How do you throttle database connections during traffic spikes to prevent pool exhaustion?

import asyncio
from fastapi import HTTPException, Depends
from sqlalchemy.ext.asyncio import AsyncSession

# Limit to 20 concurrent DB connections
db_semaphore = asyncio.Semaphore(20)

async def get_throttled_db():
    """Throttled DB session with backpressure"""
    if not await db_semaphore.acquire(timeout=0.1):  # Non-blocking acquire
        raise HTTPException(429, "DB overloaded - retry later")
    
    async with AsyncSession(engine) as session:
        try:
            yield session
        finally:
            db_semaphore.release()

@app.get("/users")
async def get_users(db: AsyncSession = Depends(get_throttled_db)):
    return await db.execute(select(User))