# CQRS Bridge Concepts (/integrations/nestjs/concepts/cqrs-bridge)



Understand how `@kubemq/nestjs-transport/cqrs` swaps the in-process `@nestjs/cqrs` buses for KubeMQ-backed publishers before wiring it up in the [how-to guide](../how-to/cqrs-bridge).

## What It Does [#what-it-does]

NestJS ships a first-class CQRS package, [`@nestjs/cqrs`](https://docs.nestjs.com/recipes/cqrs), built around three in-process buses: the `CommandBus`, the `QueryBus`, and the `EventBus`. Out of the box those buses dispatch to handlers inside the *same* process. The `KubeMQCqrsModule` from `@kubemq/nestjs-transport/cqrs` swaps each bus's internal publisher for a KubeMQ-backed one, so a dispatched command, query, or event leaves the process and travels over a KubeMQ channel to whichever service registered the matching handler.

The routing is purely convention based. When you call `CommandBus.execute(new CreateOrderCommand(...))`, the bridge resolves the message's channel segment (by default the class name, `CreateOrderCommand`) and sends it to `{commandChannelPrefix}.{CommandName}` — for example `cqrs.commands.CreateOrderCommand`. Queries and events follow the same rule against their own prefixes. The result is distributed CQRS: your application code keeps calling the familiar `CommandBus` / `QueryBus` / `EventBus` API, while the dispatch crosses a service boundary through KubeMQ.

<Mermaid
  chart="sequenceDiagram
    participant App as App Code
    participant CB as CommandBus
    participant K as KubeMQ
    participant H as @CommandHandler service
    App->>CB: execute(new CreateOrderCommand(...))
    CB->>K: sendCommand cqrs.commands.CreateOrderCommand
    K->>H: deliver on channel
    H-->>K: response body
    K-->>CB: response
    CB-->>App: return value"
/>

Internally the module replaces each bus's `publisher` with a KubeMQ adapter: `KubeMQCommandPubSub` calls `sendCommand`, `KubeMQQueryPubSub` calls `sendQuery`, and `KubeMQEventPubSub` calls `sendEvent` (or `sendEventStore` when `persistEvents` is enabled). All three share a single KubeMQ connection created by the module on startup.

<Callout type="info">
  The bridge is an **optional** feature. It lives behind the `@kubemq/nestjs-transport/cqrs` subpath entry point and requires the optional peer dependency `@nestjs/cqrs`. If you do not use NestJS CQRS, you do not need any of this — use the [handler decorators](/integrations/nestjs/how-to/usage) directly.
</Callout>

## Related [#related]

<Cards>
  <Card title="Set up the CQRS bridge" href="../how-to/cqrs-bridge" description="Install, configure, and dispatch commands, queries, and events through KubeMQ." />

  <Card title="KubeMQCqrsOptions reference" href="../reference/cqrs-options" description="Every KubeMQCqrsModule.forRoot() configuration field." />
</Cards>