Command Bus

An event bus broadcasts announcements: "Something happened." Anyone who cares can listen. Nobody is expected to do anything specific.

A command bus delivers orders: "Do this specific thing." Exactly one handler is responsible for executing it. If no handler exists, that is an error. If two handlers exist, that is also an error.

Think of it this way. An event is like a news headline: "Fire broke out on 5th Street." Anyone can react however they want. The fire department responds, journalists report, neighbors evacuate. A command is like a direct instruction to the fire department: "Dispatch Engine 7 to 5th Street." There is exactly one recipient, and they must execute it.

In software, commands represent intent: CreateOrder, CancelSubscription, SendPasswordReset. Each command has exactly one handler that knows how to fulfill it. The command bus routes the command to that handler.

This distinction (events for notifications, commands for instructions) is the foundation of CQRS (Command Query Responsibility Segregation), which separates the "do something" path from the "read something" path in your application.

A command bus has a strict contract:

• Command, an immutable object describing an action to perform (e.g., { type: 'CreateOrder', userId: '123', items: [...] })
• Handler, a function or class that executes the command (exactly one per command type)
• Bus, routes each command to its registered handler

// Define a command
class CreateOrder {
 constructor(
 public readonly userId: string,
 public readonly items: OrderItem[]
 ) {}
}

// Register the handler
commandBus.register(CreateOrder, new CreateOrderHandler());

// Dispatch the command
commandBus.dispatch(new CreateOrder('user-123', items));

Event bus vs. Command bus: