Lambda execution model in AWS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the AWS Lambda execution model?

AWS Lambda runs your code in response to events. It automatically manages the compute resources, running your code only when needed and scaling automatically.

beginner

How does AWS Lambda handle scaling when multiple events occur?

Lambda creates multiple instances of your function to handle each event concurrently, scaling automatically without manual intervention.

intermediate

What is a Lambda execution environment?

It is a temporary container where your Lambda function code runs. It includes the runtime, memory, and temporary storage, and it can be reused for multiple invocations.

intermediate

Explain the cold start in Lambda execution.

A cold start happens when Lambda creates a new execution environment to run your function for the first time or after a period of inactivity, causing a slight delay.

intermediate

What happens to the execution environment after a Lambda function finishes running?

The environment may be frozen and kept alive for some time to serve future requests quickly, or it may be discarded if not needed.

What triggers AWS Lambda to run your code?

A An event occurs

B You manually start it

C It runs continuously

D It runs on a schedule only

AWS Lambda runs your code in response to events such as file uploads, HTTP requests, or database changes.

What is a cold start in AWS Lambda?

A Lambda function timing out

B Lambda function running slowly due to code errors

C Delay when creating a new execution environment

D Lambda function running out of memory

A cold start is the delay caused by Lambda creating a new execution environment before running your function.

How does Lambda handle multiple simultaneous events?

A Runs only the first event

B Queues events and runs one at a time

C Ignores extra events

D Creates multiple execution environments to run functions concurrently

Lambda automatically scales by creating multiple environments to handle concurrent events.

What happens to the Lambda execution environment after your function finishes?

A It is always deleted immediately

B It may be frozen and reused for future calls

C It runs your code again automatically

D It sends logs to the user

Lambda may keep the environment frozen to speed up future invocations.

Which of these is NOT part of the Lambda execution environment?

A Permanent database

B Memory

C Temporary storage

D Runtime

Lambda execution environment includes runtime, memory, and temporary storage, but not a permanent database.

Describe how AWS Lambda executes your code from event to completion.

Think about what happens when an event occurs and how Lambda manages resources.

Explain what a cold start is and why it matters in Lambda execution.

Consider the impact on user experience when Lambda starts fresh.

Practice

1. What happens when an AWS Lambda function is not triggered by any event?

easy

A. The function does not run and no charges occur.

B. The function runs continuously in the background.

C. The function runs once every hour automatically.

D. The function runs only when manually started from the console.

Solution

1. Step 1: Understand Lambda's event-driven model

   A Lambda function runs only when triggered by an event, such as an API call or file upload.

2. Step 2: Consider billing and execution behavior

   If no event triggers the function, it does not execute and you are not charged.

3. Final Answer:

   The function does not run and no charges occur. -> Option A

4. Quick Check:

   Lambda runs only on events = C [OK]

Hint: Lambda runs only on events, no event means no run [OK]

Common Mistakes:

• Thinking Lambda runs continuously without events
• Assuming Lambda runs on a schedule by default
• Believing manual start is required for all executions

2. Which of the following is the correct way to define a Lambda handler function in Python?

easy

A. def handler(): print('Hello World')

B. function handler(event) { return 'Hello World'; }

C. handler(event, context) => { return 'Hello World'; }

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

Solution

1. Step 1: Recall Python Lambda handler signature

   In Python, the handler must accept two parameters: event and context.

2. Step 2: Check syntax correctness

   def handler(event, context): return 'Hello World' correctly defines a Python function with event and context and returns a string.

3. Final Answer:

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

4. Quick Check:

   Python handler(event, context) = A [OK]

Hint: Python Lambda handlers need event and context parameters [OK]

Common Mistakes:

• Using JavaScript syntax in Python handler
• Omitting context parameter
• Defining handler without parameters

3. Given this Python Lambda handler code:

def handler(event, context):
    return event['value'] * 2

What is the output if the event is {"value": 5}?

medium

A. Error: 'value' key missing

B. 5

C. 10

D. None

Solution

1. Step 1: Understand the handler logic

   The handler multiplies the 'value' from event by 2.

2. Step 2: Calculate output for event {'value': 5}

   5 * 2 = 10, so the function returns 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   5 times 2 equals 10 [OK]

Hint: Multiply event['value'] by 2 for output [OK]

Common Mistakes:

• Confusing event keys or missing them
• Assuming function returns input unchanged
• Expecting error when key exists

4. This Lambda function code causes an error when triggered:

def handler(event, context):
    return event['data'] * 3

What is the likely cause?

medium

A. The function is missing the context parameter.

B. The event dictionary does not have the 'data' key.

C. The return statement syntax is incorrect.

D. Lambda functions cannot multiply values.

Solution

1. Step 1: Identify cause of error in code

   The function tries to access event['data'], which may not exist.

2. Step 2: Understand error from missing key

   If 'data' key is missing in event, a KeyError occurs causing the function to fail.

3. Final Answer:

   The event dictionary does not have the 'data' key. -> Option B

4. Quick Check:

   Missing event key causes error = B [OK]

Hint: Check if event keys exist before using them [OK]

Common Mistakes:

• Assuming context parameter missing causes error
• Thinking return syntax is wrong
• Believing Lambda cannot do arithmetic

5. You want your Lambda function to process multiple events concurrently without waiting for previous executions. Which Lambda feature supports this behavior?

hard

A. Automatic scaling with multiple concurrent executions

B. Manual start of multiple Lambda instances

C. Scheduled triggers to queue events

D. Using a single-threaded execution model

Solution

1. Step 1: Understand Lambda concurrency

   Lambda automatically scales by running multiple instances to handle events concurrently.

2. Step 2: Eliminate incorrect options

   Manual start and single-threaded models do not support concurrency; scheduled triggers queue events but don't scale automatically.

3. Final Answer:

   Automatic scaling with multiple concurrent executions -> Option A

4. Quick Check:

   Lambda auto-scales for concurrency = D [OK]

Hint: Lambda auto-scales to run many events at once [OK]

Common Mistakes:

• Thinking manual start is needed for concurrency
• Confusing scheduling with scaling
• Assuming Lambda runs single-threaded only