Queue-triggered function on Flex Consumption won't fire without an always-ready instance

Question

Hi,

I have an Azure Function deployed with two functions: an HTTP receiver that processes a form, populates a Table, and places a message in a queue; and a queue-triggered function that picks up the message and processes the request.

The HTTP function works as expected: the data is received, placed into the table, and the message is placed in the queue.

The queue-triggered function never triggers when messages are added to the queue. There are no errors, it just never starts. However, if I go to "Scale & Concurrency" and add an always-ready instance for the function, the queue is processed immediately.

Is there any way to get a Flex Consumption queue-triggered function to actually run without configuring an always-ready instance? Is there some setting I'm missing here?

Answer 1

Hi @Nicholas Rodriguez

Thank you for reaching out to Microsoft Q&A.

It sounds like you're encountering a common scenario with Azure Functions under the Flex Consumption plan. The issue is that a queue-triggered function may not automatically start unless there's an always-ready instance, which is designed to minimize cold starts when processing messages.

Here's what you can try:

Understanding Flex Consumption: Unlike the Premium plan, Flex Consumption may require that the function be warmed up to process incoming messages. This can delay the processing of queued messages if there isn't an always-ready instance available.

Timer Trigger: One approach to mitigate this is to add a timer-triggered function that keeps the Function App "warm". This function can run at specified intervals (like every 5 minutes) to ensure that the app doesn't go idle, which helps in keeping the queue-triggered function ready to process messages.

Check Settings: Also, double-check your function app settings to ensure that:

• The function is enabled.
  • The queue trigger settings are properly configured.

Unfortunately, for now, having at least one always-ready instance might be the best solution for immediate processing without delays.

Answer 2

In Azure Functions using the Flex Consumption plan, queue-triggered functions require an always-ready instance to process messages from the queue. This is because, by default, there are no instances running to handle the queue messages, which can lead to the behavior you are experiencing where the function does not trigger without an always-ready instance configured.

The always-ready feature allows you to specify a minimum number of instances that are always running, which helps reduce cold start latency and ensures that your queue-triggered functions can execute immediately when messages are available. Without this configuration, the function will not fire as there are no active instances to process the queue messages.

Currently, there is no setting that allows queue-triggered functions to run without configuring an always-ready instance in the Flex Consumption plan. If you want your queue-triggered function to process messages without delay, you will need to keep the always-ready instance configuration.

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References:

• Azure Functions Flex Consumption plan hosting
• Set always ready instance counts