Event sourcing in Python
Project description
A library for event sourcing in Python.
"totally amazing and a pleasure to use"
Please read the docs. See also extension projects.
Installation
Use pip to install the stable distribution from the Python Package Index.
$ pip install eventsourcing
Please note, it is recommended to install Python packages into a Python virtual environment.
Example
The example below follows an outside-in approach to software development.
First, write a test.
def test_dog_school():
# Construct application object.
school = DogSchool()
# Evolve application state.
dog_id = school.register_dog('Fido')
school.add_trick(dog_id, 'roll over')
school.add_trick(dog_id, 'play dead')
# Query application state.
dog = school.get_dog(dog_id)
assert dog['name'] == 'Fido'
assert dog['tricks'] == ('roll over', 'play dead')
# Select notifications.
notifications = school.notification_log.select(start=1, limit=10)
assert len(notifications) == 3
Define application objects with the Application class.
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)}
Define aggregates with the Aggregate class and the @event decorator.
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)
Run tests with the default "in-memory" persistence module.
test_dog_school()
Configure environment variables to run applications with real databases.
import os
os.environ["PERSISTENCE_MODULE"] = 'eventsourcing.sqlite'
os.environ["SQLITE_DBNAME"] = 'dog-school.db'
test_dog_school()
Add interfaces that use your application objects. Add indexes or logs to discover aggregate IDs. See the documentation for more information.
Features
Aggregates and applications — base classes for event-sourced aggregates and applications. Suggests how to structure an event-sourced application. All classes are fully type-hinted to guide developers in using the library.
Flexible event store — flexible persistence of aggregate events. Combines an event mapper and an event recorder in ways that can be easily extended. Mapper uses a transcoder that can be easily extended to support custom model object types. Recorders supporting different databases can be easily substituted and configured with environment variables.
Application-level encryption and compression — encrypts and decrypts events inside the application. This means data will be encrypted in transit across a network ("on the wire") and at disk level including backups ("at rest"), which is a legal requirement in some jurisdictions when dealing with personally identifiable information (PII) for example the EU's GDPR. Compression reduces the size of stored aggregate events and snapshots, usually by around 25% to 50% of the original size. Compression reduces the size of data in the database and decreases transit time across a network.
Snapshotting — reduces access-time for aggregates that have many events.
Versioning - allows changes to be introduced after an application has been deployed. Both aggregate events and aggregate snapshots can be versioned.
Optimistic concurrency control — ensures a distributed or horizontally scaled application doesn't become inconsistent due to concurrent method execution. Leverages optimistic concurrency controls in adapted database management systems.
Notifications and projections — reliable propagation of application events with pull-based notifications allows the application state to be projected accurately into replicas, indexes, view models, and other applications. Supports materialized views and CQRS.
Event-driven systems — reliable event processing. Event-driven systems can be defined independently of particular persistence infrastructure and mode of running.
Detailed documentation — documentation provides general overview, introduction of concepts, explanation of usage, and detailed descriptions of library classes. All code is annotated with type hints.
Worked examples — includes examples showing how to develop aggregates, applications and systems.
Extensions
The GitHub organisation Event Sourcing in Python hosts extension projects for the Python eventsourcing library. There are projects that support ORMs such as Django and SQLAlchemy. There are projects supporting databases such as AxonDB, DynamoDB, EventStoreDB, and Apache Kafka. Another project supports transcoding domain events with Protocol Buffers rather than JSON. There are also projects that provide examples of using the library with such things as FastAPI, Flask, and serverless.
Project
This project is hosted on GitHub.
Please register questions, requests and issues on GitHub, or post in the project's Slack channel.
There is a Slack channel for this project, which you are welcome to join.
Please refer to the documentation for installation and usage guides.
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