Microsoft Azure functions (nodejs), WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE

Using consumption plan. I created a service bus nodejs function trigger app.

I do not use sessions. Tested with two topics - partitioning enabled/disabled.

const timeout = ms => new Promise(res => setTimeout(res, ms))

module.exports = async function(context, mySbMsg) {
    context.log('message start:', mySbMsg);
    await timeout(60000)
    context.log('message done:', mySbMsg);
};

host.json:

{
  "version": "2.0",
  "extensionBundle": {
    "id": "Microsoft.Azure.Functions.ExtensionBundle",
    "version": "[3.3.0, 4.0.0)"
  },
  "extensions": {
    "serviceBus": {
      "prefetchCount": 1,
      "messageHandlerOptions": {
        "autoComplete": true,
        "maxConcurrentCalls": 5,
        "maxAutoRenewDuration": "00:09:30"
      }
    }
  },
  "functionTimeout": "00:09:55"
}

With WEBSITE_MAX_DYNAMIC_APPLICATION_SCALE_OUT = 1

I expect to see 5 requests/minute per VM running.

Sending 100 messages, I expect to see 5 messages/minute.

I do see 1 vm running in the live metrics. However, I am seeing 1 message a minute in the logs.

CodePudding user response：

Yes @Teebu, "prefetchCount":1 is the culprit.

Example:

If prefetchCount is set to 200, maxConcurrentCalls is set to 16 (assume), then 200 messages will be prefetched to a specific instance, but only 16 messages processed at a time.

Prefetched Messages can be gotten by the MessageSender whereas the MaxConcurrency is only processed by the client code.

MaxConcurrentCalls - how many messages will a single MessageReceiver process at the same time.

PrefetchCount - When initiating a call to receive a message, a single MessageReceiver can retrieve up to how many messages.

To the same value, setting those two is counterproductive.

PrefetchCount should be larger than the number of messages processed concurrently.

In a simple:

The maximum number of messages prefetched by the underlying MessageReceiver utilized by Azure Functions SDK is determined by the prefetchCount.

To ensure that not too many messages are prefetched and locks are lost while waiting for processing, ensure that prefetchCount is properly configured with the value defined for maxConcurrentCalls.