Lambda execution model in AWS - Commands & Configuration

Introduction

AWS Lambda lets you run code without managing servers. It automatically runs your code when triggered and handles scaling. This model explains how Lambda runs your code behind the scenes.

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

When you need to run short tasks that start quickly and finish fast.

When you want to pay only for the compute time your code uses, not for idle servers.

When you want automatic scaling to handle many requests without manual setup.

When you want to build small, focused functions that do one job well.

Commands

This command creates a new Lambda function named 'example-function' using Python 3.9 runtime. It specifies the execution role and the handler method inside the code package.

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": "abc123def456ghi789",

    "Version": "$LATEST",

    "TracingConfig": {

        "Mode": "PassThrough"

    },

    "RevisionId": "1a2b3c4d-5678-90ab-cdef-EXAMPLE11111"

}

→

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

→

--runtime - Specifies the language runtime environment

→

--handler - Defines the entry point method in your code

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 run

This command shows the content of the output file created by the Lambda invocation, displaying the function's response.

Terminal

cat output.txt

Expected Output

"Hello from Lambda!"

Key Concept

If you remember nothing else from this pattern, remember: Lambda runs your code in short-lived containers that start on demand, run your handler, then pause or stop to save resources.

Common Mistakes

Using a handler name that does not match the code file and function.

Lambda cannot find the entry point and fails to run your code.

Ensure the handler flag matches the filename and function name exactly, like 'lambda_function.lambda_handler'.

Not assigning the correct IAM role with execution permissions.

Lambda cannot access resources or run without proper permissions, causing errors.

Create and assign an IAM role with the necessary Lambda execution permissions.

Expecting Lambda to keep running continuously like a server.

Lambda functions are short-lived and stop after execution, so persistent state is lost.

Design functions to be stateless and use external storage for persistent data.

Summary

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

Invoke the Lambda function to run your code and capture its output.

Understand that Lambda runs your code in short-lived containers that start on demand and stop after execution.

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