Timer function doens't run properly when i set Min replicas = 0

Question

Timer function doens't run properly when i set Min replicas = 0

Vitalii Koretskyi 20

I have dotnet-isolated azure function deployed to azure container apps.

User's image

The function run randomly (with delay from 30 min to 2-5 hours) if is set min replicas to 0.

[Function("EnsureSubscription")]
public async Task Run([TimerTrigger("0 1,31 * * * *")] TimerInfo myTimer)
{
}

But it works properly when min replicas is 1. It runs on 1, 31 min every hour. Why?

Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-24T07:42:10.6366667+00:00

Hello Vitalii Koretskyi,

Thanks for reaching out, when you set minReplicas = 0 in Azure Container Apps, your app is allowed to scale down to zero instances when idle. In this state, no container is running to evaluate and execute your TimerTrigger, which leads to delayed or missed executions. The timer only runs once the app is reactivated—usually due to external triggers or probes. However, when minReplicas = 1 is set, at least one instance is always running, allowing the TimerTrigger to fire reliably and on schedule.

This behavior is documented by Microsoft. In the Container Apps scaling guide, it states: “If you want to ensure that an instance of your revision is always running, set the minimum number of replicas to 1 or higher.” Additionally, the Azure Well-Architected Framework advises to “configure a minimum number of always-ready replicas” to minimize cold starts and maintain availability. Since TimerTrigger relies on an active host, setting minReplicas = 1 is the recommended approach for reliable and timely execution.
Vitalii Koretskyi 20 Reputation points

2025-06-24T08:09:55.22+00:00

But regarding this documentation "All Functions triggers are available in your containerized Functions app. However, only the following triggers can dynamically scale (from zero instances) based on received events when running in a Container Apps environment:" and timer is one of this trigger.
Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-25T00:29:05.8166667+00:00

Hello Vitalii Koretskyi

While the Azure Container Apps Functions overview indicates that Timer triggers can scale from zero using KEDA, in practice this approach comes with limitations. Timer triggers rely on KEDA’s to detect when scheduled events should run. However, if the app is scaled to zero, cold starts and missed polling windows can cause delays or skipped executions, especially for short-interval schedules.

So, Microsoft recommends setting minReplicas = 1 to avoid these issues. This ensures a container instance is always running and the TimerTrigger executes reliably on schedule. As noted in the Container Apps revision scaling guide, “If you want to ensure that an instance of your revision is always running, set the minimum number of replicas to 1 or higher.” This aligns with guidance in the Well-Architected Framework to configure always-ready replicas for critical, time-sensitive workloads.
Vitalii Koretskyi 20 Reputation points

2025-06-25T07:04:24.7933333+00:00

Hello,

This aligns with guidance in the Well-Architected Framework to configure always-ready replicas for critical, time-sensitive workloads.

The link is not correct for Well-Architected Framework (404 - not found).

Timer triggers rely on KEDA’s to detect when scheduled events should run. However, if the app is scaled to zero, cold starts and missed polling windows can cause delays or skipped executions, especially for short-interval schedules.

It will be nice to point those limitations in a documentation of azure container apps. Is it the same for azure service bus trigger?
Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-25T08:55:32.1033333+00:00

Hello Vitalii Koretskyi,

Sorry for the inconvenience this is the document which I am trying to provide you for better understanding Architecture best practices for Azure Container Apps.( This is the same document).

When using Azure Service Bus triggers with Container Apps, similar delays can occur if minReplicaCount is set to 0, due to cold starts and KEDA's polling intervals. To reduce latency and improve reliability, consider setting minReplicaCount > 0 and tuning scaler settings like pollingInterval and cooldownPeriod.
Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-26T08:41:41.4866667+00:00

Hello Vitalii Koretskyi,

I wanted to follow up to ensure the information above has helped clarify things for you. If you have any additional questions or concerns, please feel free to share them here.
Vitalii Koretskyi 20 Reputation points

2025-06-26T14:49:40.09+00:00
Well, yes basically from what u say i have 2 options:

Add min replicas to 1 for function. But it doesn't make sense for serverless to keep the container all the time to run once an hour.

Transform azure function with timer to schedule azure container job.

Am i right?

And azure function as appservice (c# dotnet isolated workder) has the same behavior?
Krishna Chowdary Paricharla 90 Reputation points Microsoft External Staff Moderator

2025-06-27T13:13:48.9233333+00:00
Hello Vitalii Koretskyi,

Thank you for following up. Yes, you’ve understood correctly:

Setting minReplicaCount: 1 in Azure Container Apps keeps one replica running at all times, which helps lower latency. However, as you pointed out, this setup may not be suitable for workloads that run infrequently, such as jobs scheduled once per hour, since it leads to continuous resource usage.

Transforming the timer-based Azure Function into a scheduled Azure Container App Job is a valid and cost-effective approach for such workloads. It maintains the serverless model and only consumes resources during execution.

For Azure Functions (.NET isolated worker), the Consumption plan does have similar cold start behavior. To address this, you might use a Premium plan, which offers Always Ready instances to help reduce startup delays.

Your answer

Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-24T07:42:10.6366667+00:00

Hello Vitalii Koretskyi,

Thanks for reaching out, when you set minReplicas = 0 in Azure Container Apps, your app is allowed to scale down to zero instances when idle. In this state, no container is running to evaluate and execute your TimerTrigger, which leads to delayed or missed executions. The timer only runs once the app is reactivated—usually due to external triggers or probes. However, when minReplicas = 1 is set, at least one instance is always running, allowing the TimerTrigger to fire reliably and on schedule.

This behavior is documented by Microsoft. In the Container Apps scaling guide, it states: “If you want to ensure that an instance of your revision is always running, set the minimum number of replicas to 1 or higher.” Additionally, the Azure Well-Architected Framework advises to “configure a minimum number of always-ready replicas” to minimize cold starts and maintain availability. Since TimerTrigger relies on an active host, setting minReplicas = 1 is the recommended approach for reliable and timely execution.
Vitalii Koretskyi 20 Reputation points

2025-06-24T08:09:55.22+00:00

But regarding this documentation "All Functions triggers are available in your containerized Functions app. However, only the following triggers can dynamically scale (from zero instances) based on received events when running in a Container Apps environment:" and timer is one of this trigger.
Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-25T00:29:05.8166667+00:00

Hello Vitalii Koretskyi

While the Azure Container Apps Functions overview indicates that Timer triggers can scale from zero using KEDA, in practice this approach comes with limitations. Timer triggers rely on KEDA’s to detect when scheduled events should run. However, if the app is scaled to zero, cold starts and missed polling windows can cause delays or skipped executions, especially for short-interval schedules.

So, Microsoft recommends setting minReplicas = 1 to avoid these issues. This ensures a container instance is always running and the TimerTrigger executes reliably on schedule. As noted in the Container Apps revision scaling guide, “If you want to ensure that an instance of your revision is always running, set the minimum number of replicas to 1 or higher.” This aligns with guidance in the Well-Architected Framework to configure always-ready replicas for critical, time-sensitive workloads.
Vitalii Koretskyi 20 Reputation points

2025-06-25T07:04:24.7933333+00:00

Hello,

This aligns with guidance in the Well-Architected Framework to configure always-ready replicas for critical, time-sensitive workloads.

The link is not correct for Well-Architected Framework (404 - not found).

Timer triggers rely on KEDA’s to detect when scheduled events should run. However, if the app is scaled to zero, cold starts and missed polling windows can cause delays or skipped executions, especially for short-interval schedules.

It will be nice to point those limitations in a documentation of azure container apps. Is it the same for azure service bus trigger?
Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-25T08:55:32.1033333+00:00

Hello Vitalii Koretskyi,

Sorry for the inconvenience this is the document which I am trying to provide you for better understanding Architecture best practices for Azure Container Apps.( This is the same document).

When using Azure Service Bus triggers with Container Apps, similar delays can occur if minReplicaCount is set to 0, due to cold starts and KEDA's polling intervals. To reduce latency and improve reliability, consider setting minReplicaCount > 0 and tuning scaler settings like pollingInterval and cooldownPeriod.
Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-26T08:41:41.4866667+00:00

Hello Vitalii Koretskyi,

I wanted to follow up to ensure the information above has helped clarify things for you. If you have any additional questions or concerns, please feel free to share them here.
Vitalii Koretskyi 20 Reputation points

2025-06-26T14:49:40.09+00:00

Well, yes basically from what u say i have 2 options:

Add min replicas to 1 for function. But it doesn't make sense for serverless to keep the container all the time to run once an hour.

Transform azure function with timer to schedule azure container job.

Am i right?

And azure function as appservice (c# dotnet isolated workder) has the same behavior?
Krishna Chowdary Paricharla 90 Reputation points Microsoft External Staff Moderator

2025-06-27T13:13:48.9233333+00:00

Hello Vitalii Koretskyi,

Thank you for following up. Yes, you’ve understood correctly:

Setting minReplicaCount: 1 in Azure Container Apps keeps one replica running at all times, which helps lower latency. However, as you pointed out, this setup may not be suitable for workloads that run infrequently, such as jobs scheduled once per hour, since it leads to continuous resource usage.

Transforming the timer-based Azure Function into a scheduled Azure Container App Job is a valid and cost-effective approach for such workloads. It maintains the serverless model and only consumes resources during execution.

For Azure Functions (.NET isolated worker), the Consumption plan does have similar cold start behavior. To address this, you might use a Premium plan, which offers Always Ready instances to help reduce startup delays.

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Timer function doens't run properly when i set Min replicas = 0

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