Lambda with S3 event triggers in AWS - Step-by-Step Execution

Process Flow - Lambda with S3 event triggers

File uploaded to S3 bucket

↓

S3 detects event: ObjectCreated

↓

S3 sends event notification

↓

Lambda function triggered

↓

Lambda executes code

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Process file or metadata

↓

Lambda finishes execution

When a file is uploaded to an S3 bucket, S3 detects the event and sends a notification that triggers a Lambda function. The Lambda runs code to process the file or its metadata.

Execution Sample

AWS

1. Upload file to S3 bucket
2. S3 triggers Lambda on ObjectCreated
3. Lambda runs handler(event)
4. Lambda processes event data
5. Lambda completes execution

This sequence shows how uploading a file to S3 triggers a Lambda function that processes the event.

Process Table

Step	Action	Event Data	Lambda State	Output/Result
1	File uploaded to S3 bucket	ObjectCreated event with file info	Idle	S3 detects new object
2	S3 sends event notification	Event sent to Lambda trigger	Idle	Lambda receives event
3	Lambda function triggered	Event passed to handler	Running	Lambda starts processing
4	Lambda processes event data	Reads file info from event	Running	Processes or logs file details
5	Lambda finishes execution	Processing complete	Idle	Returns success or error
6	Wait for next event	No event	Idle	Lambda waits for next trigger

💡 Lambda finishes execution after processing the S3 event triggered by file upload

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
event	null	ObjectCreated event with file info	Same event passed to handler	Event data processed	null after execution
Lambda State	Idle	Idle	Running	Running	Idle

Key Moments - 3 Insights

Why does Lambda start running only after the file upload?

Because S3 sends an event notification only when a new object is created, triggering Lambda as shown in execution_table step 2 and 3.

What does the Lambda 'event' variable contain?

It contains details about the S3 object created, such as bucket name and file key, as seen in execution_table step 1 and 4.

Why does Lambda return to Idle state after execution?

Lambda functions are stateless and run only when triggered; after processing, they stop running and wait for the next event, shown in step 5 and 6.

Visual Quiz - 3 Questions

Look at the execution table, what is the Lambda State at Step 3?

A Running

B Idle

C Error

D Waiting

💡 Hint

Check the 'Lambda State' column at Step 3 in the execution_table.

At which step does S3 send the event notification to Lambda?

A Step 1

B Step 4

C Step 2

D Step 5

💡 Hint

Look for the action 'S3 sends event notification' in the execution_table.

If no file is uploaded to S3, what happens to Lambda's state?

A Lambda runs continuously

B Lambda stays Idle waiting for events

C Lambda throws an error

D Lambda deletes the S3 bucket

💡 Hint

Refer to the final step in execution_table where Lambda waits for the next event.

Concept Snapshot

Lambda with S3 event triggers:
- Uploading a file to S3 triggers an ObjectCreated event.
- S3 sends this event to Lambda as a trigger.
- Lambda runs code using event data (file info).
- After processing, Lambda stops and waits for new events.
- This enables automatic processing on file uploads.

Full Transcript

When you upload a file to an S3 bucket, S3 detects this new object creation and sends an event notification. This notification triggers a Lambda function. The Lambda function receives the event data, which includes details about the uploaded file. It then runs its code to process or log this information. After finishing, the Lambda function stops running and waits for the next event. This process allows automatic reactions to file uploads without manual intervention.

Practice

1. What happens when you configure an AWS Lambda function with an S3 event trigger for object creation?

easy

A. The Lambda function runs manually and is not triggered by S3 events.

B. The Lambda function runs only when the S3 bucket is deleted.

C. The Lambda function runs automatically whenever a new file is uploaded to the S3 bucket.

D. The Lambda function runs only when an object is deleted from the S3 bucket.

Solution

1. Step 1: Understand S3 event triggers

   S3 event triggers can invoke Lambda functions automatically on specific events like object creation.

2. Step 2: Identify the event type for object creation

   When a new file is uploaded, the event type is 'ObjectCreated', which triggers the Lambda function.

3. Final Answer:

   The Lambda function runs automatically whenever a new file is uploaded to the S3 bucket. -> Option C

4. Quick Check:

   S3 ObjectCreated event triggers Lambda automatically [OK]

Hint: Remember: S3 triggers Lambda on file uploads automatically [OK]

Common Mistakes:

• Confusing object deletion with creation events
• Thinking Lambda runs manually without triggers
• Assuming triggers only work on bucket deletion

2. Which of the following is the correct JSON snippet to configure an S3 event trigger for a Lambda function on object creation?

easy

A. { "Events": ["s3:ObjectCreated:*"], "LambdaFunctionArn": "arn:aws:lambda:region:account-id:function:myFunction" }

B. { "Events": ["s3:ObjectRemoved:*"], "LambdaFunctionArn": "arn:aws:lambda:region:account-id:function:myFunction" }

C. { "Events": ["lambda:InvokeFunction"], "LambdaFunctionArn": "arn:aws:s3:::myBucket" }

D. { "Events": ["s3:ObjectCreated:Delete"], "LambdaFunctionArn": "arn:aws:lambda:region:account-id:function:myFunction" }

Solution

1. Step 1: Identify correct event type for object creation

   The event type for object creation is "s3:ObjectCreated:*" which covers all create events.

