functions-bindings-warmup.md

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, java, javascript, python	devx-track-csharp	03/04/2022	programming-languages-set-functions-lang-workers

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's run when a new instance of your function app is started. You can use a warmup trigger to pre-load custom dependencies during the pre-warming process 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. To learn more, see pre-warmed instances.

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 non-scale 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.

Example

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

[!INCLUDE functions-bindings-csharp-intro]

In-process

The following example shows a C# function that runs on each new instance when it's 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 🌞🌞🌞");
        }
    }
}

Isolated process

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

:::code language="csharp" source="~/azure-functions-dotnet-worker/samples/Extensions/Warmup/Warmup.cs" range="9-18":::

C# Script

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

Here's the function.json file:

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

For more information, see Attributes.

public static void Run(WarmupContext warmupContext, ILogger log)
{
    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 run( ExecutionContext context) {
       context.getLogger().info("Function App instance is warm 🌞🌞🌞");
}

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

The following example shows a warmup trigger in a function.json file and a JavaScript function that runs on each new instance when it's 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 (context, warmupContext) {
    context.log('Function App instance is warm 🌞🌞🌞');
};

::: 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 may 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 process C# libraries use the WarmupTrigger attribute to define the function. C# script instead uses a function.json configuration file.

In-process

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

Isolated process

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

C# script

C# script uses a function.json file for configuration instead of attributes.

The following table explains the binding configuration properties for C# script 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 name of the binding parameter, which is usually warmupContext.

---

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

Annotations

Annotations aren't required by a warmup trigger. Just use a name of warmup (case-insensitive) for the FunctionName annotation.

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

In-process

• 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.

Isolated process

• Your function must be named warmup (case-insensitive) using the FunctionName attribute.
• A return value attribute isn't required.
• You can pass an object instance to the function.

C# script

Not supported for version 1.x of the Functions runtime.

---

::: 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-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
• Learn more about Premium plan