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).You can create a C# function by using one of the following C# modes:
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 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.");
}
}
}
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 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.');
};
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,
});
TypeScript samples aren't documented for model v3.
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.
import logging
import azure.functions as func
app = func.FunctionApp()
@app.warm_up_trigger('warmup')
def warmup(warmup) -> None:
logging.info('Function App instance is warm')
For more information, see Configuration.
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.')
Warmup triggers don't require annotations. Just use a name of warmup (case-insensitive) for the FunctionName annotation.
warmupTrigger (case-insensitive).options object passed to the app.warmup() method.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 towarmupTrigger. 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. 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 towarmupTrigger. 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.
UsageThe following considerations apply to using a warmup function in C#:
warmup (case-insensitive) using the Function attribute.Microsoft.Azure.Functions.Worker.Extensions.Warmup packagewarmup (case-insensitive) using the FunctionName attribute.3.0.5 of the Microsoft.Azure.WebJobs.Extensions package, or a later version.WarmupContext instance to the function.Your function must be named warmup (case-insensitive) using the FunctionName annotation.
See the list of considerations at the top of the page for general usage advice.
The function type in function.json must be set to warmupTrigger.
The function type in function.json must be set to warmupTrigger.
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