Lambda Event Sources and Triggers

Lambda functions do not run on their own — they execute in response to events. An event source is the AWS service or custom application that generates the event and invokes your function. Understanding event sources is fundamental to building effective serverless architectures.

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Event Source Categories

Lambda event sources fall into three categories based on how invocation is managed:

Category	Who Invokes Lambda?	Examples
Push (synchronous)	The source service calls Lambda directly and waits	API Gateway, ALB, Cognito
Push (asynchronous)	The source service calls Lambda and returns immediately	S3, SNS, EventBridge, SES
Poll-based	Lambda polls the source for new records	SQS, Kinesis, DynamoDB Streams, Kafka

Push (Sync):    Source ---invoke---> Lambda ---response---> Source
Push (Async):   Source ---invoke---> Lambda (queued)
                       <---202---
Poll-based:     Lambda ---poll---> Source (SQS/Kinesis/DDB)
                Lambda <---records---

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Amazon S3 Event Notifications

S3 can trigger Lambda when objects are created, modified, or deleted.

Common S3 Event Types

Event	Trigger
s3:ObjectCreated:*	Any object creation (PUT, POST, COPY, multipart)
s3:ObjectCreated:Put	Object created via PUT
s3:ObjectRemoved:*	Any object deletion
s3:ObjectRestore:Completed	Glacier restore completed

S3 Event Structure

{
  "Records": [
    {
      "eventSource": "aws:s3",
      "eventName": "ObjectCreated:Put",
      "s3": {
        "bucket": { "name": "my-uploads" },
        "object": {
          "key": "images/photo.jpg",
          "size": 1048576
        }
      }
    }
  ]
}

Example: Image Thumbnail Generator

import boto3
from PIL import Image
import io

s3 = boto3.client('s3')

def handler(event, context):
    for record in event['Records']:
        bucket = record['s3']['bucket']['name']
        key = record['s3']['object']['key']

        # Download original image
        response = s3.get_object(Bucket=bucket, Key=key)
        image = Image.open(io.BytesIO(response['Body'].read()))

        # Create thumbnail
        image.thumbnail((200, 200))
        buffer = io.BytesIO()
        image.save(buffer, 'JPEG')
        buffer.seek(0)

        # Upload thumbnail
        thumb_key = f"thumbnails/{key.split('/')[-1]}"
        s3.put_object(Bucket=bucket, Key=thumb_key, Body=buffer)

        print(f"Created thumbnail: {thumb_key}")

Warning: Never write the output back to the same bucket and prefix that triggers the function — this creates an infinite loop.

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Amazon SQS (Simple Queue Service)

SQS is a poll-based event source. Lambda automatically polls the queue, retrieves messages in batches, and invokes your function.

Event Source Mapping Configuration

aws lambda create-event-source-mapping \
  --function-name process-orders \
  --event-source-arn arn:aws:sqs:eu-west-1:123456789012:order-queue \
  --batch-size 10 \
  --maximum-batching-window-in-seconds 5

Setting	Description	Range
batch-size	Max messages per invocation	1–10 (standard), 1–10,000 (FIFO)
maximum-batching-window	Wait time to accumulate a batch	0–300 seconds

SQS Event Structure

{
  "Records": [
    {
      "messageId": "abc-123",
      "body": "{\"orderId\": \"ORD-001\", \"amount\": 49.99}",
      "attributes": {
        "ApproximateReceiveCount": "1",
        "SentTimestamp": "1704067200000"
      }
    }
  ]
}

Partial Batch Failure Handling

By default, if your function throws an error, the entire batch returns to the queue. Enable partial batch responses to report individual failures:

export const handler = async (event) => {
  const failedItems = [];

  for (const record of event.Records) {
    try {
      const order = JSON.parse(record.body);
      await processOrder(order);
    } catch (error) {
      failedItems.push({ itemIdentifier: record.messageId });
    }
  }

  return { batchItemFailures: failedItems };
};

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Amazon DynamoDB Streams

DynamoDB Streams capture item-level changes (INSERT, MODIFY, REMOVE) and deliver them to Lambda in order.

Stream Record Structure

{
  "Records": [
    {
      "eventName": "MODIFY",
      "dynamodb": {
        "Keys": { "userId": { "S": "user-123" } },
        "OldImage": { "status": { "S": "pending" } },
        "NewImage": { "status": { "S": "confirmed" } }
      }
    }
  ]
}

Use Cases