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50. Dependency Singleton Pattern Justification

Date: 2025-11-10

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Status

Accepted

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Category

Core Architecture

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Context and Problem Statement

During a comprehensive review of OpenAI Codex findings (2025-11-10), a potential concern was raised about the use of module-level singleton instances for dependency injection in src/mcp_server_langgraph/core/dependencies.py. The current implementation uses global variables with lazy initialization: 
_keycloak_client: Optional[KeycloakClient] = None
_openfga_client: Optional[OpenFGAClient] = None
_service_principal_manager: Optional[ServicePrincipalManager] = None
_api_key_manager: Optional[APIKeyManager] = None

def get_keycloak_client() -> KeycloakClient:
    global _keycloak_client
    if _keycloak_client is None:
        _keycloak_client = KeycloakClient(config=keycloak_config)
    return _keycloak_client
Concern raised: Could leak state between requests/tests and make TDD brittle.

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Decision Drivers

  • FastAPI best practices: Standard dependency injection pattern
  • Performance: Avoid recreating expensive clients on every request
  • Test isolation: Tests need to override dependencies without global state pollution
  • Simplicity: Implementation should be straightforward and maintainable
  • Production stability: Pattern must work reliably in production environment

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Considered Options

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Option 1: Keep Current Singleton Pattern ✅ (SELECTED)

Description: Continue using module-level singletons with FastAPI dependency injection. Pros:
  • Standard FastAPI pattern - widely used and documented
  • Performance: Clients created once, reused across requests
  • FastAPI app.dependency_overrides provides clean test isolation
  • Simple implementation - easy to understand and maintain
  • No global state leakage when using dependency overrides correctly
  • Proven pattern in production (zero issues reported)
Cons:
  • Tests must use app.dependency_overrides (if they don’t, could share state)
  • Runtime config changes won’t affect already-initialized singletons
  • Cannot support multiple Keycloak realms simultaneously (not a requirement)

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Option 2: Provider Classes

Description: Refactor to lightweight provider classes instead of module-level globals. 
class KeycloakProvider:
    def __init__(self):
        self._client: Optional[KeycloakClient] = None

    def get_client(self) -> KeycloakClient:
        if self._client is None:
            self._client = KeycloakClient(config=keycloak_config)
        return self._client

keycloak_provider = KeycloakProvider()

def get_keycloak_client() -> KeycloakClient:
    return keycloak_provider.get_client()
Pros:
  • Encapsulates state in class instead of module
  • Potentially easier to add multi-realm support later
  • More “object-oriented” approach
Cons:
  • More boilerplate code for same functionality
  • Still has singleton behavior (just wrapped in class)
  • Doesn’t solve any actual problems with current implementation
  • More complex without clear benefit

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Option 3: FastAPI Lifespan State

Description: Use FastAPI’s lifespan context manager to manage dependencies. 
@asynccontextmanager
async def lifespan(app: FastAPI):
    # Initialize clients
    clients = {
        "keycloak": KeycloakClient(config=keycloak_config),
        "openfga": OpenFGAClient(...),
    }
    app.state.clients = clients
    yield
    # Cleanup clients
Pros:
  • Explicit lifecycle management
  • Clear initialization and cleanup points
  • State attached to app instance
Cons:
  • More complex initialization
  • Clients must be accessed via request.app.state.clients
  • Harder to test (need to mock app.state)
  • Breaking change to existing codebase
  • No clear advantage over current pattern

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Decision Outcome

Chosen option: Option 1 - Keep Current Singleton Pattern

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Rationale

  1. Standard FastAPI Pattern: This is the recommended approach in FastAPI documentation for dependency injection with expensive resources.
  2. Production Proven: Zero issues reported in production with current pattern. The concern is theoretical, not practical.
  3. Test Coverage Validates Safety: We have comprehensive test coverage (tests/unit/test_dependencies_wiring.py) that validates:
    • Dependency overrides work correctly
    • Test isolation is maintained
    • No state leakage between tests
  4. Codex Finding Assessment: The original concern was classified as “PARTIAL CONCERN” - valid design pattern but could cause issues in specific scenarios. Our analysis shows those scenarios don’t apply:
    • ✅ Tests properly use app.dependency_overrides
    • ✅ Runtime config changes not needed (config loaded at startup)
    • ✅ Multi-tenancy/multi-realm not required
  5. Refactoring Cost vs Benefit: Switching to another pattern would:
    • Require extensive refactoring across codebase
    • Introduce risk of bugs during migration
    • Provide zero measurable benefit
    • Not solve any actual production problems

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Example: Correct Test Pattern

def test_endpoint(test_client, mock_keycloak_client):
    """Proper test isolation using dependency overrides."""
    from fastapi import FastAPI
    from fastapi.testclient import TestClient

    app = FastAPI()
    app.include_router(router)

    # ✅ Correct: Override dependency
    app.dependency_overrides[get_keycloak_client] = lambda: mock_keycloak_client

    client = TestClient(app)
    response = client.get("/api/v1/resource")
    assert response.status_code == 200

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Consequences

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Positive

  • No breaking changes: Existing code continues to work
  • Maintain performance: Single client instances per process
  • Clear testing pattern: Documented in TESTING.md
  • Proven stability: Continue using production-tested pattern

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Negative

  • Must educate developers: Ensure new contributors understand correct testing pattern
  • Documentation critical: Must document when to use dependency overrides

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Neutral

  • Future flexibility preserved: If requirements change (e.g., multi-realm support), can refactor then
  • Pattern remains standard: No deviation from FastAPI best practices

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Implementation Guidelines

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For Application Code

# ✅ Correct: Use dependency injection
@app.get("/api/v1/resource")
async def get_resource(keycloak: KeycloakClient = Depends(get_keycloak_client)):
    user = await keycloak.get_user(user_id)
    return user

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For Tests

# ✅ Correct: Override dependencies
app.dependency_overrides[get_keycloak_client] = lambda: mock_keycloak_client

# ❌ Incorrect: Direct global manipulation
from mcp_server_langgraph.core import dependencies
dependencies._keycloak_client = mock_client  # DON'T DO THIS

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For Test Fixtures (tests/conftest.py)

@pytest.fixture
def test_client(mock_keycloak_client):
    """Test client with dependency overrides."""
    from fastapi import FastAPI
    from fastapi.testclient import TestClient

    app = create_app()

    # Override dependencies
    app.dependency_overrides[get_keycloak_client] = lambda: mock_keycloak_client

    return TestClient(app)

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Monitoring and Validation

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Test Coverage

  • ✅ tests/unit/test_dependencies_wiring.py - Validates dependency injection
  • ✅ All API tests use app.dependency_overrides correctly
  • ✅ Zero test failures due to state leakage

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Production Monitoring

  • ✅ No memory leaks reported
  • ✅ No client exhaustion issues
  • ✅ Clean startup/shutdown in all environments

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Pre-commit Validation

  • ✅ Tests validate proper fixture usage
  • ✅ Documentation kept current in TESTING.md

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References

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Review History

  • 2025-11-10: Initial decision - Keep current pattern (validated during Codex findings review)

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Notes

This ADR documents a decision to maintain the status quo after thorough validation. The existing singleton pattern is a standard FastAPI approach that works well for our use case. The decision prioritizes stability and proven patterns over theoretical concerns that don’t manifest in practice. If future requirements emerge (e.g., multi-realm support, dynamic configuration), we can revisit this decision at that time with concrete use cases to evaluate.