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.

Practice

1. What are the two main parameters that every AWS Lambda handler function must accept?

easy

A. event and context

B. request and response

C. input and output

D. data and callback

Solution

1. Step 1: Understand Lambda handler inputs

   The AWS Lambda handler function always receives two parameters: event which contains the input data, and context which provides runtime information.

2. Step 2: Match parameters to options

   Only event and context correctly lists these two parameters as event and context.

3. Final Answer:

   event and context -> Option A

4. Quick Check:

   Lambda handler parameters = event and context [OK]

Hint: Remember: Lambda handler always gets event and context [OK]

Common Mistakes:

• Confusing event with request or input
• Using callback instead of context
• Swapping parameter names

2. Which of the following is the correct syntax for a Python AWS Lambda handler function?

easy

A. function handler(event, context) { return 'Hello World'; }

B. def handler(event, context): return 'Hello World'

C. lambda handler(event, context): print('Hello World')

D. def handler(): return 'Hello World'

Solution

1. Step 1: Identify Python function syntax

   In Python, functions are defined with def and must include parameters if expected. The handler must accept event and context.

2. Step 2: Check each option

   def handler(event, context): return 'Hello World' correctly defines a Python function with two parameters and returns a string. function handler(event, context) { return 'Hello World'; } is JavaScript syntax. lambda handler(event, context): print('Hello World') misuses lambda keyword. def handler(): return 'Hello World' lacks parameters.

3. Final Answer:

   def handler(event, context): return 'Hello World' -> Option B

4. Quick Check:

   Python Lambda handler syntax = def with event, context [OK]

Hint: Python Lambda handler uses def with event and context [OK]

Common Mistakes:

• Omitting event or context parameters
• Using JavaScript syntax in Python
• Misusing lambda keyword for handler

3. Given this Python Lambda handler code:

def handler(event, context):
    return {'statusCode': 200, 'body': event['message']}

What will be the output if the event is {'message': 'Hi'}?

medium

A. {'statusCode': 404, 'body': 'Hi'}

B. Error: KeyError 'message'

C. {'statusCode': 200, 'body': 'Hi'}

D. None

Solution

1. Step 1: Analyze the return statement

   The function returns a dictionary with statusCode set to 200 and body set to the value of event['message'].

2. Step 2: Substitute event value

   Given event = {'message': 'Hi'}, event['message'] is 'Hi'. So the return is {'statusCode': 200, 'body': 'Hi'}.

3. Final Answer:

   {'statusCode': 200, 'body': 'Hi'} -> Option C

4. Quick Check:

   Return body matches event message [OK]

Hint: Return uses event['message'] directly for body [OK]

Common Mistakes:

• Assuming wrong statusCode
• Expecting error when key exists
• Returning None by mistake

4. What is wrong with this AWS Lambda handler function in Python?

def handler(event):
    return 'Done'

medium

A. Missing the required context parameter

B. Return value must be a dictionary

C. Function name must be lambda_handler

D. No error, this is valid

Solution

1. Step 1: Check function parameters

   AWS Lambda handler functions must accept two parameters: event and context. This function only has event.

2. Step 2: Validate other options

   Return type can vary by trigger, so returning a string is allowed. Function name can be configured, so handler is acceptable. Hence, only missing context is an error.

3. Final Answer:

   Missing the required context parameter -> Option A

4. Quick Check:

   Lambda handler needs event and context parameters [OK]

Hint: Always include event and context parameters [OK]

Common Mistakes:

• Omitting context parameter
• Thinking return must be dict always
• Assuming fixed function name

5. You want your AWS Lambda handler to process an event with optional user key. If user is missing, return status 400 with message 'User missing'. Otherwise, return status 200 with 'Hello, <user>'. Which handler code correctly implements this?

hard

A. def handler(event):
if 'user' in event:
return {'statusCode': 400, 'body': 'User missing'}
return {'statusCode': 200, 'body': f"Hello, {event['user']}"}

B. def handler(event, context):
return {'statusCode': 200, 'body': f"Hello, {event['user']}"}

C. def handler(event, context):
if event['user'] == None:
return {'statusCode': 400, 'body': 'User missing'}
return {'statusCode': 200, 'body': 'Hello, user'}

D. def handler(event, context):
if 'user' not in event:
return {'statusCode': 400, 'body': 'User missing'}
return {'statusCode': 200, 'body': f"Hello, {event['user']}"}

Solution

1. Step 1: Check parameter correctness

   Lambda handler must have event and context parameters.

2. Step 2: Validate logic for user key

   def handler(event, context): if 'user' not in event: return {'statusCode': 400, 'body': 'User missing'} return {'statusCode': 200, 'body': f"Hello, {event['user']}"} correctly checks if 'user' key is missing and returns 400 with message. Then returns 200 with personalized greeting. def handler(event, context): return {'statusCode': 200, 'body': f"Hello, {event['user']}"} does not check for missing user. def handler(event, context): if event['user'] == None: return {'statusCode': 400, 'body': 'User missing'} return {'statusCode': 200, 'body': 'Hello, user'} incorrectly checks if event['user'] == None which causes error if key missing. def handler(event): if 'user' in event: return {'statusCode': 400, 'body': 'User missing'} return {'statusCode': 200, 'body': f"Hello, {event['user']}"} reverses logic and misses context.

3. Final Answer:

   def handler(event, context): if 'user' not in event: return {'statusCode': 400, 'body': 'User missing'} return {'statusCode': 200, 'body': f"Hello, {event['user']}" } -> Option D

4. Quick Check:

   Check key presence before access to avoid errors [OK]

Hint: Check key exists before access; include event and context [OK]

Common Mistakes:

• Missing context parameter
• Accessing missing keys without check
• Returning wrong status codes