Understanding AWS Lambda

AWS Lambda is a serverless compute service that allows you to run code without provisioning or managing servers. It automatically scales your application by running code in response to each trigger, such as HTTP requests via Amazon API Gateway, changes in data in an Amazon S3 bucket, or updates in a DynamoDB table.

Identifying the 'Handler Not Found' Symptom

When deploying a Lambda function, you might encounter an error message stating 'Handler Not Found'. This typically occurs when AWS Lambda cannot locate the specified handler function in your code.

Common Error Message

The error message might look like this:

{
"errorMessage": "Handler 'index.handler' missing on module 'index'",
"errorType": "HandlerNotFound"
}

Exploring the Root Cause

The root cause of the 'Handler Not Found' error is usually a mismatch between the handler name specified in the Lambda configuration and the actual function in your code. This can happen due to typos, incorrect file names, or incorrect function names.

Handler Configuration

The handler is specified in the format fileName.functionName. For example, if your handler is defined as index.handler, AWS Lambda expects a file named index.js (or index.py for Python) with a function named handler.

Steps to Resolve 'Handler Not Found'

Follow these steps to resolve the 'Handler Not Found' issue:

Step 1: Verify Handler Configuration

Ensure that the handler name in your Lambda function configuration matches the actual function in your code. Check the following:

• File name matches the first part of the handler (e.g., index in index.handler).
• Function name matches the second part of the handler (e.g., handler in index.handler).

Step 2: Check File and Function Names

Open your code file and verify that the function is correctly defined. For example, in a Node.js environment, you should have:

exports.handler = async (event) => {
// Your code here
};

For Python, it should look like:

def handler(event, context):
# Your code here


Step 3: Update Lambda Configuration

If you find any discrepancies, update the Lambda function configuration to match your code. You can do this via the AWS Management Console or AWS CLI:

aws lambda update-function-configuration --function-name YourFunctionName --handler index.handler

Additional Resources

For more information on AWS Lambda handlers, you can refer to the AWS Lambda Programming Model documentation. Additionally, the AWS Lambda Homepage provides further insights into the service.