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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:
warmup
(case-insensitive).A C# function can be created by using one of the following C# modes:
Important
Support will end for the in-process model on November 10, 2026. We highly recommend that you migrate your apps to the isolated worker model for full support.
The following example shows a C# function that runs on each new instance when added to your app.
using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Logging;
namespace SampleApp
{
public static class Warmup
{
[Function(nameof(Warmup))]
public static void Run([WarmupTrigger] object warmupContext, FunctionContext context)
{
var logger = context.GetLogger(nameof(Warmup));
logger.LogInformation("Function App instance is now warm!");
}
}
}
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.");
}
The following example shows a JavaScript function with a warmup trigger that runs on each new instance when added to your app:
const { app } = require('@azure/functions');
app.warmup('warmupTrigger', {
handler: (warmupContext, context) => {
context.log('Function App instance is warm.');
},
});
The following example shows a TypeScript function with a warmup trigger that runs on each new instance when added to your app:
import { app, InvocationContext, WarmupContext } from '@azure/functions';
export async function warmupFunction(warmupContext: WarmupContext, context: InvocationContext): Promise<void> {
context.log('Function App instance is warm.');
}
app.warmup('warmup', {
handler: warmupFunction,
});
Here's the function.json file:
{
"bindings": [
{
"type": "warmupTrigger",
"direction": "in",
"name": "warmupContext"
}
]
}
PowerShell example code pending.
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.')
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.
Use the WarmupTrigger
attribute to define the function. This attribute has no parameters.
Warmup triggers don't require annotations. Just use a name of warmup
(case-insensitive) for the FunctionName
annotation.
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. |
See the Example section for complete examples.
The following considerations apply to using a warmup function in C#:
warmup
(case-insensitive) using the Function
attribute.Microsoft.Azure.Functions.Worker.Extensions.Warmup
packageYour function must be named warmup
(case-insensitive) using the FunctionName
annotation.
The function type in function.json must be set to warmupTrigger
.
Events
Mar 17, 11 PM - Mar 21, 11 PM
Join the meetup series to build scalable AI solutions based on real-world use cases with fellow developers and experts.
Register now