Singleton Azure function running as separate instances

Question

We have an Azure function that is supposed to be handling several service bus triggers at the same time and what I assume is happening is that it is being split across several instances which is causing some concurrency problems on our end.

We need our function to act as a singleton so we can process requests one at a time without any collisions. From what we looked into in this article (https://learn.microsoft.com/en-us/azure/app-service/webjobs-sdk-how-to#singleton-attribute) we should be able to accomplish this.

Our function looks like this:

[Microsoft.Azure.WebJobs.Singleton(Mode = SingletonMode.Listener)]
[FunctionName("AccountCreatedSubscriber")]
public static void Run([ServiceBusTrigger("accountscontacts-account-created", "license-keys", Connection = "FBISEventBus")]BrokeredMessage message, ILogger log)
{
    log.LogInformation($"{{ Message received from accountscontacts-account-created topic }}");

    // Do Work
    log.LogInformation($"{{ Message sent to account creation handler }}");
}

And for backup we also have this in our host.json file,

{
  "serviceBus": { "maxConcurrentCalls": 1 }
}

But for whatever reason our functions are still running parallel. Any ideas?

Answer 1

1. If your function is on Consumption plan, set WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE_OUT to 1 in Application settings.

2. Check your Azure Function runtime version in portal(Platform features> Function app settings). If it's ~2, we need to modify service bus setting in host.json as below.

   {
       "version": "2.0",
       "extensions": {
           "serviceBus": {
               "messageHandlerOptions": {
                  "maxConcurrentCalls": 1
               }
           }
       }
   }
   

Answer 2

Testing I get the following to work:

1. If your function is on Consumption plan, set WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE_OUT to 1 in Application settings.
2. [Microsoft.Azure.WebJobs.Singleton(Mode = SingletonMode.Function)]

Also since we only have one instance running (thanks to WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE_OUT = 1) the running instances are sharing the same static variables.