Run on Event

This example assumes we have a task registered on a running worker.

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:

Running a task when an ephemeral event is received, such as a webhook or a message from a queue.
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.

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.

examples/python/events/worker.py

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,
)

examples/typescript/on_event/workflow.ts

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

examples/go/workflows/on-event.go

const SimpleEvent = "simple-event:create"

func Lower(hatchet v1.HatchetClient) workflow.WorkflowDeclaration[EventInput, LowerTaskOutput] {
	return factory.NewTask(
		create.StandaloneTask{
			Name: "lower",
			// 👀 Declare the event that will trigger the workflow
			OnEvents: []string{SimpleEvent},
		}, func(ctx worker.HatchetContext, input EventInput) (*LowerTaskOutput, error) {
			// Transform the input message to lowercase
			return &LowerTaskOutput{
				TransformedMessage: strings.ToLower(input.Message),
			}, nil
		},
		hatchet,
	)
}

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

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.

Pushing an Event

You can push an event to the event queue by calling the push method on the Hatchet event client and providing the event name and payload.

examples/python/events/event.py

hatchet.event.push("user:create", {"should_skip": False})

examples/typescript/on_event/event.ts

  const res = await hatchet.events.push<Input>('simple-event:create', {
    Message: 'hello',
    ShouldSkip: false,
  });

examples/go/run/event.go

	err = hatchet.Events().Push(
		context.Background(),
		"simple-event:create",
		v1_workflows.SimpleInput{
			Message: "Hello, World!",
		},
	)

Event Filtering

Events can also be filtered in Hatchet, which allows you to push events to Hatchet and only trigger task runs from them in certain cases. If you enable filters on a workflow, your workflow will be triggered once for each matching filter on any incoming event with a matching scope (more on scopes below).

Basic Usage

There are two ways to create filters in Hatchet.

Default filters on the workflow

The simplest way to create a filter is to register it declaratively with your workflow when it’s created. For example:

examples/python/events/worker.py

event_workflow_with_filter = hatchet.workflow(
    name="EventWorkflow",
    on_events=[EVENT_KEY, SECONDARY_KEY, WILDCARD_KEY],
    input_validator=EventWorkflowInput,
    default_filters=[
        DefaultFilter(
            expression="true",
            scope="example-scope",
            payload={
                "main_character": "Anna",
                "supporting_character": "Stiva",
                "location": "Moscow",
            },
        )
    ],
)

examples/typescript/on_event/workflow.ts

export const lowerWithFilter = hatchet.workflow<Input, LowerOutput>({
  name: 'lower',
  // 👀 Declare the event that will trigger the workflow
  onEvents: ['simple-event:create'],
  defaultFilters: [
    {
      expression: 'true',
      scope: 'example-scope',
      payload: {
        mainCharacter: 'Anna',
        supportingCharacter: 'Stiva',
        location: 'Moscow',
      },
    },
  ],
});

examples/go/workflows/on-event.go

func LowerWithFilter(hatchet v1.HatchetClient) workflow.WorkflowDeclaration[EventInput, LowerTaskOutput] {
	return factory.NewTask(
		create.StandaloneTask{
			Name: "lower",
			// 👀 Declare the event that will trigger the workflow
			OnEvents: []string{SimpleEvent},
			DefaultFilters: []types.DefaultFilter{{
				Expression: "true",
				Scope:      "example-scope",
				Payload: map[string]interface{}{
					"main_character":       "Anna",
					"supporting_character": "Stiva",
					"location":             "Moscow"},
			}},
		}, accessFilterPayload,
		hatchet,
	)
}

In each of these cases, we register a filter with the workflow. Note that these “declarative” filters are overwritten each time your workflow is updated, so the ids associated with them will not be stable over time. This allows you to modify a filter in-place or remove a filter, and not need to manually delete it over the API.

