buslane

Simple message (event/command) bus.

Installation

To install buslane, simply use pip (or pipenv):

$ pip install buslane

Requirements

Minimum Python version supported by buslane is 3.6.

Quick start

from dataclasses import dataclass

from buslane.commands import Command, CommandHandler, CommandBus


@dataclass(frozen=True)
class RegisterUserCommand(Command):
    email: str
    password: str


class RegisterUserCommandHandler(CommandHandler[RegisterUserCommand]):

    def handle(self, command: RegisterUserCommand) -> None:
        assert command == RegisterUserCommand(
            email='john@lennon.com',
            password='secret',
        )


command_bus = CommandBus()
command_bus.register(handler=RegisterUserCommandHandler())
command_bus.execute(command=RegisterUserCommand(
    email='john@lennon.com',
    password='secret',
))

About

This library makes it easier to create solutions based on messages. If you want to split event occurrence from its handling, buslane is the way to go. It supports commands (single handler) and events (0 or multiple handlers) approach.

Motivation

This package could be probably replaced with a simple Python dictionary with messages classes as keys and ordinary functions as values. Python is a dynamic language and we can implement such solution very easy, without any classes, inheritance, methods overriding and so one. So why you should use buslane? Is it the pythonic approach?

First of all, buslane is very simple and tiny project. I was copying these few lines from project to project, so now I don't have to.

Secondly, I'm a huge fan of type annotations. This a game changer in a project with a huge codebase. buslane has type hints everywhere and it is based on Python generics. In combination with such tools like mypy you will be sure that you are doing (from types point of view) everything OK.

Message handler is a class instead of function, because you can easily inject your dependencies via __init__ parameters. Such class is very easy to test, you don't have to mock.patch everything. The interface is clear.

Last but not least - the buslane API is simple and well defined. You can extend it easily, e.g. log all of your messages or store them in a database.

It can be used as a foundation of your CQRS-based system.

Reference

buslane uses Python type annotations to properly register handler. To create your message you have to inherit from Event or Command class. I recommend to use dataclasses module from Python 3.7 (or from PyPI) - command/event should be just a simple bundle of immutable data, dataclass decorator makes it easy to create such class.

Handler should inherit from EventHandler[T] or CommandHandler[T], where T is a class of your message.

Events

You can register multiple or none handlers for a single event.

Classes:

• Event
• EventHandler[Event]
• EventBus

Exceptions:

• WrongHandlerException

Example

from buslane.events import Event, EventHandler, EventBus


class SampleEvent(Event):
    pass


class SampleEventHandler(EventHandler[SampleEvent]):

    def handle(self, event: SampleEvent) -> None:
        pass


event_bus = EventBus()
event_bus.register(handler=SampleEventHandler())
event_bus.publish(event=SampleEvent())

Commands

You have to register only single handler for the given command.

Classes:

• Command
• CommandHandler[Command]
• CommandBus

Exceptions:

• MissingCommandHandlerException
• CommandHandlerRegisteredException
• WrongHandlerException

Example

from buslane.commands import Command, CommandHandler, CommandBus


class SampleCommand(Command):
    pass


class SampleCommandHandler(CommandHandler[SampleCommand]):

    def handle(self, command: SampleCommand) -> None:
        pass


command_bus = CommandBus()
command_bus.register(handler=SampleCommandHandler())
command_bus.execute(command=SampleCommand())

Customizations

If you want to customize behaviour of your bus, you can override handle method from EventBus / CommandBus class.

The following example shows a bus which logs every event and process it in a thread.

import logging
from concurrent.futures import ThreadPoolExecutor


class CustomEventBus(EventBus):

    def __init__(self, workers: int) -> None:
        super().__init__()
        self.logger = logging.getLogger()
        self.executor = ThreadPoolExecutor(max_workers=workers)

    def handle(self, event: Event, handler: EventHandler) -> None:
        self.logger.info(f'Handling event {event} by {handler}')
        self.executor.submit(handler.handle, event)

Note: This type of customization will be deprecated in the next release in favor of a plugins architecture.

Authors

Created by Konrad Hałas.