# Map commands to events

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**Added in** `eventcatalog@4.2.0`

Commands often produce one of several events. For example, a `CreateOrder` command may produce `OrderCreated` when it succeeds or `OrderRejected` when it fails.

EventCatalog lets you document these relationships so teams can see which events to expect from a command and which commands can produce a specific event.

![Cancel Order command mapped to the Order Cancelled event through the Order Service](/assets/images/cancel-order-command-to-event-614636bc24c4c7f031dded118498fcf9.png)

## Connect a command to events[​](#connect-a-command-to-events "Direct link to Connect a command to events")

Commands are `received` from particular resources in EventCatalog (e.g services). When you receive a command, you can also specify what events are triggered downstream, you do this by using the `triggers` property.

In the example below, the `OrderService` receives the `CreateOrder` (command) and we also specify that htis command is linked to and triggers the `OrderCreated` and `OrderRejected` event.

/services/OrderService/index.mdx

```
---
id: OrderService
version: 1.0.0
name: Order service
receives:
  # receives the CreateOrder Command
  - id: CreateOrder
    version: 1.0.0
    # Tell EventCatalog that the CreateOrderCommand
    # in this context will trigger the OrderCreated and Rejeected events.
    triggers:
      - id: OrderCreated
        version: 1.0.0
      - id: OrderRejected
        version: 1.0.0
---
```

The command and every event it triggers must already exist in your catalog before EventCatalog can resolve the relationship.

## Describe different scenarios[​](#describe-different-scenarios "Direct link to Describe different scenarios")

Add an optional `condition` to explain the scenario in which an event is produced. Conditions are useful for success and failure outcomes, business-rule branches, or different paths that produce the same event.

/services/OrderService/index.mdx

```
---
id: OrderService
version: 1.0.0
name: Order service
receives:
  - id: CreateOrder
    version: 1.0.0
    triggers:
      - id: OrderCreated
        version: 1.0.0
        # specify the scenario when this event would be triggered
        condition: When payment is authorized and stock is available
---
```

This lets you document common command-handling scenarios such as:

* A command producing a success event or a failure event.
* A command producing different events based on a business rule.
* Multiple paths producing the same event for different reasons.
* Different services producing different events from the same command.

| Property    | Type     | Required | Description                                                 |
| ----------- | -------- | -------- | ----------------------------------------------------------- |
| `id`        | `string` | Yes      | Id of the event produced by the command.                    |
| `version`   | `string` | No       | Version or semver range of the event. Defaults to `latest`. |
| `condition` | `string` | No       | Scenario in which the command produces the event.           |

The `version` on the received command also defaults to `latest` when omitted.

## Explore command and event relationships[​](#explore-command-and-event-relationships "Direct link to Explore command and event relationships")

The command page lists its events under **Triggers**. Each event page lists the commands that can produce it under **Triggered by**. Message maps also show the command, the resource handling it, and the resulting events.

The example below shows the `Cancel Order` command mapped to the `Order Cancelled` event through the `Order Service`, including the scenario in which the event is produced.

![Cancel Order command mapped to the Order Cancelled event through the Order Service](/assets/images/cancel-order-command-to-event-614636bc24c4c7f031dded118498fcf9.png)

Select **Map commands to events** under **Architecture** to compare these relationships. EventCatalog draws each command-to-event path through the receiving service or domain and groups repeated paths into selectable scenarios.

The **Map commands to events** link is only generated for messages that participate in at least one resolved relationship.