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Events

Events are defined as properties on your API classes. Events are useful when:

1. You need asynchronous communication between services
2. You wish to loosely-couple your services
3. You need a service to perform a task in the background

See event considerations in the explanations section for further discussion.

Events provide at-least-once delivery semantics. Given this, your event handlers should be idempotent.

Defining events

You can define events using the Event class. For example, you could define the following bus.py in your authenication service:

# auth_service/bus.py
from lightbus import Api, Event


class AuthApi(Api):
    user_created = Event(parameters=('username', 'email'))
    user_updated = Event(parameters=('username', 'new_email'))
    user_deleted = Event(parameters=('username'))

    class Meta:
        name = 'auth'

Firing events

You can fire events as follows:

# Anywhere in your code

# Import your project's bus instance
from bus import bus

bus.auth.user_created.fire(username='adam', password='adam@example.com')

Firing events (asynchronously)

You can also fire events asynchronously using asyncio:

# Anywhere in your code

# Import your project's bus instance
from bus import bus

await bus.auth.user_created.fire_async(
    username='adam',
    password='adam@example.com'
)

Listening for events

Listening for events is typically a long-running background activity, and is therefore dealt with by the lightbus run command.

You can setup event listeners in your services' bus module as follows:

# other_service/bus.py
import lightbus

bus = lightbus.create()
user_db = {}


def handle_created(username, email):
    user_db[username] = email
    print(user_db)


def handle_updated(username, email):
    user_db[username] = email
    print(user_db)


def handle_deleted(username, email):
    user_db.pop(username)
    print(user_db)


@bus.client.on_start()
def on_start():
    # Bus client has started up, so register our listeners

    bus.auth.user_created.listen(
        handle_created,
        listener_name="user_created"
    )
    bus.auth.user_updated.listen(
        handle_updated,
        listener_name="user_updated",
    )
    bus.auth.user_deleted.listen(
        handle_deleted,
        listener_name="user_deleted"
    )

Specifying listener_name is required in order to ensure each listeners receives all events it is due. See the events explanation page for further discussion.

---

You cannot have two listeners with the same name for the same API. For example, this will not work (it will raise a DuplicateListenerName exception:

bus.competitor_prices.changed.listen(
    send_price_alerts,
    listener_name="price_listener",
)

### ERROR ##
# This will raise a DuplicateListenerName exception because  
# we have already created a listener named 'price_listener'
# on the 'competitor_prices' API (above).
bus.competitor_prices.changed.listen(
    update_prices,
    listener_name="price_listener",  # ⇠ DuplicateListenerName exception
)

This restriction only applies to listeners within the same service.

Errors in event listeners

By default, the Lightbus worker will exit if an event listener encounters an error. The event being handled will remain unacknowledged, and will be re-attempted after acknowledgement_timeout seconds.

You can customise this behaviour by passing the on_error parameter to listen(). Possible values are:

• lightbus.OnError.SHUTDOWN (Default) - Shutdown the Lightbus work upon encountering an error
• lightbus.OnError.ACKNOWLEDGE_AND_LOG (Default) - Acknowledge erroring messages as being successfully processed, and log the error.

For example:

# Taken from the above example

# Worker shutdown on error (the default)
bus.auth.user_created.listen(
    handle_created,
    listener_name="user_created",
    on_error=lightbus.OnError.SHUTDOWN,
)

# Or, log the error and acknowlegde the message as being processed
bus.auth.user_created.listen(
    handle_created,
    listener_name="user_created",
    on_error=lightbus.OnError.ACKNOWLEDGE_AND_LOG,
)

Type hints

See the typing reference.