Lambda Warm-Up: Keep Functions Hot with CloudWatch Events (2026)

Lambda functions can become sluggish for the first request after a period of inactivity because the underlying execution environment needs to be initialized.

Here’s how you can keep your Lambda functions "warm" and ready to go using CloudWatch Events.

Imagine you have a critical Lambda function that processes incoming webhook requests. If this function hasn’t been invoked in a while, the very first request might take several seconds longer than usual to complete. This is because AWS Lambda needs to:

1. Allocate an execution environment: Find a container to run your code.
2. Download your code: Transfer your function’s deployment package.
3. Initialize the runtime: Start the language runtime (e.g., Node.js, Python).
4. Run your initialization code: Execute any code outside of your main handler function.

All these steps contribute to the "cold start" latency. For latency-sensitive applications, this can be unacceptable.

The Solution: Scheduled "Pings"

The most straightforward way to combat cold starts is to proactively invoke your Lambda function on a schedule. This keeps the execution environment active. CloudWatch Events (now EventBridge) is perfect for this.

Let’s say you want to keep your my-critical-lambda function warm.

Step 1: Create a CloudWatch Event Rule

You’ll create a rule that triggers on a schedule. A common schedule is every 5 minutes, but you can adjust this based on your function’s inactivity period and tolerance for cold starts.

Here’s how you’d create this rule using the AWS CLI:

aws events put-rule \
    --name "KeepMyCriticalLambdaWarm" \
    --schedule-expression "rate(5 minutes)" \
    --state ENABLED

This command creates a rule named KeepMyCriticalLambdaWarm that will fire every 5 minutes.

Step 2: Add a Target to the Rule

Now, you need to tell the rule what to do when it fires. In this case, it will invoke your Lambda function.

aws events put-targets \
    --rule "KeepMyCriticalLambdaWarm" \
    --targets "Id"="1", "Arn"="arn:aws:lambda:us-east-1:123456789012:function:my-critical-lambda"

Replace us-east-1 and 123456789012 with your actual AWS region and account ID, and my-critical-lambda with your function’s name.

This command configures the KeepMyCriticalLambdaWarm rule to invoke my-critical-lambda.

Step 3: Grant Permissions

Your CloudWatch Events rule needs permission to invoke your Lambda function. You can grant this permission using the Lambda add-permission command:

aws lambda add-permission \
    --function-name my-critical-lambda \
    --statement-id "EventInvokePermission" \
    --action "lambda:InvokeFunction" \
    --principal "events.amazonaws.com" \
    --source-arn "arn:aws:events:us-east-1:123456789012:rule/KeepMyCriticalLambdaWarm"

Again, update the region, account ID, and function name. The source-arn is crucial here, as it specifies which CloudWatch Event rule is allowed to invoke the function.

What Happens Internally?

When the CloudWatch Event rule triggers, it sends an event payload to your Lambda function. This payload is typically a JSON object describing the event itself. For a scheduled event, it might look something like this:

{
  "version": "0",
  "id": "EXAMPLE1-1234-1234-1234-EXAMPLE123456",
  "detail-type": "Scheduled Event",
  "source": "aws.events",
  "account": "123456789012",
  "time": "2023-10-27T10:00:00Z",
  "region": "us-east-1",
  "resources": [
    "arn:aws:events:us-east-1:123456789012:rule/KeepMyCriticalLambdaWarm"
  ],
  "detail": {}
}

Your Lambda function’s handler will receive this event. You don’t necessarily need to do anything with this event data within your handler. The mere act of invoking the function, even with an empty payload, causes Lambda to reuse an existing execution environment if one is available or spin up a new one if not. This keeps the environment warm.

Fine-Tuning and Considerations

• Frequency: How often should you ping? If your function is invoked, say, once every 10 minutes, pinging it every 5 minutes is overkill. If your function is only invoked every hour, pinging it every 5 minutes might be too aggressive for cost optimization, but effective for latency. A good starting point is to ping at a frequency that is half of your typical longest inactivity period.
• Payload: While you can send specific payloads to your "warm-up" invocations, for simple warming, an empty or generic payload is sufficient. Your handler code should be written to gracefully accept any event structure, or at least the structure of your scheduled event.
• Cost: You are billed for Lambda invocations. These scheduled invocations are billed as normal Lambda executions. If your function is very large or complex, the initialization time for a cold start can be significant, and the cost of keeping it warm might be less than the cost of frequent cold starts.
• Execution Role: Ensure the Lambda function’s execution role has the necessary permissions if your warm-up invocation does need to perform actions (e.g., accessing a database).
• Regionality: Remember to set up these rules and permissions in the same AWS region as your Lambda function.

The "No-Op" Handler

Often, you’ll want your warm-up handler to do as little as possible. Here’s a simple Python example:

def lambda_handler(event, context):
    # This function is just for warming up the environment.
    # No actual work needs to be done here.
    print("Lambda function warmed up successfully.")
    return {
        'statusCode': 200,
        'body': 'Warm'
    }

This handler logs a message and returns a simple success response. It doesn’t perform any external API calls or heavy computation, minimizing the cost and potential failure points of the warm-up invocation itself.

The Next Hurdle: Concurrency Limits

Once you’ve successfully kept your critical functions warm, you might start noticing issues with other, less critical functions experiencing their own cold starts, or even hitting concurrency limits if your warm-up strategy is applied too broadly.