Event triggers for Lambda in AWS - Step-by-Step Execution

Process Flow - Event triggers for Lambda

Event Occurs

↓

Event Source Detects

↓

Trigger Lambda Function

↓

Lambda Executes Code

↓

Lambda Returns Result

↓

Process Complete

When an event happens, the event source detects it and triggers the Lambda function to run its code and return a result.

Execution Sample

AWS

1. S3 bucket receives a file upload
2. S3 triggers Lambda
3. Lambda processes the file
4. Lambda returns success

This example shows how an S3 upload event triggers a Lambda function to process the file.

Process Table

Step	Event Source	Event Detected	Lambda Triggered	Lambda Action	Result
1	S3 Bucket	File uploaded	Yes	Start Lambda function	Lambda starts execution
2	Lambda	Running	N/A	Process file content	File processed
3	Lambda	Execution complete	N/A	Return success	Success response sent
4	System	No new event	No	No action	Waiting for next event

💡 No new event detected, so Lambda is not triggered again.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Event Detected	No	Yes	Yes	Yes	No
Lambda State	Idle	Triggered	Running	Completed	Idle
Result	None	None	Processing	Success	Success

Key Moments - 3 Insights

Why does Lambda only run after the event source detects an event?

Because Lambda functions are event-driven, they only start when the event source signals an event, as shown in execution_table step 1.

What happens if no event occurs?

Lambda stays idle and does not run, as shown in execution_table step 4 where no new event means no trigger.

Does Lambda run continuously or only on events?

Lambda runs only when triggered by events, not continuously, as seen by the state changes in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Lambda start processing the file?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Lambda Action' column in execution_table row for Step 2.

According to variable_tracker, what is the Lambda State after Step 3?

A Idle

B Running

C Completed

D Triggered

💡 Hint

Look at the 'Lambda State' row under 'After Step 3' in variable_tracker.

If no file is uploaded, what will happen to the 'Event Detected' variable?

A It stays No

B It becomes Processing

C It becomes Yes

D It becomes Success

💡 Hint

Refer to the 'Event Detected' row in variable_tracker and the exit_note in execution_table.

Concept Snapshot

Event triggers start Lambda functions automatically when something happens.
Common triggers: file upload, HTTP request, database change.
Lambda runs code only after trigger.
No event means no Lambda run.
This makes Lambda efficient and cost-effective.

Full Transcript

This visual execution shows how AWS Lambda functions are triggered by events. When an event occurs, such as a file upload to an S3 bucket, the event source detects it and triggers the Lambda function. The Lambda function then runs its code to process the event and returns a result. If no event happens, Lambda remains idle and does not run. This event-driven model ensures Lambda runs only when needed, saving resources.

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