You are maybe wondering, why a blog post about scaling Azure Functions? Isn't the whole point of Azure Functions that they scale automatically based on the number of incoming events?
And indeed, you are right this is exactly what Azure Functions does for you out-of-the-box.
I want to talk about how you can put a limit on the scale out. In our case we have a CosmosDB instance and we want to avoid a 429 response (and also save some Request Units along the way) by limiting the througput.
By default, Consumption plan functions scale out to as many as 200 instances, and Premium plan functions will scale out to as many as 100 instances.
To specify a lower maximum for a specific app, we need to modify the functionAppScaleLimit value:
az resource update --resource-type Microsoft.Web/sites -g <RESOURCE_GROUP> -n <FUNCTION_APP-NAME>/config/web --set properties.functionAppScaleLimit=<SCALE_LIMIT>
Another way to limit the scale out behavior is by controlling the concurrency level. The way to control this differs per trigger type. For example, the Service Bus trigger provides both a MaxConcurrentCalls and a MaxConcurrentSessions setting in the host.json file: