title	description	keywords	ms.service	ms.topic	ms.devlang	ms.custom	ms.date	zone_pivot_groups
Azure Functions warmup trigger	Understand how to use the warmup trigger in Azure Functions.	azure functions, functions, event processing, warmup, cold start, premium, dynamic compute, serverless architecture	azure-functions	reference	csharp	devx-track-csharp, devx-track-extended-java, devx-track-js, devx-track-python, devx-track-ts	09/04/2023	programming-languages-set-functions

Azure Functions warmup trigger

This article explains how to work with the warmup trigger in Azure Functions. A warmup trigger is invoked when an instance is added to scale a running function app. The warmup trigger lets you define a function that runs when a new instance of your function app is started. You can use a warmup trigger to preload custom dependencies so your functions are ready to start processing requests immediately. Some actions for a warmup trigger might include opening connections, loading dependencies, or running any other custom logic before your app begins receiving traffic.

The following considerations apply when using a warmup trigger:

The warmup trigger isn't available to apps running on the Consumption plan.
The warmup trigger isn't supported on version 1.x of the Functions runtime.
Support for the warmup trigger is provided by default in all development environments. You don't have to manually install the package or register the extension.
There can be only one warmup trigger function per function app, and it can't be invoked after the instance is already running.
The warmup trigger is only called during scale-out operations, not during restarts or other nonscaling startups. Make sure your logic can load all required dependencies without relying on the warmup trigger. Lazy loading is a good pattern to achieve this goal.
Dependencies created by warmup trigger should be shared with other functions in your app. To learn more, see Static clients.
If the built-in authentication (also known as Easy Auth) is used, HTTPS Only should be enabled for the warmup trigger to get invoked.

Example

::: zone pivot="programming-language-csharp"

[!INCLUDE functions-bindings-csharp-intro-with-csx]

[!INCLUDE functions-in-process-model-retirement-note]

Isolated worker model

The following example shows a C# function that runs on each new instance when added to your app.

:::code language="csharp" source="~/azure-functions-dotnet-worker/samples/Extensions/Warmup/Warmup.cs" range="4-18":::

In-process model

The following example shows a C# function that runs on each new instance when added to your app.

using Microsoft.Azure.WebJobs;
using Microsoft.Extensions.Logging;
 
namespace WarmupSample
{

    //Declare shared dependencies here

    public static class Warmup
    {
        [FunctionName("Warmup")]
        public static void Run([WarmupTrigger()] WarmupContext context,
            ILogger log)
        {
            //Initialize shared dependencies here
            
            log.LogInformation("Function App instance is warm.");
        }
    }
}

::: zone-end ::: zone pivot="programming-language-java"

The following example shows a warmup trigger that runs when each new instance is added to your app.

@FunctionName("Warmup")
public void warmup( @WarmupTrigger Object warmupContext, ExecutionContext context) {
    context.getLogger().info("Function App instance is warm.");
}

::: zone-end
::: zone pivot="programming-language-javascript"

Model v4

The following example shows a JavaScript function with a warmup trigger that runs on each new instance when added to your app:

:::code language="javascript" source="~/azure-functions-nodejs-v4/js/src/functions/warmupTrigger1.js" :::

Model v3

The following example shows a warmup trigger in a function.json file and a JavaScript function that runs on each new instance when added to your app.

Here's the function.json file:

{
    "bindings": [
        {
            "type": "warmupTrigger",
            "direction": "in",
            "name": "warmupContext"
        }
    ]
}

The configuration section explains these properties.

Here's the JavaScript code:

module.exports = async function (warmupContext, context) {
    context.log('Function App instance is warm.');
};

::: zone-end ::: zone pivot="programming-language-typescript"

Model v4

The following example shows a TypeScript function with a warmup trigger that runs on each new instance when added to your app:

:::code language="typescript" source="~/azure-functions-nodejs-v4/ts/src/functions/warmupTrigger1.ts" :::

Model v3

TypeScript samples aren't documented for model v3.

