Amazon EventBridge

Many systems are easier to build if parts announce what happened instead of calling each other directly. An order was placed, a file was uploaded, a server stopped. EventBridge is the AWS service that carries those announcements, called events: small JSON messages that go into an event bus.

On the bus you set rules. Each rule describes a pattern, like "events from the orders app where the total is above 1000", and one or more targets to send matching events to: an AWS Lambda function, an Amazon SQS queue, an AWS Step Functions workflow, another account. The app that emits the event does not know or care who is listening. Adding a new reaction, say a fraud check, means adding a rule, not changing the orders app.

AWS services already publish their own events to a default bus (an EC2 machine changed state, a backup finished), and many SaaS tools can send theirs too. EventBridge also includes a scheduler for running something at a set time or on a repeating schedule, which replaced the old trick of cron jobs on a server. It used to be called CloudWatch Events, and some older documentation still uses that name.