Approach for creating saga on concurrent messages of the same type
M___
Hello,
I am currently implementing really simple saga being able to group messages of one type and actually trigger some events depending on their amount, for instance publish the message on first message of type correlating on some property. My current simplified implementation looks like this:
'''cs
public class FileUploadSaga : MassTransitStateMachine<FileUploadSagaData>
{
public FileUploadSaga()
{
}
public FileUploadSaga()
{
Configure();
}
private void Configure()
{
InstanceState(x => x.CurrentState);
Event(() => FileSavedEvent,
x => x.CorrelateBy(data => data.FilePath, ctx => ctx.Message.FilePath)
.SelectId(ctx => Guid.NewGuid()));
Initially(
When(FileSavedEvent)
.Then(ctx =>
{
ctx.Instance.FilePath = ctx.Data.FilePath;
ctx.Instance.Updated = DateTime.UtcNow;
})
.Publish(...)
.TransitionTo(SavedOnce));
During(SavedOnce,
When(FileSavedEvent)
.Then(ctx =>
{
ctx.Instance.Updated = DateTime.UtcNow;
})
.Finalize());
SetCompletedWhenFinalized();
}
public State SavedOnce { get; private set; }
public Event<IFileSavedEvent> FileSavedEvent { get; set; }
}
'''
The problem is that because of concurrency the Initially() handlers will be called twice if messages will come at the same time (and it is not very rare case in my application because I have huge amount of workers each of them reporting that their work is done, file saved to their place). After the handlers being executed the exception will occur on database level (insert). I was really wondering what is the recommended way to implement such functionality?
Yes, the message will be retried eventually, but the duplicate message from Publish delegate will already be sent at that point. Sending some in-memory event to already created saga seem a big unreliable as well as if I understand correctly message will be marked as ACKed but the event will not finish execution at that point, too?
I am sorry if this is a duplicate already, but I spent quite a while trying to find an answer to the question and wasn't able to. There was a discussion about Initialize method a while ago but there was no further information on this available.
Thanks!
Chris Patterson
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Chris Patterson
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