Azure function does not trigger

Question

Azure function does not trigger

Anca Corbos 25

Hello,

we have an Azure Function on plan EP2, which is triggered by a queue. Sometimes the function is not triggered until we visit the function in the Azure Portal.

The queue maxPollingInterval is set to 2 seconds. There should always be an instace ready in EP2.

Today, for example, we found 3 messages waiting in the queue. As soon as I went to see the last invocations of the function in the portal, they were picked up.

You see the delay in the screenshot: 41 minutes, 28 minutes, repectively 25 minutes (local time is UTC+1 hour)

User's image

Can someone please explain?

Here is an invocation ID for Microsoft to quickly find our data: 438942da-ce72-4ba4-9f36-77f1216ce8fe

Thank you,

Anca

Khadeer Ali 5,990 Reputation points Microsoft External Staff Moderator

2025-03-17T16:22:36.7666667+00:00

@Anca Corbos ,

Thank you for reaching out. We acknowledge the delay in the function execution and understand the impact this may have on your workflow. We will analyze the invocation logs and function runtime behavior to determine the root cause and will get back to you on this.
Jadapalli, Srikanth 0 Reputation points

2025-03-18T11:02:38.3266667+00:00

We were also facing the exact same issue in one of the critical solution where AZ function app do processing of sales orders.

We are eager to know the root cause of the issue
RithwikBojja 3,055 Reputation points Microsoft External Staff Moderator

2025-03-19T03:26:13.2533333+00:00

This issue occurs because of function app cold start
RithwikBojja 3,055 Reputation points Microsoft External Staff Moderator

2025-03-19T04:36:41.5333333+00:00

Hi @Anca Corbos,

This issue occurs because of cold start of Azure Function App. So after opening the function app and checking the details, then function gets triggered. This clearly explains that when you open the function app, it gets warm. Before opening it was in Cold Start State. Be it any plan one need to make sure function is always warm to get triggered. Even though EP2 does not scale to zero, it still stops running when idle for too long.

To keep function warm, you can use timer trigger function (running every 5 min or every other min) which always runs and make sure that function is warm. Ms-Doc. This is like a keep alive function.

You can increase Always ready instances in Scale Out:

Whereas in Premium V3 plan you will have a always on option, which keeps the function app always warm.

Hope this helps.

If the answer is helpful, please click Accept Answer and kindly upvote it. If you have any further questions about this answer, please click Comment.
Anca Corbos 25 Reputation points

2025-03-19T07:47:48.88+00:00
"Be it any plan one need to make sure function is always warm to get triggered. Even though EP2 does not scale to zero, it still stops running when idle for too long."

This must be a joke.

Why do we pay premium then?

From the documentation you linked:

"Premium plan hosting provides the following benefits to your functions:

Avoid cold starts with warm instances. [...]"

"Always ready instances

In the Premium plan, you can have your app always ready on a specified number of instances. Your app runs continuously on those instances, regardless of load."

Why should I "increase Always ready instances in Scale Out" if there is one per default and that was not not used at the moment?
Deleted

This comment has been deleted due to a violation of our Code of Conduct. The comment was manually reported or identified through automated detection before action was taken. Please refer to our Code of Conduct for more information.
Deleted

This comment has been deleted due to a violation of our Code of Conduct. The comment was manually reported or identified through automated detection before action was taken. Please refer to our Code of Conduct for more information.
Anca Corbos 25 Reputation points

2025-03-19T10:21:41.3033333+00:00
I need an official answer from Microsoft, respecting the Developer Support Plan we booked additionally before starting this inquiry.

We have an Azure Function in the Premiun Plan, SKU EP2.

In the Overview of plans they say about the premium plan:

"Automatically scales based on demand using prewarmed workers, which run applications with no delay after being idle, runs on more powerful instances, and connects to virtual networks."

Also, in the Azure Portal, on the function app, blade Scale out, the tooltip for Always ready instances says "The number of instances that are always ready and warm for this function app"

We are not on a Dedicated Plan! Premium V3, which was proposed here, is not possible in the Premium Plan, it is a pricing tier on the Dedicated Plan.

But anyway, let's look at the documentation for the Dedicated plan/Always on option https://learn.microsoft.com/en-us/azure/azure-functions/dedicated-plan#always-on

"When you run your app on an App Service plan, you should enable the Always on setting so that your function app runs correctly. On an App Service plan, the Functions runtime goes idle after a few minutes of inactivity. The Always on setting is available only on an App Service plan. In other plans, the platform activates function apps automatically. If you choose not to enable Always on, you can reactivate an idled app in these ways:

Send a request to an HTTP trigger endpoint or any other endpoint on the app. Even a failed request should wake up your app.

