Event triggers for Lambda in AWS - Mini Project: Build & Apply

Event triggers for Lambda

📖 Scenario: You are setting up a simple AWS Lambda function that runs automatically when a new file is uploaded to an S3 bucket. This helps automate tasks like image processing or data analysis without manual intervention.

🎯 Goal: Create an AWS Lambda function and configure an S3 bucket event trigger so the Lambda runs whenever a new object is created in the bucket.

📋 What You'll Learn

Create an AWS Lambda function named ProcessNewFile

Create an S3 bucket named my-upload-bucket

Configure the S3 bucket to trigger the Lambda function on ObjectCreated events

Use AWS CloudFormation YAML syntax for the infrastructure as code

💡 Why This Matters

🌍 Real World

Automating workflows by running code automatically when files are uploaded to cloud storage is common in data processing, image handling, and backups.

💼 Career

Understanding how to connect AWS Lambda with event sources like S3 is essential for cloud engineers and developers building serverless applications.

1

Create the AWS Lambda function resource

Write a CloudFormation resource named ProcessNewFile of type AWS::Lambda::Function with the following properties: FunctionName set to ProcessNewFile, Runtime set to python3.12, Handler set to index.handler, and Role set to arn:aws:iam::123456789012:role/lambda-execution-role.

AWS

# Your code here

Hint

Define the Lambda function resource with the exact properties given.

2

Create the S3 bucket resource

Add a CloudFormation resource named MyUploadBucket of type AWS::S3::Bucket with the BucketName set to my-upload-bucket.

AWS

Resources:
  ProcessNewFile:
    Type: AWS::Lambda::Function
    Properties:
      FunctionName: ProcessNewFile
      Runtime: python3.12
      Handler: index.handler
      Role: arn:aws:iam::123456789012:role/lambda-execution-role

# Your code here

Hint

Define the S3 bucket resource with the exact bucket name.

3

Add permission for S3 to invoke Lambda

Add a CloudFormation resource named LambdaInvokePermission of type AWS::Lambda::Permission with these properties: FunctionName set to !Ref ProcessNewFile, Action set to lambda:InvokeFunction, Principal set to s3.amazonaws.com, and SourceArn set to !GetAtt MyUploadBucket.Arn.

AWS

Resources:
  ProcessNewFile:
    Type: AWS::Lambda::Function
    Properties:
      FunctionName: ProcessNewFile
      Runtime: python3.12
      Handler: index.handler
      Role: arn:aws:iam::123456789012:role/lambda-execution-role
  MyUploadBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: my-upload-bucket

# Your code here

Hint

Grant S3 permission to invoke the Lambda function using the exact properties.

4

Configure S3 bucket notification to trigger Lambda

Add a NotificationConfiguration property to the MyUploadBucket resource. Inside it, add a LambdaConfigurations list with one item that has Event set to s3:ObjectCreated:* and Function set to !GetAtt ProcessNewFile.Arn.

AWS

Resources:
  ProcessNewFile:
    Type: AWS::Lambda::Function
    Properties:
      FunctionName: ProcessNewFile
      Runtime: python3.12
      Handler: index.handler
      Role: arn:aws:iam::123456789012:role/lambda-execution-role
  MyUploadBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: my-upload-bucket

# Add NotificationConfiguration here
  LambdaInvokePermission:
    Type: AWS::Lambda::Permission
    Properties:
      FunctionName: !Ref ProcessNewFile
      Action: lambda:InvokeFunction
      Principal: s3.amazonaws.com
      SourceArn: !GetAtt MyUploadBucket.Arn

Hint

Add the notification configuration exactly as described to trigger Lambda on new S3 objects.

Practice

1. Which AWS service event can automatically start a Lambda function when a new file is uploaded?

easy

A. AWS CloudTrail log event

B. Amazon EC2 instance launch event

C. Amazon S3 object upload event

D. Amazon RDS backup event

Solution

1. Step 1: Understand Lambda triggers

   Lambda functions can be triggered by events from various AWS services.

2. Step 2: Identify common event sources

   Amazon S3 triggers Lambda when a new object is uploaded to a bucket.