Filters feature client

You also can create event filters by using the filters clients on the SDKs:

examples/python/events/filter.py

hatchet.filters.create(
    workflow_id=event_workflow.id,
    expression="input.should_skip == false",
    scope="foobarbaz",
    payload={
        "main_character": "Anna",
        "supporting_character": "Stiva",
        "location": "Moscow",
    },
)

examples/typescript/on_event/filter.ts

hatchet.filters.create({
  workflowId: lower.id,
  expression: 'input.ShouldSkip == false',
  scope: 'foobarbaz',
  payload: {
    main_character: 'Anna',
    supporting_character: 'Stiva',
    location: 'Moscow',
  },
});

examples/go/run/event.go

	payload := map[string]interface{}{
		"main_character":       "Anna",
		"supporting_character": "Stiva",
		"location":             "Moscow",
	}

	_, err = hatchet.Filters().Create(
		context.Background(),
		rest.V1CreateFilterRequest{
			WorkflowId: uuid.New(),
			Expression: "input.shouldSkip == false",
			Scope:      "foobarbaz",
			Payload:    &payload,
		},
	)

⚠️

Note the scope argument to the filter. When you create a filter, it must be given a scope which will be used by Hatchet internally to look it up. When you push events that you want filtered, you must provide a scope with those events that matches the scope sent with the filter. If you do not, the filter will not apply.

Then, push an event that uses the filter to determine whether or not to run. For instance, this run will be skipped, since the payload does not match the expression:

examples/python/events/filter.py

hatchet.event.push(
    event_key=EVENT_KEY,
    payload={
        "should_skip": True,
    },
    options=PushEventOptions(
        scope="foobarbaz",
    ),
)

examples/typescript/on_event/filter.ts

hatchet.events.push(
  SIMPLE_EVENT,
  {
    Message: 'hello',
    ShouldSkip: true,
  },
  {
    scope: 'foobarbaz',
  }
);

examples/go/run/event.go

	skipPayload := map[string]interface{}{
		"shouldSkip": true,
	}
	skipScope := "foobarbaz"
	err = hatchet.Events().Push(
		context.Background(),
		"simple-event:create",
		skipPayload,
		client.WithFilterScope(&skipScope),
	)

But this one will be triggered since the payload does match the expression:

examples/python/events/filter.py

hatchet.event.push(
    event_key=EVENT_KEY,
    payload={
        "should_skip": True,
    },
    options=PushEventOptions(
        scope="foobarbaz",
    ),
)

examples/typescript/on_event/filter.ts

hatchet.events.push(
  SIMPLE_EVENT,
  {
    Message: 'hello',
    ShouldSkip: false,
  },
  {
    scope: 'foobarbaz',
  }
);

examples/go/run/event.go

	triggerPayload := map[string]interface{}{
		"shouldSkip": false,
	}
	triggerScope := "foobarbaz"
	err = hatchet.Events().Push(
		context.Background(),
		"simple-event:create",
		triggerPayload,
		client.WithFilterScope(&triggerScope),
	)

In Hatchet, filters are “positive”, meaning that we look for matches to the filter to determine which tasks to trigger.

Accessing the filter payload

You can access the filter payload by using the Context in the task that was triggered by your event:

examples/python/events/worker.py

@event_workflow_with_filter.task()
def filtered_task(input: EventWorkflowInput, ctx: Context) -> None:
    print(ctx.filter_payload)

examples/typescript/on_event/workflow.ts

lowerWithFilter.task({
  name: 'lowerWithFilter',
  fn: (input, ctx) => {
    console.log(ctx.filterPayload());
  },
});

examples/go/workflows/on-event.go

func accessFilterPayload(ctx worker.HatchetContext, input EventInput) (*LowerTaskOutput, error) {
	fmt.Println(ctx.FilterPayload())
	return &LowerTaskOutput{
		TransformedMessage: strings.ToLower(input.Message),
	}, nil
}

Advanced Usage

In addition to referencing input in the expression (which corresponds to the event payload), you can also reference the following fields:

payload corresponds to the filter payload (which was part of the request when the filter was created).
additional_metadata allows for filtering based on additional_metadata sent with the event.
event_key allows for filtering based on the key of the event, such as user:created.

Cron Trigger Bulk Run Many

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Run on Event

Declaring Event Triggers

Pushing an Event

Event Filtering

Basic Usage

Default filters on the workflow

Filters feature client

Accessing the filter payload

Advanced Usage