Lambda handler function structure in AWS - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the correct AWS Lambda handler signature

Which of the following AWS Lambda handler function signatures is correct for Node.js?

A function handler(event) { return 'done'; }

B exports.handler = (context, event) => { return 'done'; }

C exports.handler = async (event, context) => { return 'done'; }

D handler = (event, context, callback) => { return 'done'; }

💡 Hint

Remember the order of parameters: event first, then context.

The correct AWS Lambda handler in Node.js receives event as the first parameter and context as the second. It can be async and return a value or use a callback.

❓ Configuration

intermediate

Determine the output of a Python Lambda handler

What is the output when this AWS Lambda Python handler is invoked with {"key": "value"} as event?

AWS

def lambda_handler(event, context):
    return event.get('key', 'default')

A "value"

B "default"

C None

D KeyError

💡 Hint

Check how dict.get() works in Python.

The get method returns the value for the key if it exists; otherwise, it returns the default. Here, 'key' exists with value 'value'.

❓ Architecture

advanced

Choose the best event source for a Lambda function triggered by file uploads

You want to run a Lambda function automatically whenever a new file is uploaded to an S3 bucket. Which event source configuration is correct?

A Set up CloudWatch Events to poll the S3 bucket every minute

B Configure S3 bucket event notifications to trigger Lambda on 's3:ObjectCreated:*' events

C Use API Gateway to invoke Lambda when files are uploaded

D Manually invoke Lambda from an EC2 instance after upload

💡 Hint

Think about native S3 event triggers for Lambda.

S3 can directly trigger Lambda functions on object creation events, making option B the best and most efficient choice.

❓ security

advanced

Identify the least privilege IAM policy for Lambda execution

Which IAM policy grants the least privilege necessary for a Lambda function to write logs to CloudWatch?

A { "Effect": "Allow", "Action": ["logs:CreateLogGroup", "logs:CreateLogStream", "logs:PutLogEvents"], "Resource": "arn:aws:logs:*:*:*" }

B { "Effect": "Allow", "Action": "*", "Resource": "*" }

C { "Effect": "Allow", "Action": ["logs:PutLogEvents"], "Resource": "*" }

D { "Effect": "Allow", "Action": ["logs:CreateLogGroup", "logs:CreateLogStream"], "Resource": "arn:aws:logs:*:*:*" }

💡 Hint

Consider all actions needed to create and write logs.

Option A includes all necessary actions with resource restriction, following least privilege. Option A is too broad. Option A misses creating groups and streams. Option A misses putting log events.

❓ service_behavior

expert

Predict Lambda function behavior with asynchronous invocation and error handling

A Lambda function is invoked asynchronously. It throws an exception during execution. What happens to the event and error?

A The event is lost immediately with no retries or notifications

B The error is returned immediately to the caller

C The function retries indefinitely until it succeeds

D The event is retried twice by default, then sent to a dead-letter queue if configured

💡 Hint

Think about Lambda's default retry behavior for asynchronous calls.

For asynchronous invocations, Lambda retries failed executions twice by default. If a dead-letter queue is set, the event is sent there after retries fail.

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