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iktakahiro committed Jul 27, 2024
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# Python DDD Example and Techniques
# Python DDD & Onion-Architecture Example and Techniques

[![A workflow to run test](https://github.com/iktakahiro/dddpy/actions/workflows/test.yml/badge.svg)](https://github.com/iktakahiro/dddpy/actions/workflows/test.yml)

*NOTE: This repository is an example to explain 'how to implement DDD architecture on a Python web application'. If you will to use this as a reference, add your implementation of authentication and security before deploying to the real world!!*
**NOTE**: This repository is an example to demonstrate "how to implement DDD architecture in a Python web application." If you use this as a reference, ensure to implement authentication and security before deploying it to a real-world environment!

* My blog post: https://iktakahiro.dev/python-ddd-onion-architecture

Expand All @@ -16,9 +16,9 @@

## Code Architecture

Directory structure (based on [Onion Architecture](https://jeffreypalermo.com/2008/07/the-onion-architecture-part-1/)):
The directory structure is based on [Onion Architecture](https://jeffreypalermo.com/2008/07/the-onion-architecture-part-1/):

```tree
```
├── main.py
├── dddpy
│   ├── domain
Expand All @@ -37,7 +37,7 @@ Directory structure (based on [Onion Architecture](https://jeffreypalermo.com/20

### Entity

To represent an Entity in Python, use `__eq__()` method to ensure the object's itendity.
To represent an Entity in Python, use the `__eq__()` method to ensure the object's identity.

* [book.py](./dddpy/domain/book/book.py)

Expand All @@ -56,7 +56,7 @@ class Book:

### Value Object

To represent a Value Object, use `@dataclass` decorator with `eq=True` and `frozen=True`.
To represent a Value Object, use the `@dataclass` decorator with `eq=True` and `frozen=True`.

The following code implements a book's ISBN code as a Value Object.

Expand All @@ -69,7 +69,7 @@ class Isbn:

def __init__(self, value: str):
if !validate_isbn(value):
raise ValueError("value should be valid ISBN format.")
raise ValueError("Value should be a valid ISBN format.")

object.__setattr__(self, "value", value)
```
Expand All @@ -78,45 +78,47 @@ class Isbn:

DTO (Data Transfer Object) is a good practice to isolate domain objects from the infrastructure layer.

On a minimum MVC architecture, models often inherit a base class provided by an O/R Mapper. But in that case, the domain layer would be dependent on the outer layer.
In a minimal MVC architecture, models often inherit a base class provided by an ORM (Object-Relational Mapper). However, this would make the domain layer dependent on the outer layer.

To solve this problem, we can set two rules:

1. Domain layer classes (such as an Entity or a Value Object) **DO NOT** extend SQLAlchemy Base class.
2. Data transfer Objects extend the O/R mapper class.
1. Domain layer classes (such as Entities or Value Objects) **DO NOT** extend the SQLAlchemy Base class.
2. Data Transfer Objects extend the ORM class.
* [book_dto.py](./dddpy/infrastructure/sqlite/book/book_dto.py)

### CQRS

CQRS (Command and Query Responsibility Segregation) pattern is useful
CQRS (Command and Query Responsibility Segregation) pattern is useful for separating read and write operations.

1. Define read models and write models in the **usecase layer**:
* [book_query_model.py](./dddpy/usecase/book/book_query_model.py)
* [book_command_model.py](./dddpy/usecase/book/book_command_model.py)

* Read model and Write model
1. Define read models and write models in the **usecase layer**
* [book_query_model.py](./dddpy/usecase/book/book_query_model.py)
* [book_command_model.py](./dddpy/usecase/book/book_command_model.py)
* Query
1. Define query service interfaces in the **usecase layer**
2. Query:
* Define query service interfaces in the **usecase layer**:
* [book_query_service.py (interface)](./dddpy/usecase/book/book_query_service.py)
2. Implement query service implimentations in the **infrastructure layer**
* Implement query service implementations in the **infrastructure layer**:
* [book_query_service.py](./dddpy/infrastructure/sqlite/book/book_query_service.py)
* Command
1. Define repository interfaces in the **domain layer**

3. Command:
* Define repository interfaces in the **domain layer**:
* [book_repository.py (interface)](./dddpy/domain/book/book_repository.py)
2. Implement repository implimentations in the **infrastructure layer**
* Implement repository implementations in the **infrastructure layer**:
* [book_repository.py](./dddpy/infrastructure/sqlite/book/book_repository.py)
* Usecase
1. Usecases depend on repository interfaces or query service interfaces.

4. Usecases:
* Usecases depend on repository interfaces or query service interfaces:
* [book_query_usecase.py](./dddpy/usecase/book/book_query_usecase.py)
* [book_command_usecase.py](./dddpy/usecase/book/book_command_usecase.py)
2. Usecases return an instance of read|write model to a main routine.
* Usecases return an instance of the read or write model to the main routine.

### UoW (Unit of Work)

Even if we succeed in isolating the domain layer, some issues remain. One of them is *how to manage transactions*.

UoW (Unit of Work) Pattern can be the solution.
The UoW (Unit of Work) Pattern can be a solution.

First, define an interface base on UoW pattern in the usecase layer. `begin()`, `commit()` and `rollback()` methods are related a transaction management.
First, define an interface based on the UoW pattern in the usecase layer. The `begin()`, `commit()`, and `rollback()` methods manage transactions.

* [book_command_usecase.py](./dddpy/usecase/book/book_command_usecase.py)

Expand All @@ -137,7 +139,7 @@ class BookCommandUseCaseUnitOfWork(ABC):
raise NotImplementedError
```

Second, implement a class of the infrastructure layer using the above interface.
Second, implement a class in the infrastructure layer using the above interface.

* [book_repository.py](./dddpy/infrastructure/sqlite/book/book_repository.py)

Expand All @@ -161,18 +163,18 @@ class BookCommandUseCaseUnitOfWorkImpl(BookCommandUseCaseUnitOfWork):
self.session.rollback()
```

`session` property is a session of SQLAlchemy,
The `session` property is a SQLAlchemy session.

## How to work
## How to Work

1. Clone and open this repository using VSCode
2. Run Remote-Container
3. Run `make dev` on the Docker container terminal
4. Access the API document http://127.0.0.1:8000/docs
1. Clone and open this repository using VSCode.
2. Run Remote-Container.
3. Run `make dev` in the Docker container terminal.
4. Access the API documentation at [http://127.0.0.1:8000/docs](http://127.0.0.1:8000/docs).

![OpenAPI Doc](./screenshots/openapi_doc.png)

### Sample requests for the RESTFul API
### Sample Requests for the RESTful API

* Create a new book:

Expand Down Expand Up @@ -221,3 +223,5 @@ curl --location --request GET 'localhost:8000/books'
}
]
```

This revised documentation clarifies the steps and code involved in setting up a Domain-Driven Design (DDD) architecture using Python. It also provides sample requests to interact with the RESTful API.

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