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Event sourcing and handling

Project description

==========================================
django-spark - Event sourcing and handling
==========================================

.. image:: https://travis-ci.org/matthiask/django-spark.png?branch=master
:target: https://travis-ci.org/matthiask/django-spark

Version |release|

This is not supposed to be real documentation; it's more a reminder for
myself.

The idea is that there are event sources and event handlers. Event
sources may create a stream of ``spark.api.Event`` instances, where each
event must have a ``group`` and a ``key``. Additional data may be added
to the ``Event`` as well. Keys are globally unique -- events with the
same key are still only processed exactly once. Groups are used to
determine which handlers handle a certain event.

Event handlers are functions which are called once per
``spark.api.Event`` instance if the event's group matches the event
handler's regex.


Some usage example code
=======================

Given a challenge, create events for the challenge (the specifics do not
matter):

.. code-block:: python

from datetime import date
from spark import api

def events_from_challenge(challenge):
if not challenge.is_active:
return

yield api.Event(
group='challenge_created',
key='challenge_created_%s' % challenge.pk,
# Attach any metadata to the Event (it is a types.SimpleNamespace)
challenge=challenge,
)

if (date.today() - challenge.start_date).days > 2:
if challenge.donations.count() < 2:
yield api.Event(
group='challenge_inactivity_2d',
key='challenge_inactivity_2d_%s' % challenge.pk,
challenge=challenge,
)

if (challenge.end_date - date.today()).days <= 2:
yield api.Event(
group='challenge_ends_2d',
key='challenge_ends_2d_%s' % challenge.pk,
challenge=challenge,
)

if challenge.end_date < date.today():
yield api.Event(
group='challenge_ended',
key='challenge_ended_%s' % challenge.pk,
challenge=challenge,
)


Send mails related to challenges (uses django-authlib's
``render_to_mail``):

.. code-block:: python

from authlib.email import render_to_mail

def send_challenge_mails(event):
render_to_mail(
# Different mail text per event group:
'challenges/mails/%s' % event.group,
{
'challenge': event.challenge,
},
to=[event.challenge.user.email],
).send(fail_silently=True)


Register the handlers:

.. code-block:: python

class ChallengesConfig(AppConfig):
def ready(self):
# Prevent circular imports:
from spark import api

api.register_group_handler(
handler=send_challenge_mails,
group=r'^challenge',
)

Challenge = self.get_model('Challenge')

# All this does right now is register a post_save signal
# handler which runs the challenge instance through
# events_from_challenge:
api.register_model_event_source(
sender=Challenge,
source=events_from_challenge,
)


Now, events are generated and handled directly in process.
Alternatively, you might want to handle events outside the
request-response cycle. This can be achieved by only registering the
model event source e.g. in a management command, and then sending all
model instances through all event sources, and directly processing those
events, for example like this:

.. code-block:: python

from spark import api

api.register_model_event_source(...)

# Copied from the process_spark_sources management command inside
# this repository
for model, sources in api.MODEL_SOURCES.items():
for instance in model.objects.all():
for source in sources:
api.process_events(source(instance))


- `Documentation <https://django-spark.readthedocs.io>`_
- `Github <https://github.com/matthiask/django-spark/>`_


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