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Serverless Patterns in Cloud Computing

Learn about Serverless Patterns in this comprehensive Cloud Computing tutorial. S3 and API Gateway integration.

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1The Infinite Loop Anti-Pattern

A common mistake when setting up S3 triggers is having the Lambda function save its processed output back into the EXACT SAME S3 bucket that triggered it. This causes the new file to trigger the Lambda again, creating an infinite loop that will cost thousands of dollars. Always write output to a different bucket or use a specific prefix/folder rule.

2Step-by-Step Breakdown

Event-Driven Architecture. Lambda acts as the glue between AWS services. Instead of constantly polling databases or queues for changes, Lambda reacts instantly to events.

S3 Event Notifications. You can configure an S3 bucket to trigger a Lambda function whenever an object is created, deleted, or modified.

The 'event' Object. When triggered by S3, Lambda receives a JSON payload (the 'event' object) containing the Bucket Name and the exact Object Key (filename) that triggered the execution.

S3 Trigger Pattern. A classic pattern: User uploads a large image to an S3 bucket. S3 triggers a Lambda. The Lambda resizes the image and saves the thumbnail to a different S3 bucket.

Knowledge Check. When Amazon S3 triggers a Lambda function, which argument inside your Lambda code contains the name of the uploaded file?

→The 'context' object
→The 'event' object

Amazon API Gateway. Lambda cannot be accessed directly via a public URL by default. API Gateway allows you to create RESTful and HTTP APIs to securely expose your Lambda functions to the internet.

Lambda Proxy Integration. The most common API Gateway setup. It passes the entire HTTP request (headers, query string parameters, body) directly into the Lambda 'event' object.

Returning HTTP Responses. When using API Gateway Proxy Integration, your Lambda function MUST return a specific JSON dictionary containing a 'statusCode' and a 'body' string.

Synchronous vs Asynchronous. API Gateway triggers Lambda *synchronously* (the API waits for Lambda to finish and return a response). S3 triggers Lambda *asynchronously* (S3 fires the event and immediately forgets about it).

Summary. Use S3 triggers for background processing, and API Gateway for user-facing REST APIs.

Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Granting an S3 event trigger permission to invoke Lambda but forgetting the resource-based policy

aws lambda add-permission --function-name my-fn --statement-id s3invoke \
  --action lambda:InvokeFunction --principal s3.amazonaws.com --source-arn arn:aws:s3:::my-bucket

The Solution //

Setting up an S3 event notification pointing at a Lambda function isn't enough on its own — Lambda's resource-based policy must explicitly allow the S3 bucket to invoke it, or every invocation attempt fails silently with an AccessDenied that never reaches your logs the way you'd expect.

The Error //

Creating a recursive S3-to-Lambda trigger loop

// Trigger notification filtered to prefix "uploads/", output written to "processed/"
// so the write doesn't re-trigger the same function

The Solution //

A Lambda function triggered on S3 object creation that itself writes a new object back to the same bucket (e.g. writing a processed/thumbnail version to the same prefix) can re-trigger itself indefinitely, racking up invocations and cost. Write outputs to a different bucket or a prefix explicitly excluded from the trigger.