Understanding Serverless Computing with AWS Lambda: A Practical Guide for Modern Developers

📘 Chapter 2: Triggers, Events, and Integrations

🔍 Overview

Serverless functions like those powered by AWS Lambda are not standalone—they are event-driven. Lambda gets its real power when triggered by events from other AWS services such as API Gateway, S3, DynamoDB, or scheduled events. In this chapter, you’ll learn how triggers work, how to set them up, and how to integrate Lambda with a wide range of AWS services to build real-world event-driven applications.

📡 1. What Is a Lambda Trigger?

A trigger is a service or event that invokes a Lambda function. Once configured, the trigger automatically runs your Lambda function whenever the event occurs—without requiring manual interaction.

✅ Key Characteristics

• Event source driven
• Fully managed invocation
• Asynchronous or synchronous execution
• Integrates natively with many AWS services

📬 2. Synchronous vs. Asynchronous Triggers

🧩 3. Common Lambda Triggers & Use Cases

✅ Amazon API Gateway

• RESTful APIs (GET, POST, PUT, etc.)
• Integrate Lambda as the backend
• Add authentication (via Cognito or JWT)
• Synchronous execution

Sample Event Object:

json

{

  "httpMethod": "POST",

  "body": "{\"name\":\"Alice\"}"

}

✅ Amazon S3

• Trigger Lambda on file uploads, deletions, or versioning
• Process images, generate thumbnails, scan for viruses

Example Setup (Console):

• Go to S3 → Select bucket → Properties
• Enable Event Notification → Select "Lambda Function"

S3 Event Format:

json

{

  "Records": [

    {

      "s3": {

        "bucket": { "name": "my-bucket" },

        "object": { "key": "example.png" }

      }

    }

  ]

}

✅ Amazon DynamoDB Streams

• React to table item changes (INSERT, MODIFY, REMOVE)
• Ideal for audit logs, cache updates, downstream workflows

CLI Example:

bash

aws lambda create-event-source-mapping \

  --function-name ProcessDynamoUpdates \

  --event-source arn:aws:dynamodb:us-east-1:123456789012:table/MyTable/stream/2022-10-10T00:00:00.000 \

  --batch-size 100 \

  --starting-position LATEST

✅ Amazon EventBridge (CloudWatch Events)

• Run Lambda on scheduled cron jobs
• Capture events from over 90 AWS services

Sample Schedule Expression:

bash

rate(5 minutes) or cron(0 12 * * ? *)

✅ Amazon SNS / SQS

• Process notifications or messages in queues
• Asynchronous and highly scalable
• Ideal for decoupled microservices

Sample SNS Trigger Event:

json

{

  "Records": [

    {

      "Sns": {

        "Message": "{\"type\":\"alert\",\"severity\":\"high\"}"

      }

    }

  ]

}

✅ AWS Cognito

• Execute Lambda at key authentication events
• Customize signup, sign-in, and token issuance

✅ Application Load Balancer (ALB)

• Invoke Lambda via HTTP/S listener rules
• Use with REST or GraphQL APIs
• Supports authentication, WAF, and SSL offloading

🔄 4. Integrating Multiple Triggers

A single Lambda function can be invoked by multiple event sources. Example:

• S3 (image upload)
• EventBridge (every 6 hours)
• API Gateway (manual re-trigger)

You can even reuse the same function with minor logic branching based on the event source.

🧪 5. Testing Lambda Triggers

✅ API Gateway Test

Use the “Test” button in API Gateway or send a curl/Postman request:

bash

curl -X POST https://{api-id}.execute-api.us-east-1.amazonaws.com/prod/hello \

  -d '{"name":"Bob"}'

✅ S3 Trigger Test

Upload a file to the configured S3 bucket and observe logs via CloudWatch.

🔐 6. IAM Permissions for Triggering Services

Each event source needs to invoke Lambda securely. You must:

• Attach InvokeFunction permissions
• Grant trust relationships via execution role
• Use resource-based policies (e.g., S3 to invoke Lambda)

✅ Sample S3 Invocation Permission

bash

aws lambda add-permission \

  --function-name ImageProcessor \

  --action lambda:InvokeFunction \

  --principal s3.amazonaws.com \

  --source-arn arn:aws:s3:::my-bucket \

  --statement-id s3invoke

📊 7. Monitoring Event Sources

Use CloudWatch Metrics for:

• Number of invocations
• Throttles (due to limits)
• Errors per trigger
• Event source mapping status (e.g., SQS, DynamoDB)

🧠 8. Best Practices

• Use dedicated Lambda functions for different triggers
• Handle event versioning and schema changes
• Avoid tight coupling with upstream services
• Add retry logic and dead-letter queues for failed invocations
• Enable CloudTrail to audit who/what triggered functions
• Log and parse event payloads carefully for debugging

📋 Summary Table – Lambda Event Sources