Limit Number of Concurrent Calls Across Azure Function Hosts

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To limit the number of open connections to our database I need to limit the number of simultaneous function calls.

MassTransit has the UseConcurrentMessageLimit which apparently restricts the number of messages each host instance will process simultaneously, is this the same as maxConcurrentCalls from host.json?

The Azure portal has the App Scale Out -> Maximum Scale Out Limit which restricts the number of hosts that will be created.

To limit the number of simultaneous function calls to n should I set Maximum Scale Out Limit * UseConcurrentMessageLimit or maxCurrentCalls = n?

Can I achieve this with one parameter in MassTransit or the Azure Functions configuration?

azureservicebus azure-functions masstransit
1 Answers

With MassTransit, for a given receive endpoint, ConcurrentMessageLimit should be used to limit the number of concurrent consumers executing on that particular receive endpoint. This setting only affects the receive endpoint on that service instance.

If you have multiple instances, and a hard limit of say 100 connections to your back-end database, and each instance has a ConcurrentMessageLimit of 20, then you could have a maximum number of 5 instances to stay within that 100 connection limit (assuming each consumer uses a single connection to the database).

Both UseConcurrentMessageLimit and MaxConcurrentCalls are deprecated and retained only for backwards compatibility. The only configuration value that should be used for current applications is ConcurrentMessageLimit. This is covered in this video.

Azure Functions

Oi, you're talking about Azure Functions. MassTransit has no control over the receiver used to pull messages and deliver them to consumers. I think host.json is where you need to configure it, and it's probably maxConcurrentCalls that would be similar.

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