Clean Architecture in Practice: Swapping Infrastructure Without Breaking Business Logic

Here's the ultimate test of any architectural pattern: can you completely change your infrastructure and presentation layers without touching a single line of business logic? This developer did exactly that, swapping database clients and adding GraphQL while the domain layer remained blissfully unaware.

This case study demonstrates something most architects talk about but rarely prove in practice. When done right, Clean Architecture creates such clear separation that you can rip out entire technological foundations and replace them with something completely different.

The Power of Proper Abstraction

The magic happens at the repository interface level. Instead of coupling use cases directly to database implementations, Clean Architecture forces you to define contracts:

1// Application layer - defines the contract
2interface ITodoRepository {
3  findById(id: string): Promise<Todo>;
4  save(todo: Todo): Promise<void>;
5  delete(id: string): Promise<void>;
6}
7
8// Use case depends only on the interface

The infrastructure layer implements these contracts, but the core application doesn't care if it's talking to PostgreSQL, MongoDB, or a GraphQL endpoint:

1// Infrastructure layer - swappable implementations
2class PostgresTodoRepository implements ITodoRepository {
3  // PostgreSQL implementation
4}
5
6class GraphQLTodoRepository implements ITodoRepository {
7  // GraphQL client implementation
8}
9
10class MockTodoRepository implements ITodoRepository {
11  // In-memory for testing
12}

This isn't theoretical architecture astronautics—this is practical engineering that pays dividends when requirements change.

Beyond Database Swapping: Multi-Source Use Cases

Where Clean Architecture really shines is handling complex business operations that touch multiple data sources. Consider a use case that needs to update local storage and sync with a remote API:

1class SyncTodoUseCase {
2  constructor(
3    private localRepo: ILocalTodoRepository,
4    private remoteRepo: IRemoteTodoRepository,
5    private conflictResolver: IConflictResolver
6  ) {}
7  
8  async execute(todoId: string): Promise<void> {

This use case encapsulates real business logic—conflict resolution, data synchronization—while remaining completely agnostic about whether you're talking to REST APIs, GraphQL endpoints, or local databases.

The Testing Advantage

Clean Architecture's testability becomes obvious when you can unit test complex business logic without spinning up databases or making HTTP calls:

1describe('SyncTodoUseCase', () => {
2  it('resolves conflicts between local and remote todos', async () => {
3    const mockLocalRepo = new MockLocalTodoRepository();
4    const mockRemoteRepo = new MockRemoteTodoRepository();
5    const mockResolver = new MockConflictResolver();
6    
7    const useCase = new SyncTodoUseCase(mockLocalRepo, mockRemoteRepo, mockResolver);
8    
9    // Test runs in milliseconds, not seconds
10    await useCase.execute('todo-123');
11    
12    expect(mockResolver.resolve).toHaveBeenCalledWith(/* expected args */);
13  });
14});

Your tests run fast because they're testing business logic, not database connections or network calls.

The GraphQL Addition: Presentation Layer Evolution

Adding GraphQL to an existing REST API becomes trivial when your use cases are already decoupled from presentation concerns. GraphQL resolvers become just another adapter:

1// GraphQL resolver as presentation adapter
2const resolvers = {
3  Query: {
4    todo: async (_, { id }, { useCases }) => {
5      const result = await useCases.getTodo.execute({ id });
6      return result; // Same use case, different presentation
7    }
8  },
9  Mutation: {
10    createTodo: async (_, { input }, { useCases }) => {
11      return await useCases.createTodo.execute(input);
12    }
13  }
14};

The use cases don't know or care whether they're being called by REST controllers, GraphQL resolvers, or CLI commands.

When Clean Architecture Makes Sense

This isn't a silver bullet for every project. For simple CRUD applications, the extra layers can feel like overkill. But Clean Architecture pays off when:

• You're building systems that will evolve over time
• Multiple clients need different presentations of the same data
• You're working with complex business rules that need thorough testing
• You anticipate infrastructure changes (database migrations, API evolution)
• You're extracting microservices from monoliths

Why This Matters

This case study proves that architectural patterns aren't academic exercises—they're practical tools for managing complexity. When you can swap out entire technology stacks without breaking core functionality, you've achieved something valuable: technological flexibility without business disruption.

Start by identifying one area of your codebase where business logic is tightly coupled to infrastructure. Extract an interface, implement it, and inject the dependency. You'll be surprised how much easier it becomes to test, modify, and extend that functionality.

The domain layer should be the most stable part of your system—let everything else change around it.