3. Final Answer:

   Amazon S3 object upload event -> Option C

4. Quick Check:

   S3 upload triggers Lambda = A [OK]

Hint: Remember: S3 uploads often trigger Lambda automatically [OK]

Common Mistakes:

• Confusing EC2 events as Lambda triggers
• Thinking CloudTrail logs directly trigger Lambda
• Assuming RDS backups trigger Lambda

2. Which of the following is the correct way to configure an SQS queue as an event source for a Lambda function?

easy

A. Enable CloudWatch Events on the SQS queue

B. Add an S3 trigger to the Lambda function

C. Attach an IAM role to the SQS queue

D. Create an event source mapping between the SQS queue and Lambda

Solution

1. Step 1: Know how Lambda connects to SQS

   Lambda uses event source mappings to poll messages from SQS queues.

2. Step 2: Identify correct configuration

   Creating an event source mapping links the SQS queue to Lambda for automatic invocation.

3. Final Answer:

   Create an event source mapping between the SQS queue and Lambda -> Option D

4. Quick Check:

   SQS triggers Lambda via event source mapping = D [OK]

Hint: Event source mapping links SQS to Lambda directly [OK]

Common Mistakes:

• Confusing S3 triggers with SQS triggers
• Attaching IAM roles to SQS instead of Lambda
• Using CloudWatch Events incorrectly for SQS

3. Given this Lambda trigger configuration snippet:

{
  "EventSourceArn": "arn:aws:sqs:us-east-1:123456789012:MyQueue",
  "FunctionName": "ProcessQueueMessages",
  "Enabled": true
}

What happens when a new message arrives in the SQS queue?

medium

A. The Lambda function ProcessQueueMessages is invoked automatically

B. The message is deleted from the queue without processing

C. The Lambda function is not triggered until manually invoked

D. An error occurs because the configuration is incomplete

Solution

1. Step 1: Understand event source mapping

   The snippet shows an event source mapping linking an SQS queue to a Lambda function.

2. Step 2: Effect of Enabled=true

   With Enabled set to true, Lambda polls the queue and invokes the function on new messages.

3. Final Answer:

   The Lambda function ProcessQueueMessages is invoked automatically -> Option A

4. Quick Check:

   Enabled event source mapping triggers Lambda = C [OK]

Hint: Enabled event source mapping means automatic Lambda invocation [OK]

Common Mistakes:

• Assuming manual invocation is needed
• Thinking messages get deleted without processing
• Believing configuration is incomplete without batch size

4. You configured a DynamoDB stream as a trigger for your Lambda function, but the function is not invoked when the table changes. What is the most likely cause?

medium

A. The DynamoDB stream is disabled or not enabled

B. The DynamoDB table has no data

C. The Lambda function code is empty

D. The Lambda execution role lacks permission to read the DynamoDB stream

Solution

1. Step 1: Check DynamoDB stream status

   For Lambda to trigger, the DynamoDB stream must be enabled on the table.

2. Step 2: Confirm stream is active

   If the stream is disabled or not enabled, no events are generated to invoke Lambda.

3. Final Answer:

   The DynamoDB stream is disabled or not enabled -> Option A

4. Quick Check:

   DynamoDB stream must be enabled for Lambda trigger = A [OK]

Hint: Ensure DynamoDB stream is enabled to trigger Lambda [OK]

Common Mistakes:

• Ignoring stream enablement status
• Assuming empty table stops triggers
• Blaming Lambda code without checking stream

5. You want a Lambda function to run every day at 6 AM UTC without any external event. Which AWS service and configuration should you use to trigger the Lambda?

hard

A. Configure an S3 bucket event to trigger Lambda at 6 AM

B. Create a CloudWatch Events (EventBridge) rule with a cron expression to trigger Lambda

C. Set up an SQS queue with a message sent daily at 6 AM

D. Enable DynamoDB Streams to trigger Lambda daily

Solution

1. Step 1: Identify scheduled triggers for Lambda

   To run Lambda on a schedule without external events, use CloudWatch Events (EventBridge) rules.

2. Step 2: Use cron expression in EventBridge

   Create a rule with a cron expression for 6 AM UTC daily to trigger the Lambda function automatically.

3. Final Answer:

   Create a CloudWatch Events (EventBridge) rule with a cron expression to trigger Lambda -> Option B

4. Quick Check:

   Scheduled Lambda uses EventBridge cron rule = B [OK]

Hint: Use EventBridge cron rules for scheduled Lambda runs [OK]

Common Mistakes:

• Trying to trigger Lambda via S3 or SQS for schedule
• Assuming DynamoDB Streams can schedule events
• Not using cron expressions for timing