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To support zero-downtime deployments, configure a health check endpoint in your FastAPI application. This endpoint allows Sevalla to verify that your app is running correctly before routing traffic to it. There are multiple ways to implement a health check in FastAPI, depending on the level of control and visibility required.

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Option 1: Use an existing endpoint

If you prefer not to create a separate health check path, you can use any existing public endpoint in your app. Choose a path that will return an error if a critical service, like your database, is not functioning correctly.

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Option 2: Use a dedicated health check endpoint

A dedicated health check endpoint allows you to explicitly verify the health of your app’s dependencies. For example, you can check database connectivity by executing a simple query such as session.exec(text("SELECT 1")). If the connection fails, the endpoint will return an error. 
# app/main.py
from fastapi import FastAPI
from app.core.config import settings
from app.api.deps import SessionDep
from sqlalchemy import text

app = FastAPI()

@app.get("/health_check")
def health_check(session: SessionDep):
    session.exec(text("SELECT 1"))
    return "OK"
You can extend this endpoint to include additional checks for other services your app depends on.

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Option 3: Use a health check library

Health check libraries provide ready-made solutions for monitoring the state of your app’s dependencies, such as databases, task queues, caches, and email services. One popular library for FastAPI is FastAPI Health, which simplifies the creation of robust health check endpoints.