Assign a priority on subscribers of a Queue using Azure Service Bus

Question

I know it is possible to assign a priority to messages but this is not what I am looking for, I need to assign a priority to subscribers.

For example, 2 services (A and B) are subscribed on the same queue but I would like to have 1 subscriber (A) at higher priority. This means that service A would process all messages that enter the queue when the 2 services are available, and only when service A is busy then service B would process new messages.

Is that possible using .NET and Azure Service Bus?

More details:

Each message takes a good amount of time to process (30-60 seconds) so a service is considered busy as soon as it is processing a single message and each service has a single processor.

One subscriber (A) is enough when the message flow is really low (less than 1 message per minute) but when the flow increases, more services (B, C, etc.) will be booted up to satisfy the demand.

I would like to always prioritize service A so it is 100% busy, then use service B if 1 is not enough, then use C if 2 are not enough, etc.

As the demand lowers, I would like to stop the services in the reverse order (C first, then B). For that, subscribers priority is needed, so that C stops receiving messages when 2 services is enough for the current load.

Edit

Thanks to the current answer, I found out more about Azure Autoscale and the usage of terminate notifications: https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-terminate-notification

This is all great, interesting and of consideration, but I would still like to know the answer to my original question:

Is it possible to assign a priority on a subscriber, when subscribing to an Azure Service Bus Queue?

Answer 1

You do not specify what you mean by the term “service”, but one way to achieve a scalable yet affordable implementation of multiple consuming apps from a queue is to host them in function apps on an app service plan and add a scale out rule based on the queue length to the plan. This is described here https://learn.microsoft.com/en-us/azure/azure-monitor/autoscale/autoscale-get-started