system.rst

~eventsourcing.system --- Event-driven systems

This module shows how event-sourced applications </topics/application> can be combined to make an event driven system.

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System of applications

The library's system class...

from eventsourcing.system import System

from uuid import uuid4

from eventsourcing.domain import Aggregate, event

class Dog(Aggregate):
    @event('Registered')
    def __init__(self, name):
        self.name = name
        self.tricks = []

    @event('TrickAdded')
    def add_trick(self, trick):
        self.tricks.append(trick)

Now let's define an application...

from eventsourcing.application import Application

class DogSchool(Application):
    def register_dog(self, name):
        dog = Dog(name)
        self.save(dog)
        return dog.id

    def add_trick(self, dog_id, trick):
        dog = self.repository.get(dog_id)
        dog.add_trick(trick)
        self.save(dog)

    def get_dog(self, dog_id):
        dog = self.repository.get(dog_id)
        return {'name': dog.name, 'tricks': tuple(dog.tricks)}

Now let's define an analytics application...

from uuid import uuid5, NAMESPACE_URL

class Counter(Aggregate):
    def __init__(self, name):
        self.name = name
        self.count = 0

    @classmethod
    def create_id(cls, name):
        return uuid5(NAMESPACE_URL, f'/counters/{name}')

    @event('Incremented')
    def increment(self):
        self.count += 1

from eventsourcing.application import AggregateNotFound
from eventsourcing.system import ProcessApplication
from eventsourcing.dispatch import singledispatchmethod

class Counters(ProcessApplication):
    @singledispatchmethod
    def policy(self, domain_event, process_event):
        """Default policy"""

    @policy.register(Dog.TrickAdded)
    def _(self, domain_event, process_event):
        trick = domain_event.trick
        try:
            counter_id = Counter.create_id(trick)
            counter = self.repository.get(counter_id)
        except AggregateNotFound:
            counter = Counter(trick)
        counter.increment()
        process_event.collect_events(counter)

    def get_count(self, trick):
        counter_id = Counter.create_id(trick)
        try:
            counter = self.repository.get(counter_id)
        except AggregateNotFound:
            return 0
        return counter.count

system = System(pipes=[[DogSchool, Counters]])

Single-threaded runner

from eventsourcing.system import SingleThreadedRunner

runner = SingleThreadedRunner(system)
runner.start()

school = runner.get(DogSchool)
counters = runner.get(Counters)

dog_id1 = school.register_dog('Billy')
dog_id2 = school.register_dog('Milly')
dog_id3 = school.register_dog('Scrappy')

school.add_trick(dog_id1, 'roll over')
school.add_trick(dog_id2, 'roll over')
school.add_trick(dog_id3, 'roll over')

assert counters.get_count('roll over') == 3
assert counters.get_count('fetch ball') == 0
assert counters.get_count('play dead') == 0

school.add_trick(dog_id1, 'fetch ball')
school.add_trick(dog_id2, 'fetch ball')

assert counters.get_count('roll over') == 3
assert counters.get_count('fetch ball') == 2
assert counters.get_count('play dead') == 0

school.add_trick(dog_id1, 'play dead')

assert counters.get_count('roll over') == 3
assert counters.get_count('fetch ball') == 2
assert counters.get_count('play dead') == 1

runner.stop()

Multi-threaded runner

from eventsourcing.system import MultiThreadedRunner

runner = MultiThreadedRunner(system)
runner.start()

school = runner.get(DogSchool)
counters = runner.get(Counters)

dog_id1 = school.register_dog('Billy')
dog_id2 = school.register_dog('Milly')
dog_id3 = school.register_dog('Scrappy')

school.add_trick(dog_id1, 'roll over')
school.add_trick(dog_id2, 'roll over')
school.add_trick(dog_id3, 'roll over')

school.add_trick(dog_id1, 'fetch ball')
school.add_trick(dog_id2, 'fetch ball')

school.add_trick(dog_id1, 'play dead')

from time import sleep

sleep(0.01)

assert counters.get_count('roll over') == 3
assert counters.get_count('fetch ball') == 2
assert counters.get_count('play dead') == 1

runner.stop()

Classes

eventsourcing.system