Event triggers for Lambda in AWS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

How does an S3 event trigger a Lambda function?

When a new file is uploaded to an S3 bucket, which event trigger configuration will cause a Lambda function to run automatically?

A Configure the Lambda function with an S3 trigger for the 's3:ObjectCreated:*' event on the bucket.

B Set up a CloudWatch alarm on the bucket and link it to the Lambda function.

C Create an SNS topic subscribed by the Lambda function and publish messages manually.

D Enable versioning on the bucket and link it to the Lambda function.

💡 Hint

Think about which S3 event directly relates to new file uploads and can invoke Lambda automatically.

The 's3:ObjectCreated:*' event triggers Lambda when a new object is created in the bucket. CloudWatch alarms and SNS require manual or indirect setup. Versioning alone does not trigger Lambda.

❓ Architecture

intermediate

Which AWS service can trigger Lambda on a scheduled basis?

You want a Lambda function to run every day at midnight automatically. Which AWS service should you use to trigger it?

A AWS Config

B AWS CloudTrail

C Amazon SNS topic with daily messages

D Amazon EventBridge (CloudWatch Events) with a scheduled rule

💡 Hint

Consider which service is designed to run tasks on a schedule.

Amazon EventBridge (formerly CloudWatch Events) supports scheduled rules to trigger Lambda functions at fixed times. CloudTrail and Config track changes but don't schedule tasks. SNS requires manual message publishing.

❓ security

advanced

What IAM permission is required for S3 to invoke a Lambda function?

To allow an S3 bucket to trigger a Lambda function on object creation, which IAM permission must be granted?

A s3:PutObject permission granted to the Lambda execution role

B lambda:InvokeFunction permission granted to the S3 service principal

C lambda:CreateFunction permission granted to the S3 bucket

D iam:PassRole permission granted to the Lambda function

💡 Hint

Think about what permission allows one service to call another.

S3 needs permission to invoke the Lambda function, which is granted by allowing the 'lambda:InvokeFunction' action to the S3 service principal. The other permissions are unrelated or incorrect for this trigger.

✅ Best Practice

advanced

How to avoid duplicate Lambda executions from SQS triggers?

You configured an SQS queue to trigger a Lambda function. Sometimes the function processes the same message twice. What is the best way to reduce duplicates?

A Delete messages from the queue immediately after Lambda invocation starts.

B Increase the Lambda timeout to avoid retries.

C Enable Lambda's event source mapping with batch size 1 and use idempotent processing in the function.

D Disable the SQS queue's dead-letter queue.

💡 Hint

Consider how Lambda processes messages and what can cause duplicates.

Setting batch size to 1 and making the function idempotent helps handle retries and avoid duplicate effects. Deleting messages too early or disabling dead-letter queues can cause message loss or unhandled failures.

🧠 Conceptual

expert

What happens if a Lambda function triggered by DynamoDB Streams fails repeatedly?

A Lambda function is triggered by DynamoDB Streams on table updates. If the function keeps failing for the same batch of records, what is the expected behavior?

A Lambda retries processing the batch until successful or the data expires in the stream (24 hours).

B Lambda skips the failed batch and moves to the next batch immediately.

C DynamoDB deletes the failed records from the stream after 5 retries.

D The Lambda function is disabled automatically after 3 failures.

💡 Hint

Think about how Lambda handles stream event retries and retention.

Lambda retries the same batch until it succeeds or the records expire from the DynamoDB stream (usually 24 hours). It does not skip or delete records automatically, nor disables itself.

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