python 34 lines · 6 steps

Liveness vs readiness health checks in FastAPI

Two endpoints separate whether the process is alive from whether its dependencies are actually reachable.

Explained by highlit
FastAPI Intermediate health-checks dependency-injection async error-handling observability
1from fastapi import APIRouter, status
2from fastapi.responses import JSONResponse
3from sqlalchemy import text
4from sqlalchemy.exc import SQLAlchemyError
5from sqlalchemy.ext.asyncio import AsyncSession
6from fastapi import Depends
7 
8from app.db import get_session
9 
10router = APIRouter(tags=["health"])
11 
12 
13@router.get("/health/live", status_code=status.HTTP_200_OK)
14async def liveness() -> dict[str, str]:
15 return {"status": "ok"}
16 
17 
18@router.get("/health/ready")
19async def readiness(session: AsyncSession = Depends(get_session)) -> JSONResponse:
20 checks: dict[str, str] = {}
21 
22 try:
23 await session.execute(text("SELECT 1"))
24 checks["database"] = "ok"
25 except SQLAlchemyError as exc:
26 checks["database"] = f"error: {exc.__class__.__name__}"
27 
28 healthy = all(value == "ok" for value in checks.values())
29 code = status.HTTP_200_OK if healthy else status.HTTP_503_SERVICE_UNAVAILABLE
30 
31 return JSONResponse(
32 status_code=code,
33 content={"status": "ok" if healthy else "degraded", "checks": checks},
34 )
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Liveness answers whether the process runs; readiness answers whether it can serve traffic.
  2. 2Probing dependencies with a cheap query surfaces outages without failing the whole request.
  3. 3Mapping check results to HTTP status codes lets orchestrators route traffic automatically.

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