What's the easiest way to trigger an AWS Lambda function when a file is uploaded to S3?

You can trigger a Lambda function when a file is uploaded to an S3 bucket by setting up an S3 event notification. Here's how to do it:

1. First, create your Lambda function that will process the S3 event. Your function will receive an event object containing details about the uploaded file, including the bucket name and object key.

2. Configure an S3 event notification on your bucket that points to your Lambda function:

• In the S3 console, navigate to your bucket
• Go to the "Properties" tab
• Find the "Event notifications" section and create a new notification
• Select "All object create events" (or a specific event like PUT, POST, etc.)
• Choose Lambda as the destination
• Select your Lambda function

When your Lambda function is triggered, it will receive an S3Event object containing information about the uploaded file. You can then use the AWS SDK to retrieve the file and process it as needed.

Your Lambda function code will need to:

1. Extract the bucket name and object key from the event
2. Use the S3 client to retrieve the object
3. Process the object as needed

If you need to trigger your Lambda only when specific files are uploaded, you can use prefix and suffix filters in the S3 event notification configuration to narrow down which uploads will trigger your function.

Note that if you need your Lambda to trigger only when multiple files are present (like both test1.csv and test2.csv), you'll need to implement additional logic in your Lambda function to check for the existence of all required files before proceeding with processing.
Sources
trigger lambda only if 2 files are placed in S3 path | AWS re:Post
Invoke a Lambda function from an Amazon S3 trigger - AWS Lambda
Invoke a Lambda function from an Amazon S3 trigger - Amazon Simple Storage Service