Access your app in the Azure portal."

Again: "In other plans, the platform activates function apps automatically"

In other words: Always on is not an option in the Premium Plan because the always ready instances should be by default always on.

At least that's what one understands from the documentation.
VenkateshDodda-MSFT 24,951 Reputation points Microsoft Employee Moderator

2025-03-19T11:09:21.11+00:00
@Anca Corbos Thanks for your patience on this.

Based on the shared innvocationId , I have looked the backend logs, and I can see function app is running with high CPU utilization the above-mentioned time frame.

You can troubleshoot this by using the built-in detectors (Function App down or Reporting Errors & Function Logs detectors) under diagnose and solve problems of your function app.

Reference troubleshooting guide to troubleshoot high cpu :

https://azure.github.io/AppService/2020/10/05/Diagnose-App-with-High-CPU.html

https://www.youtube.com/watch?v=2kewsEVn9I4

Could you please check the below

On an average How, much time does you function takes to execute individual message

Did you have observed the same behaviour when you are running the function locally. As we can see high cpu utilization around 90% of the cpu cycles.

I would suggest you add additional logging to know how much time an individual thread is taking to execute or if there any wait and retry logic in your application code is causing this.

I would you scaling up to next tiers and check whether that would help in fixing the High CPU issues.
Anca Corbos 25 Reputation points

2025-03-19T11:47:17.11+00:00

@VenkateshDodda-MSFT thank you for the hints!

Yes, sometimes we're at the CPU limit. It depends on the amount of data we're processing.

Hard to say the average, sometimes 2 seconds, but it can also be over 10 minutes (the case at 9:52). And yes, it's the same locally.

Is this the reason why the function is not triggered?
VenkateshDodda-MSFT 24,951 Reputation points Microsoft Employee Moderator

2025-03-24T10:15:30.85+00:00

@Anca Corbos Thanks for your response and follow up questions on this. This could be one of the reason which we have seen in the past.

Accepted answer

0 additional answers

Your answer

Khadeer Ali 5,990 Reputation points Microsoft External Staff Moderator

2025-03-17T16:22:36.7666667+00:00

@Anca Corbos ,

Thank you for reaching out. We acknowledge the delay in the function execution and understand the impact this may have on your workflow. We will analyze the invocation logs and function runtime behavior to determine the root cause and will get back to you on this.
Jadapalli, Srikanth 0 Reputation points

2025-03-18T11:02:38.3266667+00:00

We were also facing the exact same issue in one of the critical solution where AZ function app do processing of sales orders.

We are eager to know the root cause of the issue
RithwikBojja 3,055 Reputation points Microsoft External Staff Moderator

2025-03-19T03:26:13.2533333+00:00

This issue occurs because of function app cold start
RithwikBojja 3,055 Reputation points Microsoft External Staff Moderator

2025-03-19T04:36:41.5333333+00:00

Hi @Anca Corbos,

This issue occurs because of cold start of Azure Function App. So after opening the function app and checking the details, then function gets triggered. This clearly explains that when you open the function app, it gets warm. Before opening it was in Cold Start State. Be it any plan one need to make sure function is always warm to get triggered. Even though EP2 does not scale to zero, it still stops running when idle for too long.

To keep function warm, you can use timer trigger function (running every 5 min or every other min) which always runs and make sure that function is warm. Ms-Doc. This is like a keep alive function.

You can increase Always ready instances in Scale Out:

Whereas in Premium V3 plan you will have a always on option, which keeps the function app always warm.

Hope this helps.

If the answer is helpful, please click Accept Answer and kindly upvote it. If you have any further questions about this answer, please click Comment.
Anca Corbos 25 Reputation points

2025-03-19T07:47:48.88+00:00

"Be it any plan one need to make sure function is always warm to get triggered. Even though EP2 does not scale to zero, it still stops running when idle for too long."

This must be a joke.

Why do we pay premium then?

From the documentation you linked:

"Premium plan hosting provides the following benefits to your functions:

Avoid cold starts with warm instances. [...]"

"Always ready instances

In the Premium plan, you can have your app always ready on a specified number of instances. Your app runs continuously on those instances, regardless of load."