2. Step 2: Check Lambda function ARN placement

   The LambdaFunctionArn must point to the Lambda function's ARN, not the bucket ARN.

3. Final Answer:

   { "Events": ["s3:ObjectCreated:*"], "LambdaFunctionArn": "arn:aws:lambda:region:account-id:function:myFunction" } -> Option A

4. Quick Check:

   Use "s3:ObjectCreated:*" for creation triggers [OK]

Hint: Use "s3:ObjectCreated:*" for all object creation events [OK]

Common Mistakes:

• Using ObjectRemoved event instead of ObjectCreated
• Putting bucket ARN instead of Lambda ARN
• Using invalid event names like ObjectCreated:Delete

3. Given this Lambda function code triggered by S3 object creation:

def lambda_handler(event, context):
    bucket = event['Records'][0]['s3']['bucket']['name']
    key = event['Records'][0]['s3']['object']['key']
    return f"File {key} uploaded to {bucket}"

What will be the output if a file named 'photo.jpg' is uploaded to bucket 'my-images'?

medium

A. "File uploaded to my-images"

B. "File photo.jpg uploaded to my-images"

C. "File my-images uploaded to photo.jpg"

D. Runtime error due to missing event keys

Solution

1. Step 1: Extract bucket and key from event

   The code accesses event['Records'][0]['s3']['bucket']['name'] as bucket name and event['Records'][0]['s3']['object']['key'] as file key.

2. Step 2: Substitute values from the upload event

   For file 'photo.jpg' uploaded to 'my-images', bucket='my-images' and key='photo.jpg'.

3. Final Answer:

   "File photo.jpg uploaded to my-images" -> Option B

4. Quick Check:

   Event keys map file and bucket correctly [OK]

Hint: Check event['Records'][0]['s3'] keys for bucket and object [OK]

Common Mistakes:

• Mixing bucket name and object key positions
• Assuming event keys are missing causing errors
• Confusing string formatting output

4. You configured an S3 bucket to trigger a Lambda function on object creation, but the Lambda is not invoked. What is the most likely cause?

medium

A. The S3 bucket notification configuration is missing permission for S3 to invoke Lambda.

B. The S3 bucket is in a different AWS region than the Lambda function.

C. The Lambda function code has a syntax error.

D. The Lambda function's execution role lacks permission to read from S3.

Solution

1. Step 1: Check permissions for S3 to invoke Lambda

   S3 must have permission to invoke the Lambda function via a resource-based policy on Lambda.

2. Step 2: Understand common misconfigurations

   Missing this permission causes Lambda not to trigger even if bucket notifications are set.

3. Final Answer:

   The S3 bucket notification configuration is missing permission for S3 to invoke Lambda. -> Option A

4. Quick Check:

   S3 needs invoke permission on Lambda [OK]

Hint: Grant S3 permission to invoke Lambda in Lambda's resource policy [OK]

Common Mistakes:

• Confusing Lambda execution role with S3 invoke permission
• Assuming code errors prevent invocation
• Thinking region mismatch blocks invocation

5. You want a Lambda function to process only PNG files uploaded to an S3 bucket. How should you configure the S3 event trigger to achieve this?

hard

A. Set the event type to 's3:ObjectCreated:Put' without any filters.

B. Set the event type to 's3:ObjectRemoved:*' and add a prefix filter '.png'.

C. Configure the Lambda function code to ignore non-PNG files without using filters.

D. Set the event type to 's3:ObjectCreated:*' and add a filter with suffix '.png' in the bucket notification configuration.

Solution

1. Step 1: Use event type for object creation

   Use 's3:ObjectCreated:*' to capture all create events including uploads.

2. Step 2: Apply suffix filter for '.png'

   Configure the bucket notification with a filter to trigger Lambda only for files ending with '.png'.

3. Final Answer:

   Set the event type to 's3:ObjectCreated:*' and add a filter with suffix '.png' in the bucket notification configuration. -> Option D

4. Quick Check:

   Use event filters to limit Lambda triggers by file type [OK]

Hint: Use suffix filter '.png' in S3 event trigger for file type filtering [OK]

Common Mistakes:

• Using ObjectRemoved event instead of ObjectCreated
• Relying only on Lambda code filtering without event filters
• Using prefix filter with file extension instead of suffix