How does S3 trigger a Lambda function?

S3 Event-Driven Invocation of Lambda

Amazon S3 can trigger a Lambda function automatically in response to specific events like object uploads, deletions, or updates. This allows developers to build serverless, event-driven architectures where file operations directly invoke backend logic without manual polling.

Common S3 Events That Can Trigger Lambda

1
s3:ObjectCreated:* – triggers when an object is uploaded or copied
2
s3:ObjectRemoved:* – triggers when an object is deleted
3
s3:ObjectRestore:* – triggers when an archived object is restored
4
s3:Replication:* – triggers when an object is replicated

Steps to Configure S3 to Trigger Lambda

1
Go to the S3 bucket in the AWS Console
2
Select the 'Properties' tab and find 'Event notifications'
3
Create a new event and specify the event type (e.g., ObjectCreated)
4
Choose 'Lambda function' as the destination
5
Select the Lambda function and grant it permission to be invoked by S3

Sample S3 Event Payload Sent to Lambda

Difficulty: 5/10

Topics: S3 event notifications, Lambda permissions, Cross-account triggers

Scenario Questions

0-2 years experience

1
You need to run a Lambda function every time a new object is uploaded to an S3 bucket. Walk me through the steps you would take to set that up.
2
If you accidentally set the event notification on the wrong prefix, what would happen when objects are added elsewhere in the bucket?

2-5 years experience

1
Our team noticed that a Lambda function isn’t being invoked for certain S3 PUT events. How would you troubleshoot the issue?
2
Explain the trade‑offs between using S3 event notifications versus S3 Object Lambda for processing new objects.

5-8 years experience

1
Design a solution to reliably process millions of S3 object creation events per hour with Lambda, ensuring ordering and handling throttling.
2
How would you secure cross‑account S3 to Lambda triggers while minimizing permissions and avoiding circular dependencies?

8+ years experience

1
We are migrating a legacy data pipeline that currently uses S3 notifications to trigger a fleet of Lambda functions across multiple regions. What architectural changes would you recommend to improve scalability, observability, and fault tolerance?
2
Discuss the long‑term maintenance implications of using S3 event notifications versus an event‑bus (e.g., EventBridge) for triggering Lambda, especially in a multi‑team environment.

Follow-up Questions

❓What limits should we be aware of when using S3‑to‑Lambda triggers?
❓How does Lambda handle retries if the function fails?
❓Can you change the trigger without redeploying the function?