Architecture Patterns with Python

1. Domain modeling is the foundation of clean, maintainable software architecture

A domain model is not a data model—we're trying to capture the way the business works: workflow, rules around state changes, messages exchanged; concerns about how the system reacts to external events and user input.

Domain-Driven Design (DDD) emphasizes creating a rich domain model that reflects the business logic and processes. This approach separates core business rules from infrastructure concerns, making the system more flexible and easier to maintain. Key concepts include:

• Entities: Objects with a distinct identity that persists over time
• Value Objects: Immutable objects defined by their attributes
• Aggregates: Clusters of related objects treated as a unit for data changes
• Domain Events: Represent significant occurrences within the domain

By focusing on the domain model, developers can create a shared language with stakeholders, improving communication and ensuring the software accurately represents the business requirements.

2. Repository and Unit of Work patterns decouple the domain from infrastructure

The Repository pattern is an abstraction over persistent storage, allowing us to decouple our model layer from the data layer.

Repository pattern provides a collection-like interface for accessing domain objects, hiding the details of data access. Unit of Work pattern maintains a list of objects affected by a business transaction and coordinates the writing out of changes. Together, they offer several benefits:

• Separation of concerns: Domain logic remains pure, free from infrastructure details
• Testability: Easier to mock or fake for unit testing
• Flexibility: Ability to switch between different storage mechanisms without affecting the domain

These patterns create a clear boundary between the domain and data access layers, allowing each to evolve independently and promoting a more modular architecture.

3. Service Layer pattern orchestrates use cases and defines system boundaries

The Service Layer pattern is an abstraction over domain logic that defines the application's use cases and what they require from the domain model.

Service Layer acts as a facade for the domain model, encapsulating application-specific logic and orchestrating the execution of use cases. It provides several advantages:

• Clear API: Defines the operations the application can perform
• Separation of concerns: Keeps domain logic separate from application logic
• Testability: Enables high-level unit tests without need for integration tests

By implementing a Service Layer, developers can create a clear boundary between the application's external interfaces (e.g., API, CLI) and its internal domain logic, making the system easier to understand and maintain.

4. Event-driven architecture enables loose coupling and scalability

Events can help us to keep things tidy by separating primary use cases from secondary ones. We also use events for communicating between aggregates so that we don't need to run long-running transactions that lock against multiple tables.

Event-driven architecture uses events to trigger and communicate between decoupled services. This approach offers several benefits:

• Loose coupling: Services can evolve independently
• Scalability: Easier to scale individual components
• Flexibility: Simplifies adding new features or changing business processes

Key components of event-driven systems include:

• Domain Events: Represent significant changes in the domain
• Message Bus: Routes events to appropriate handlers
• Event Handlers: React to specific events and perform actions

This architecture enables systems to handle complex workflows and integrate multiple services while maintaining modularity and scalability.

5. Command-Query Responsibility Segregation (CQRS) optimizes read and write operations

Reads and writes are different, so they should be treated differently (or have their responsibilities segregated, if you will).

CQRS separates the read and write models of an application, allowing each to be optimized independently. This pattern is particularly useful for complex domains or high-performance systems. Benefits include:

• Performance optimization: Read and write models can be scaled separately
• Simplified models: Each model focuses on a single responsibility
• Flexibility: Enables use of different data stores for reads and writes

Implementation strategies:

• Separate read and write models
• Use different databases for reads and writes
• Implement eventual consistency between read and write sides

While CQRS adds complexity, it can significantly improve performance and scalability in the right scenarios.

6. Dependency Injection promotes flexibility and testability

Dependency injection (DI) is a technique whereby an object's dependencies are provided to it, rather than the object itself creating or managing those dependencies.

Dependency Injection is a design pattern that improves code modularity, testability, and flexibility. Key benefits include:

• Loose coupling: Objects don't need to know how their dependencies are created
• Testability: Easy to swap real implementations with test doubles
• Flexibility: Simplifies changing implementations without modifying dependent code

Implementing DI:

• Constructor injection: Dependencies provided through the constructor
• Property injection: Dependencies set through public properties
• Method injection: Dependencies provided as method parameters

By using DI, developers can create more modular and maintainable code, especially when combined with other patterns like Repository and Unit of Work.

7. Validation at the system's edge ensures data integrity and simplifies the domain

Validate at the edge when possible. Validating required fields and the permissible ranges of numbers is boring, and we want to keep it out of our nice clean codebase. Handlers should always receive only valid messages.

Edge validation involves verifying inputs at the system's entry points before they reach the domain logic. This approach offers several advantages:

• Clean domain model: Domain logic focuses on business rules, not input validation
• Improved security: Catches malformed or malicious inputs early
• Better user experience: Provides immediate feedback on invalid inputs

Types of validation:

• Syntactic: Ensures correct data structure and types
• Semantic: Verifies the meaning and consistency of data
• Pragmatic: Applies business rules in the context of the operation

By implementing thorough validation at the system's edge, developers can create more robust and maintainable applications while keeping the domain model focused on core business logic.

Last updated: March 11, 2025