What is an Event Bus?

In a tightly coupled system, one service calling another means both must be online, compatible, and reachable at the same moment. An event bus decouples that: a service publishes "order created" onto the bus and moves on, while any number of subscribers — a billing service, a notification service, an analytics pipeline — pick it up independently, on their own schedule, without the publisher ever knowing they exist. This makes it far easier to add new consumers of an event later without touching the code that produces it. Event buses typically guarantee at-least-once delivery and preserve some ordering within a topic or partition, and durable implementations (Kafka, SNS/SQS, cloud pub/sub services) persist events so a subscriber that was briefly offline can catch up rather than losing messages. The trade-off is eventual consistency and more complex debugging — tracing a chain of cause and effect across asynchronous subscribers is harder than following a single synchronous call stack, so good tracing and idempotent handlers matter.