Best way to ignore ‘duplicate’ commands

1

Apologies if this has been covered before but my searches haven’t uncovered anything
I have a scheduler which sends a SyncCommand to an endpoint every 5 mins. The command has one of many possible TenantId’s as a property, but each command has a unique commandID
The saga should only do single processing per tenant (multiple tenant processing simultaneously though) and may take a short time or occasionally much longer than 5 mins
What is the best way of ignoring or discarding incoming commands if that tenant is currently processing?
I have looked at outbox but not sure how that would be utilised
Is uniform session applicable?
Or do I just match saga on TenantId and somehow consume the multiple incoming commands, but basically ignoring them since the saga is still processing for that tenantID?

2

@Gobba,

to better understand the scenario, how do you handle the case when the saga processing exceeds the transport timeout?
What transport and persistence are you using?

.m

3

Thanks for your reply

For timeouts, I check if it’s still processing (regularly updating a timestamp in the state during valid processing) and extend the timeout if needed
For valid timeouts I markascomplete, publishing an error notification event

Am using SqlPersistance
Using Azure ServiceBus in production and RabbitMQ for local development

4

You probably need to design a sort of semaphore saga that acts like a decision-maker and a blocker if the saga that does the actual work is still running.

  • The scheduler sends a message (the command with a unique ID and a tenant ID)
  • The semaphore saga is triggered and is mapped to the unique command ID
    • It checks (by looking at the saga data) is a long-running job for the combination to tenant and command is already running
    • If it is, it rejects the command; otherwise, it pulls off the long-running job saga by sending an internal command
  • When the long-running job saga finishes, it replies back to the semaphore saga that removes from its saga data the tenant and command combination acting like a semaphore for simultaneous executions

Does it make sense to you, @Gobba?

.m

5

Thanks for your reply, it does make sense. It sounds a little complicated so will have to digest it further
Much appreciated
Alan

6

@Gobba,

if you need to get on a call to discuss this further, please do not hesitate to contact me. We can go through your requirements and brainstorm options together.

.m

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