Event Bus

Picture an office with 20 departments. The HR department needs to announce a new hire. Should they walk to every single department and tell them individually? That is 19 visits. And if a new department opens next month, HR has to remember to add them to the list.

Now picture a bulletin board in the lobby. HR posts the announcement once. Every department checks the board when they walk by. Marketing sees the new hire and starts onboarding materials. IT sees it and provisions a laptop. Payroll sees it and sets up direct deposit. Nobody coordinates with each other. Nobody needs to know who else is reading.

That bulletin board is an event bus. It is a central communication channel where any component can publish events, and any other component can subscribe to the events it cares about. The publisher does not know who is listening. The subscribers do not know who published.

Event buses are everywhere, from in-process event systems in frontend frameworks (React context, Vue's event bus, Angular's EventEmitter) to distributed message buses connecting microservices (Azure Service Bus, NServiceBus, MassTransit). The pattern is the same at every scale.

An event bus has three parts:

• Events, named messages carrying data (e.g., UserCreated { userId: "abc", email: "..." })
• Publishers, components that emit events to the bus
• Subscribers, components that register handlers for specific event types

Component A --publish--> [Event Bus] --deliver--> Handler X
Component B --publish-->              --deliver--> Handler Y
Component C --publish-->              --deliver--> Handler Z

In-process event bus (within a single application):

// Publisher
eventBus.publish('order.placed', { orderId: '123', amount: 99.99 });

// Subscribers (registered elsewhere)
eventBus.subscribe('order.placed', sendConfirmationEmail);
eventBus.subscribe('order.placed', updateInventory);
eventBus.subscribe('order.placed', triggerAnalytics);

The event bus maintains a mapping of event names to handler functions. When an event is published, the bus iterates through all registered handlers for that event type and invokes them.

Distributed event bus (across services):

The same concept, but the bus is an external system. Kafka, RabbitMQ, Azure Service Bus. Events travel over the network. Publishers and subscribers are separate processes, possibly on different machines. The semantics are identical. The transport changes.