Every time a new order comes in, you have to update inventory, notify shipping, send a confirmation email, and log it for analytics. Change any one of those? You're editing the order system.
Your systems are tangled together like Christmas lights. Touch one, and three others break.
Adding a new feature means finding every place that needs to know about it. And you always miss one.
Tightly coupled systems mean every change ripples through everything. Event buses let systems talk without knowing each other exist.
LAYER 1 - Event buses are the nervous system of modern architectures. They turn point-to-point spaghetti into a clean broadcast network.
An event bus is like a radio station. When something happens in your system - an order placed, a customer updated, a payment received - you broadcast it once. Any system that cares can tune in. Systems that don't care ignore it.
This is fundamentally different from direct integration. Instead of System A calling System B, C, and D whenever something happens, System A just announces 'hey, this happened.' B, C, and D decide for themselves whether to react.
The magic is in the decoupling. The order system doesn't know or care that five other systems are listening. Add a sixth? The order system doesn't change. Remove one? Still doesn't change. The publishers and subscribers never talk directly - they just share a channel.
When you find yourself building a system that needs to 'notify' multiple other systems, you're looking at an event bus problem. The question isn't whether to use one - it's how soon you'll regret not using one.
Event buses solve a universal problem: how do you let multiple systems react to the same thing without welding them together? The answer is always the same pattern - publish events to a shared channel, let interested parties subscribe.
Publishers emit events without knowing who's listening. Subscribers receive events without knowing who sent them. The bus handles all the routing. This is why you can add new subscribers without changing publishers.
Toggle between approaches, then click "Place Order" to see the difference.
One event, many subscribers
When an event is published, every subscriber receives a copy. The order system publishes 'OrderPlaced' once, and inventory, shipping, email, and analytics all get their own copy to process independently.
Subscribe to specific event types
Subscribers choose which event types they care about. The inventory system subscribes to 'orders.*' events, the billing system to 'payments.*' events. Each only receives what's relevant.
Events as the source of truth
Instead of storing current state, you store every event that happened. 'OrderPlaced', 'ItemAdded', 'OrderShipped'. Current state is derived by replaying events. New systems can catch up by processing historical events.
When an order comes in, five systems need to know. Without an event bus, the order service calls each one and waits. With pub/sub, it publishes 'OrderPlaced' once. Everyone who cares subscribes.
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You routed OrderPlaced events through a function that calculates tax before forwarding. Now the bus is doing business logic, and you've recreated the coupling you were trying to avoid.
Instead: Events should be pure data. Subscribers do the logic. The bus just routes.
OrderPlaced and OrderCancelled events arrive at the warehouse system. But they arrive out of order, so the warehouse thinks there's an order to ship. Now you have orphaned packages.
Instead: Include sequence numbers or timestamps. Build subscribers that handle out-of-order delivery.
A subscriber crashes while processing an event. The event is gone. You have no idea what failed or how to recover. The customer's order vanishes into the void.
Instead: Failed events go to a dead letter queue for investigation and replay.
You've learned how to decouple systems with event-driven communication. The next step is understanding how to process continuous streams of events at scale.