# Event Trigger

> This example assumes we have a [task](/v1/tasks) registered on a running [worker](/v1/workers).

Run-on-event allows you to trigger one or more tasks when a specific event occurs. This is useful when you need to execute a task in response to an ephemeral event where the result is not important. A few common use cases for event-triggered task runs are:

1. Running a task when an ephemeral event is received, such as a webhook or a message from a queue.
2. When you want to run multiple independent tasks in response to a single event. For instance, if you wanted to run a `send_welcome_email` task, and you also wanted to run a `grant_new_user_credits` task, and a `reward_referral` task, all triggered by the signup. In this case, you might declare all three of those tasks with an event trigger for `user:signup`, and then have them all kick off when that event happens.

> **Info:** Event triggers evaluate tasks to run at the time of the event. If an event is
>   received before the task is registered, the task will not be run.

## Declaring Event Triggers

To run a task on an event, you need to declare the event that will trigger the task. This is done by declaring the `on_events` property in the task declaration.

#### Python

```python
EVENT_KEY = "user:create"
SECONDARY_KEY = "foobarbaz"
WILDCARD_KEY = "subscription:*"


class EventWorkflowInput(BaseModel):
    should_skip: bool


event_workflow = hatchet.workflow(
    name="EventWorkflow",
    on_events=[EVENT_KEY, SECONDARY_KEY, WILDCARD_KEY],
    input_validator=EventWorkflowInput,
)
```

#### Typescript

```typescript
export const lower = hatchet.workflow({
  name: 'lower',
  // 👀 Declare the event that will trigger the workflow
  onEvents: ['simple-event:create'],
});
```

#### Go

```go
const SimpleEvent = "simple-event:create"

func Lower(client *hatchet.Client) *hatchet.StandaloneTask {
	return client.NewStandaloneTask(
		"lower", func(ctx hatchet.Context, input EventInput) (*LowerTaskOutput, error) {
			return &LowerTaskOutput{
				TransformedMessage: strings.ToLower(input.Message),
			}, nil
		},
		hatchet.WithWorkflowEvents(SimpleEvent),
	)
}
```

#### Ruby

```ruby
EVENT_KEY = "user:create"
SECONDARY_KEY = "foobarbaz"
WILDCARD_KEY = "subscription:*"

EVENT_WORKFLOW = HATCHET.workflow(
  name: "EventWorkflow",
  on_events: [EVENT_KEY, SECONDARY_KEY, WILDCARD_KEY]
)
```

> **Info:** Note: Multiple tasks can be triggered by the same event.

> **Info:** As of engine version 0.65.0, Hatchet supports wildcard event triggers using
>   the `*` wildcard pattern. For example, you could register `subscription:*` as
>   your event key, which would match incoming events like `subcription:create`,
>   `subscription:renew`, `subscription:cancel`, and so on.