Lambda handler function structure in AWS - Commands & Configuration

Introduction

A Lambda handler function is the starting point for your AWS Lambda code. It processes events and returns responses, letting you run code without managing servers.

When you want to run code in response to events like file uploads or API calls without managing servers

When you need to process data quickly and automatically in the cloud

When you want to build a simple backend function for a web or mobile app

When you want to automate tasks triggered by cloud services like S3 or DynamoDB

When you want to run code that scales automatically with demand

Commands

This command creates a new Lambda function named 'example-function' using Python 3.9 runtime. It specifies the IAM role for permissions, the handler function, and uploads the zipped code.

Terminal

aws lambda create-function --function-name example-function --runtime python3.9 --role arn:aws:iam::123456789012:role/lambda-execution-role --handler lambda_function.lambda_handler --zip-file fileb://function.zip

Expected Output

{

    "FunctionName": "example-function",

    "FunctionArn": "arn:aws:lambda:us-east-1:123456789012:function:example-function",

    "Runtime": "python3.9",

    "Role": "arn:aws:iam::123456789012:role/lambda-execution-role",

    "Handler": "lambda_function.lambda_handler",

    "CodeSize": 1234,

    "Description": "",

    "Timeout": 3,

    "MemorySize": 128,

    "LastModified": "2024-06-01T12:00:00.000+0000",

    "CodeSha256": "abc123def456ghi789jkl012mno345pqr678stu901vwx234yz567",

    "Version": "$LATEST",

    "TracingConfig": {

        "Mode": "PassThrough"

    },

    "RevisionId": "1a2b3c4d-5678-90ab-cdef-1234567890ab"

}

→

--function-name - Sets the name of the Lambda function

→

--runtime - Specifies the programming language runtime

→

--handler - Defines the file and function name that Lambda calls

This command runs the Lambda function named 'example-function' and saves the output to a file named output.txt.

Terminal

aws lambda invoke --function-name example-function output.txt

Expected Output

{

    "StatusCode": 200,

    "ExecutedVersion": "$LATEST"

}

→

--function-name - Specifies which Lambda function to invoke

This command displays the content of the output.txt file, showing the Lambda function's response.

Terminal

cat output.txt

Expected Output

{"statusCode": 200, "body": "Hello from Lambda!"}

Key Concept

If you remember nothing else from this pattern, remember: the handler function is the entry point AWS Lambda uses to run your code when triggered.

Common Mistakes

Using the wrong handler name in the create-function command

Lambda cannot find the function to run, causing invocation errors

Ensure the handler is in the format 'filename.functionname' matching your code exactly

Not packaging the code correctly into a zip file before upload

Lambda fails to load your code, resulting in deployment errors

Zip your code files properly and reference the zip file in the command

Invoking the function without checking the output file

You miss the function's response and cannot verify it worked

Always check the output file content after invocation to confirm success

Summary

Create a Lambda function with the correct runtime, role, and handler using the AWS CLI.

Invoke the Lambda function and save its output to a file.

Check the output file to see the function's response and verify it ran successfully.