What triggers can invoke a serverless function?

Serverless functions can be triggered by a wide variety of events: (1) HTTP/API — API Gateway, Application Load Balancer, Function URLs (direct HTTPS endpoint); (2) Object storage — S3 file upload/delete events; (3) Databases — DynamoDB Streams (CDC), RDS Proxy, Aurora zero-ETL; (4) Messaging — SQS (queue messages), SNS (notifications), Kinesis (data streams), EventBridge (event bus); (5) Schedulers — EventBridge Scheduler, CloudWatch Events (cron expressions); (6) Authentication — Cognito user pool triggers (pre/post authentication, token generation); (7) IoT — AWS IoT Core rule actions; (8) Serverless-to-serverless — Lambda invoking Lambda, Step Functions state machine transitions; (9) GraphQL — AppSync resolvers; (10) Email — SES receipt rules. This breadth of triggers is what makes serverless ideal for event-driven, loosely coupled architectures.