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I'm posting this question to see if I'm understanding parallelism in Azure Functions correctly, and particularly Durable Functions.

The ability to set max degree of parallelism was recently added to Azure Functions using az cli: https://github.com/Azure/azure-functions-host/issues/1207

az resource update --resource-type Microsoft.Web/sites -g <resource_group> -n <function_app_name>/config/web --set properties.functionAppScaleLimit=<scale_limit>

I've applied this to my Function App, but what I'm unsure of is how this plays with the MaxConcurrentOrchestratorFunctions and MaxConcurrentActivityFunctions settings for Durable Functions.

Would the below lead to a global max of 250 concurrent activity functions?

  • functionAppScaleLimit: 5
  • MaxConcurrentOrchestratorFunctions: 5
  • MaxConcurrentActivityFunctions: 10
Joe Eng
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2 Answers2

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Referring to the link you shared to limit scaling this functionAppScaleLimit will help you to specify the maximum number of instances for your function. Now coming to MaxConcurrentOrchestratorFunctions : sets the maximum number of orchestrator functions that can be processed concurrently on a single host instance and MaxConcurrentActivityFunctions the maximum number of activity functions that can be processed concurrently on a single host instance. Refer to this

Now, I am explaining what MaxConcurrentOrchestratorFunctions does , which would help you understand how it works:

MaxConcurrentOrchestratorFunctions controls how many orchestrator functions can be loaded into memory at any given time. If you set concurrency to 1 and then start 10 orchestrator functions, only one will be loaded in memory at a time. Remember that if an orchestrator function calls an activity function, the orchestrator function will unload from memory while it waits for a response. During this time, another orchestrator function may start. The effect is that you will have as many as 10 orchestrator functions running in an interleaved way, but only 1 should actually be executing code at a time.

The motivation for this feature is to limit CPU and memory used by orchestrator code. It's not going to be useful for implementing any kind of singleton pattern. If you want to limit the number of active orchestrations, then you will need to implement this.

JayaChatterjee-MSFT
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  • Okay, so it sounds like global max of 250 concurrent activity functions based on my example would still be the case, correct? – Joe Eng Aug 26 '20 at 23:25
  • I believe https://github.com/Azure/azure-functions-durable-extension/issues/1272#issuecomment-606789083 - might help you. – JayaChatterjee-MSFT Aug 27 '20 at 06:53
  • Also, "These settings are useful to help manage memory and CPU usage on a single VM. However, when scaled out across multiple VMs, each VM has its own set of limits. These settings can't be used to control concurrency at a global level." - check the notes part - https://learn.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-perf-and-scale#functions-20 Please 'Accept as answer' and ‘Up-vote’ if it helped you so that it can help others in the community looking for help on similar topics. – JayaChatterjee-MSFT Aug 27 '20 at 06:53
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    @JoeyEng, your global max of activity functions would be 50. This is based on 5 app instances as specified by `functionAppScaleLimit` and 10 activity functions as specified by `MaxConcurrentActivityFunctions`. The relationship between the number of orchestrator function executions and activity function executions depends entirely on your specific implementation. You could have1-1,000 orchestration(s) that spawn 1-1,000 activities. Regardless, the settings you propose will ensure there are never more than 5 orchestrations and 10 activities running concurrently on a single function instance. – LeastOne Dec 22 '20 at 19:32
  • @LeastOne this is old but if you post your comment as an answer I'll accept it. – Joe Eng May 13 '21 at 18:37
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Your global max of activity functions would be 50. This is based on 5 app instances as specified by functionAppScaleLimit and 10 activity functions as specified by MaxConcurrentActivityFunctions. The relationship between the number of orchestrator function executions and activity function executions depends entirely on your specific implementation. You could have 1-1,000 orchestration(s) that spawn 1-1,000 activities. Regardless, the settings you propose will ensure there are never more than 5 orchestrations and 10 activities running concurrently on a single function instance.

LeastOne
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