Event triggers for Lambda in AWS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an event trigger in AWS Lambda?

An event trigger is a source that causes a Lambda function to run automatically when a specific event happens, like a file upload or a message arrival.

beginner

Name three common AWS services that can trigger Lambda functions.

Amazon S3 (file uploads), Amazon DynamoDB (table updates), and Amazon API Gateway (HTTP requests) are common event triggers for Lambda.

beginner

How does Amazon S3 trigger a Lambda function?

When a file is added, modified, or deleted in an S3 bucket, it can send an event notification that triggers a Lambda function to run.

intermediate

What is the role of Amazon EventBridge in Lambda triggers?

EventBridge can route events from many AWS services or custom apps to Lambda functions, enabling complex event-driven workflows.

intermediate

Can a Lambda function have multiple event triggers? Explain.

Yes, a Lambda function can be triggered by multiple event sources, allowing it to respond to different types of events in one place.

Which AWS service can trigger a Lambda function when a new file is uploaded?

A Amazon VPC

B Amazon EC2

C Amazon RDS

D Amazon S3

Amazon S3 can send event notifications to Lambda when files are uploaded.

What type of event can Amazon DynamoDB send to trigger Lambda?

A Table updates like inserts or deletes

B EC2 instance launches

C S3 bucket creation

D CloudWatch alarm

DynamoDB streams capture table changes and can trigger Lambda functions.

Which service routes events from many sources to Lambda for complex workflows?

A Amazon EventBridge

B Amazon SNS

C Amazon CloudFront

D AWS IAM

EventBridge routes events from multiple sources to Lambda.

Can a Lambda function be triggered by an HTTP request?

A Yes, but only from Amazon S3

B No, Lambda only triggers on file uploads

C Yes, via Amazon API Gateway

D No, Lambda only triggers on database changes

API Gateway can trigger Lambda functions on HTTP requests.

What happens when multiple event triggers are set for one Lambda function?

A The function runs only on the first trigger

B The function runs when any trigger event occurs

C The function runs only if all triggers happen simultaneously

D The function cannot have multiple triggers

Lambda runs whenever any configured event trigger occurs.

Explain how AWS Lambda uses event triggers to run functions automatically.

Think about what causes Lambda to start without manual action.

List and describe at least three AWS services that can trigger Lambda functions and the types of events they send.

Consider storage, database, and web services.

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