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

Process Flow - Lambda with S3 event triggers

File uploaded to S3 bucket

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S3 detects event: ObjectCreated

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S3 sends event notification

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Lambda function triggered

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Lambda executes code

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Process file or metadata

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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.