fastapi-dependency-injection

FastAPI Dependency Injection & Architecture Rules

🚨 CRITICAL: Prevent Circular Import Dependencies

Sprint 10 suffered from circular import issues between main.py and router files. ALWAYS use dependency injection.

✅ MANDATORY PATTERN: Dependency Injection

1. Dependencies Module Setup

# backend/dependencies.py
from qdrant_client import QdrantClient
from sentence_transformers import SentenceTransformer

class AppState:
    """Centralized application state container."""
    def __init__(self):
        self.qdrant_client: QdrantClient | None = None
        self.ml_model: SentenceTransformer | None = None
        self.active_collection: str | None = None

# Global instance - safe because it's explicitly typed
app_state = AppState()

# Dependency provider functions
def get_qdrant_client() -> QdrantClient:
    """Dependency function to get the initialized Qdrant client."""
    if app_state.qdrant_client is None:
        raise RuntimeError("Qdrant client has not been initialized.")
    return app_state.qdrant_client

def get_ml_model() -> SentenceTransformer:
    """Dependency function to get the initialized ML model."""
    if app_state.ml_model is None:
        raise RuntimeError("ML model has not been initialized.")
    return app_state.ml_model

def get_active_collection() -> str:
    """Dependency function to get the currently active collection name."""
    if app_state.active_collection is None:
        return "default_collection"  # Or raise HTTPException
    return app_state.active_collection

2. Main App Initialization

# backend/main.py
from contextlib import asynccontextmanager
from fastapi import FastAPI
from .dependencies import app_state
from .routers import search, images, collections

@asynccontextmanager
async def lifespan(app: FastAPI):
    """Application lifespan manager for startup/shutdown."""
    # --- Startup ---
    print("INFO:     Starting up services...")
    
    # Initialize services into app_state
    app_state.qdrant_client = QdrantClient(host="localhost", port=6333)
    app_state.ml_model = SentenceTransformer("clip-ViT-B-32")
    app_state.active_collection = "default_collection"
    
    print("INFO:     Startup complete.")
    yield
    
    # --- Shutdown ---
    print("INFO:     Shutting down services...")

# Create app with lifespan
app = FastAPI(lifespan=lifespan)

# Include routers - NO circular imports
app.include_router(search.router)
app.include_router(images.router)
app.include_router(collections.router)