::: zone-end ::: zone pivot="programming-language-powershell"
Here's the function.json file:

{
    "bindings": [
        {
            "type": "warmupTrigger",
            "direction": "in",
            "name": "warmupContext"
        }
    ]
}

PowerShell example code pending.

::: zone-end
::: zone pivot="programming-language-python"

The following example shows a warmup trigger in a function.json file and a Python function that runs on each new instance when it'is added to your app.

Your function must be named warmup (case-insensitive) and there can only be one warmup function per app.

Here's the function.json file:

{
    "bindings": [
        {
            "type": "warmupTrigger",
            "direction": "in",
            "name": "warmupContext"
        }
    ]
}

For more information, see Configuration.

Here's the Python code:

import logging
import azure.functions as func


def main(warmupContext: func.Context) -> None:
    logging.info('Function App instance is warm.')

::: zone-end
::: zone pivot="programming-language-csharp"

Attributes

Both in-process and isolated worker process C# libraries use the WarmupTrigger attribute to define the function. C# script instead uses a function.json configuration file.

Isolated worker model

Use the WarmupTrigger attribute to define the function. This attribute has no parameters.

In-process model

Use the WarmupTrigger attribute to define the function. This attribute has no parameters.

::: zone-end
::: zone pivot="programming-language-java"

Annotations

Warmup triggers don't require annotations. Just use a name of warmup (case-insensitive) for the FunctionName annotation.

::: zone-end
::: zone pivot="programming-language-javascript,programming-language-typescript"

Configuration

Model v4

There are no properties that need to be set on the options object passed to the app.warmup() method.

Model v3

The following table explains the binding configuration properties that you set in the function.json file.

function.json property	Description
type	Required - must be set to `warmupTrigger`.
direction	Required - must be set to `in`.
name	Required - the variable name used in function code. A `name` of `warmupContext` is recommended for the binding parameter.

::: zone-end ::: zone pivot="programming-language-powershell,programming-language-python"

Configuration

The following table explains the binding configuration properties that you set in the function.json file.

function.json property	Description
type	Required - must be set to `warmupTrigger`.
direction	Required - must be set to `in`.
name	Required - the variable name used in function code. A `name` of `warmupContext` is recommended for the binding parameter.

::: zone-end

See the Example section for complete examples.

Usage

::: zone pivot="programming-language-csharp"
The following considerations apply to using a warmup function in C#:

Isolated worker model

Your function must be named warmup (case-insensitive) using the Function attribute.
A return value attribute isn't required.
Use the Microsoft.Azure.Functions.Worker.Extensions.Warmup package
You can pass an object instance to the function.

In-process model

Your function must be named warmup (case-insensitive) using the FunctionName attribute.
A return value attribute isn't required.
You must be using version 3.0.5 of the Microsoft.Azure.WebJobs.Extensions package, or a later version.
You can pass a WarmupContext instance to the function.

::: zone-end
::: zone pivot="programming-language-java" Your function must be named warmup (case-insensitive) using the FunctionName annotation. ::: zone-end ::: zone pivot="programming-language-javascript,programming-language-typescript"

Model v4

See the list of considerations at the top of the page for general usage advice.

Model v3

The function type in function.json must be set to warmupTrigger. ::: zone-end ::: zone pivot="programming-language-powershell,programming-language-python" The function type in function.json must be set to warmupTrigger. ::: zone-end

Next steps

Learn more about Azure functions triggers and bindings
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functions-bindings-warmup.md

functions-bindings-warmup.md

Azure Functions warmup trigger

Example

Isolated worker model

In-process model

Model v4

Model v3

Model v4

Model v3

Attributes

Isolated worker model

In-process model

Annotations

Configuration

Model v4

Model v3

Configuration

Usage

Isolated worker model

In-process model

Model v4

Model v3

Next steps

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functions-bindings-warmup.md

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Azure Functions warmup trigger

Example

Attributes

Annotations

Configuration

Configuration

Usage

Next steps