---
url: /guide/command.md
description: >-
  Enter Wow commands through definition, in-process sending, HTTP invocation,
  service clients, completion semantics, and troubleshooting.
---

# Commands

A command expresses one business intent to change aggregate state. Define its payload and target aggregate first, then send it through an in-process `CommandGateway`, an aggregate HTTP route, or the global command facade, and wait only for the completion stage the caller actually needs.

A command starts as business intent and connects to downstream collaboration through durable facts and observable completion signals.

```mermaid
flowchart TB
    Intent["Business intent"] --> Definition["Define command"]
    Definition --> Send["Send command"]
    Send --> Process["Process in aggregate"]
    Process --> Append["Append domain events"]
    Append --> Completion["Observe completion stages"]
    Completion --> Collaboration["Events and collaboration"]
```

## Quick Paths

| Goal | Entry point |
| --- | --- |
| Define and send a command | Read [Define Commands](./definition.md), then [Send Commands](./sending.md) |
| Call another service | Use the [API Client](./api-client.md) with the global command facade |
| Choose a completion stage | Read [Completion Semantics](./completion.md) |
| Diagnose timeouts, duplicate requests, and downstream failures | Read [Failures and Idempotency](./reliability.md) |

## Application Use

1. Use [Define Commands](./definition.md) to establish the payload, target aggregate, and handler.
2. Use [Send Commands](./sending.md) to choose among the local Gateway, aggregate routes, and the global facade.
3. For service-to-service calls, use the [API Client](./api-client.md) and accept its current final-result-only boundary.
4. Use [Completion Semantics](./completion.md) to choose the earliest stage that satisfies the response contract.

This track answers how an application invokes commands. It does not expand Dispatcher, Filter, WaitState, or notifier internals.

## How It Works

To understand the runtime, start with [Command Processing Pipeline](./internals/pipeline.md), then read [Command Wait Runtime](./internals/wait-runtime.md) and [Command Transport and Routing](./internals/transport.md). They own the sequence from Gateway through aggregate processing, event append, and stage signals; application pages consume those results without duplicating the state machine.

For failures, return to [Failures and Idempotency](./reliability.md) and collect evidence by `commandId`, `requestId`, aggregate identity, `stage`, and error code.