Lamda Function

Flow:
User uploads file → S3 Bucket
S3 event triggers → Lambda Function
Lambda processes file (log, move, resize, etc.)

Step 1: Create S3 Bucket
Go to AWS Consolet
Open S3
Click Create Buckett
Give name: my-upload-bucket-demo
Keep default settingst
Create buckett

Step 2: Create Lambda Function
Go to Lambda
Click Create Function
Choose → Author from scratch
Function name: s3-file-trigger
Runtime: Python 3.x

Step 3: Add S3 Trigger to Lambda
Open your Lambda function
Click Add Trigger
Select → S3
Choose your bucket
Event type:
PUT (Object Created)
Add trigger
Now whenever a file is uploaded → Lambda runs automatically.

Step 4: Sample Lambda Code (Read Uploaded File Info)
import json
def lambda_handler(event, context):
print("Event: ", json.dumps(event))
for record in event['Records']:
bucket_name = record['s3']['bucket']['name']
file_key = record['s3']['object']['key']
print(f"File uploaded: {file_key}")
print(f"Bucket: {bucket_name}")
return {
'statusCode': 200,
'body': json.dumps('File processed successfully!')
}

Deploy this code.

Step 5: Upload File to S3
Open your S3 bucket
Click Upload
Select any file (example: Hello.txt)
Upload

Check Lambda Logs
Go to Lambda
Click Monitor
Open CloudWatch Logs
You will see:
File uploaded: Hello.txt
Bucket: my-upload-bucket-demo