Fractal

Fractal is a scaffolding toolkit for building SOLID logic for your Python applications.

Installation

pip install fractal-toolkit

Usage

• Fractal can be used inside large Python applications to isolate certain (logical related) behaviour from the rest of the application.
• Fractal is ideal for refactoring large applications into smaller parts
• Fractal applications by design are microservices
  • Just wrap the app in an HTTP framework (like FastAPI, see contrib module) and expose with Docker
  • Other usages apart from HTTP (and Docker) are also possible
    • Like subscribing to a data stream or pub/sub channel

Architecture

Applications that use Fractal can be built in many ways, including a non-SOLID architecture. The Fractal toolkit tries to make it easier to go for the SOLID approach.

To start a Fractal project, the first class to make derives from fractal.Fractal. It should provide a fractal.core.utils.settings.Settings object and a fractal.core.utils.application_context.ApplicationContext object, which should also be derived from.

The Settings class provides all static configuration for the application; it's the place where environment variables are loaded. The class creates a singleton object.

The Context class provides the dynamic configuration of the application, using the Settings object. In the Context all dependencies will be injected.

Hexagonal Architecture (ports and adapters)

In Hexagonal Architecture, together with Domain Driven Design principles, the core of the application, is the bounded context containing the domain objects (entities, repositories, services, etc.) but without specific implementation details. Just the domain logic. From now on we call the core the domain.

This is (loosely) enforced by not allowing dependencies to external packages inside the domain. This, in turn, is the dependency inversion principle of SOLID.

The repositories and services inside the domain are interfaces or abstract classes. These are known as ports.

Next to the domain there are the adapters. Each interface or port needs an adapter to function at runtime. Adapters are allowed to depend on external packages.

At runtime, in the application Context, based on Settings, the appropriate adapter will be set for each port.

Basic application structure

A typical application folder structure using Fractal looks like:

app/
├── adapters/
│   └── users.py
├── domain/
│   └── users.py
├── context.py
├── main.py
└── settings.py

With this, a fully functional Fractal application can be built having a Python interface. That is, the logic of the application can only be reached by invoking methods on Python level.

Such Fractal applications might be used as part of larger (Python) applications to isolate or encapsulate certain behaviour. The larger application itself can also be a Fractal application, and so on. Hence the name: Fractal.

While using Fractal as a way to have separation of concerns with separate isolated bounded contexts in Python applications, it's also possible to wrap Fractal in a small application and expose as REST API using, for example, FastAPI, Flask or Django. Next that application can be deployed again in a Docker environment. This makes Fractal a perfect fit for microservices as well.

Example file contents

main.py

from app.service.context import ApplicationContext
from app.service.settings import Settings
from fractal import Fractal


class ApplicationFractal(Fractal):
    settings = Settings()
    context = ApplicationContext()

settings.py

import os

from fractal.core.utils.settings import Settings as BaseSettings


class Settings(BaseSettings):
    BASE_DIR = os.path.dirname(__file__)
    ROOT_DIR = os.path.dirname(os.path.dirname(BASE_DIR))
    APP_NAME = os.getenv("APP_NAME", "user_system")

    def load(self):
        self.USER_REPOSITORY_BACKEND = os.getenv("USER_REPOSITORY_BACKEND", "")

context.py

from app.service.settings import Settings
from fractal.core.utils.application_context import ApplicationContext as BaseContext


class ApplicationContext(BaseContext):
    def load_repositories(self):
        if Settings().USER_REPOSITORY_BACKEND == "sql":
            '''some sql adapter code'''
        elif Settings().USER_REPOSITORY_BACKEND == "file":
            '''some file adapter code'''
        else:
            from app.service.adapters.users import InMemoryUserRepository

            self.user_repository: UserRepository = self.install_repository(
                InMemoryUserRepository(),
            )

domain/users.py

from abc import ABC
from dataclasses import dataclass

from fractal.core.models import Model
from fractal.core.repositories import Repository


@dataclass
class User(Model):
    id: str
    name: str


class UserRepository(Repository[User], ABC):
    pass

adapters/users.py

from app.service.domain.users import User, UserRepository
from fractal.core.repositories.inmemory_repository_mixin import InMemoryRepositoryMixin


class InMemoryUserRepository(UserRepository, InMemoryRepositoryMixin[User]):
    pass

Advanced features

Command bus pattern

A command is a container to invoke actions in the domain, from inside and outside of the domain. A command has a one-to-one relation with a handler, a so called command handler. The command handler can be seen as a single transaction, e.g., to a database.

The code in the command handler should just be doing the thing that is necessary to be inside the transaction. A transaction can go wrong and can be rolled back, so it's important to prevent side effects from happening and include only the code that needs to go in the same transaction and thus will be rolled back as a whole when going wrong. Secondary actions that need to take place after the action has been done, should be outside of scope of the command handler.

After a command handler has been completed successfully, that is, when the transaction is persisted, an event will be published. This event is the trigger for all secondary actions, which in turn can be commands again.

TODO

Event bus

Next to the command bus, Fractal provides an event bus. Events are published onto the event bus by the command handlers. Event projectors may be used to subscribe to certain events to be able to perform a new action, a side effect. A projector can be used to publish events externally as well, for example, onto event streaming platforms. Other applications (or Fractal services) can subscribe to these events again and project them internally.

TODO

Eventual consistency

TODO

Event sourcing

TODO

Specification pattern

TODO

FastAPI + Docker

TODO

Request contract, together with URI parameters and authentication token payload can be processed by the application by using the command bus. The command can ingest the separate variables and/or domain objects (entities).

Response contract might be different from the domain object that is affected by the request.

Authentication

TODO