Azure function Flex Consumption delay in service bus trigger

Question

I've got a V4 function app triggering on service bus running on the flex consumption plan with 2 x always ready instances.

I've recently noticed slow processing times for the azure function to pickup the messages from the service bus ranging from 20ms up to 15 seconds. Even though the function is configured with 2 x always ready instances it does not appear to make use of the ready instances as the trace logs show new instances being used.

Is there any recommendations to resolve this?

Answer 1

@Shaun Squires ,

Thank you for reaching out.

You're seeing inconsistent trigger latency (20ms to 15s) in your Azure Function V4 app using Service Bus on the Flex Consumption plan, despite having 2 always ready instances configured.

After reviewing the current platform behavior and configuration options, here’s what we can confirm:

• Always Ready Instances: These are designed to reduce cold start latency. However, under certain conditions—such as scaling events or platform-level decisions—new instances may still be initialized, which can explain the behavior you're observing.
• Concurrency Settings: Review the concurrency settings for your function. If the concurrency is set too low, it may limit the number of messages processed simultaneously, leading to delays. Always ready instances
• Performance Optimization: Try using the Performance Optimizer tool (available via Azure Load Testing) to simulate and tune your configuration for optimal responsiveness.

If latency is a critical concern, you may also consider the Premium Plan, which offers more predictable warm instance behavior and scaling control.

Hope this helps. Do let us know if you have any further queries.

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