Why should I "increase Always ready instances in Scale Out" if there is one per default and that was not not used at the moment?
Deleted

This comment has been deleted due to a violation of our Code of Conduct. The comment was manually reported or identified through automated detection before action was taken. Please refer to our Code of Conduct for more information.
Anca Corbos 25 Reputation points

2025-03-19T10:21:41.3033333+00:00

I need an official answer from Microsoft, respecting the Developer Support Plan we booked additionally before starting this inquiry.

We have an Azure Function in the Premiun Plan, SKU EP2.

In the Overview of plans they say about the premium plan:

"Automatically scales based on demand using prewarmed workers, which run applications with no delay after being idle, runs on more powerful instances, and connects to virtual networks."

Also, in the Azure Portal, on the function app, blade Scale out, the tooltip for Always ready instances says "The number of instances that are always ready and warm for this function app"

We are not on a Dedicated Plan! Premium V3, which was proposed here, is not possible in the Premium Plan, it is a pricing tier on the Dedicated Plan.

But anyway, let's look at the documentation for the Dedicated plan/Always on option https://learn.microsoft.com/en-us/azure/azure-functions/dedicated-plan#always-on

"When you run your app on an App Service plan, you should enable the Always on setting so that your function app runs correctly. On an App Service plan, the Functions runtime goes idle after a few minutes of inactivity. The Always on setting is available only on an App Service plan. In other plans, the platform activates function apps automatically. If you choose not to enable Always on, you can reactivate an idled app in these ways:

Send a request to an HTTP trigger endpoint or any other endpoint on the app. Even a failed request should wake up your app.

Access your app in the Azure portal."

Again: "In other plans, the platform activates function apps automatically"

In other words: Always on is not an option in the Premium Plan because the always ready instances should be by default always on.

At least that's what one understands from the documentation.
VenkateshDodda-MSFT 24,951 Reputation points Microsoft Employee Moderator

2025-03-19T11:09:21.11+00:00

@Anca Corbos Thanks for your patience on this.

Based on the shared innvocationId , I have looked the backend logs, and I can see function app is running with high CPU utilization the above-mentioned time frame.

You can troubleshoot this by using the built-in detectors (Function App down or Reporting Errors & Function Logs detectors) under diagnose and solve problems of your function app.

Reference troubleshooting guide to troubleshoot high cpu :

https://azure.github.io/AppService/2020/10/05/Diagnose-App-with-High-CPU.html

https://www.youtube.com/watch?v=2kewsEVn9I4

Could you please check the below

On an average How, much time does you function takes to execute individual message

Did you have observed the same behaviour when you are running the function locally. As we can see high cpu utilization around 90% of the cpu cycles.

I would suggest you add additional logging to know how much time an individual thread is taking to execute or if there any wait and retry logic in your application code is causing this.

I would you scaling up to next tiers and check whether that would help in fixing the High CPU issues.
Anca Corbos 25 Reputation points

2025-03-19T11:47:17.11+00:00

@VenkateshDodda-MSFT thank you for the hints!

Yes, sometimes we're at the CPU limit. It depends on the amount of data we're processing.

Hard to say the average, sometimes 2 seconds, but it can also be over 10 minutes (the case at 9:52). And yes, it's the same locally.

Is this the reason why the function is not triggered?
VenkateshDodda-MSFT 24,951 Reputation points Microsoft Employee Moderator

2025-03-24T10:15:30.85+00:00

@Anca Corbos Thanks for your response and follow up questions on this. This could be one of the reason which we have seen in the past.

Answer 1

@Anca Corbos Thanks for your patience on this. I am summarizing the offline investigations and posting the same answer for the benefit of other community members.

Issue: Azure Function took around 40 minutes to consume the message from storage queue.

Root Cause:

During the specified time frame, we observed high CPU usage, which affected the function host's ability to execute background tasks like queue polling. Queue polling relies on asynchronous I/O operations, and excessive CPU-bound tasks can exhaust the .NET thread pool, preventing new threads from being scheduled for polling.
To identify the cause of the 100% CPU usage and host thread starvation during the first message execution, review the Application Insights logs. Also, check the application code whether you are creating multiple new threads instead of completing the existing thread to complete the operation this might be one of the causes for high CPU.
Finally, it is recommended to disable Dynamic Concurrency to avoid the known issue of Azure functions getting stuck running at 100% CPU Azure function gets stuck running to 100% CPU.

Hope this helps, let me know if you have any further questions on this.

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Azure function does not trigger

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