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