server, ISpecifyMessageHandlerOrdering
{
public void SpecifyOrder(Order order)
{
order.Specify(First<PrimaryCommand>.Then<SecondaryCommand>());
}
}
Because the explicit order of messages is not known, one message, InitialAuditMessage is the initial message, and inherits from PrimaryCommand.
Other messages which are allowed to be received at a later stage inherit from SecondaryCommand.
public class StartAuditMessage : PrimaryCommand
public class UpdateAudit1Message : SecondaryCommand
public class UpdateAudit2Message : SecondaryCommand
public class ProcessUpdateMessage : SecondaryCommand
This works in controlling the handling order of messages where they are sent from the same thread.
This breaks down however, if the messages are sent from separate threads or processes, which makes sense as there is nothing to link the messages as related.
How can I link the messages, say through an ID of some sort so that they are not processed out of order when sent from separate threads? Is this a use case for Sagas?
Also, with regard to status update messages, how can I ensure that messages of the same type are processed in the order in which they were sent?

Answer 1

Whenever you have a requirement for ordered processing you cannot avoid the conclusion that at some point in your processing you need to restrict everything down to a single thread. The single thread guarantees the order in which things are processed.
In some cases you can "scale out" the single thread into multiple threads by splitting the processing by a correlating identifier. The correlation ID allows you to define a logical grouping of messages within which order must be maintained. This allows you to have concurrent threads each performing ordered processing which is more efficient.