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