Mysterious Database Connection Failure

Help
1

Hello everyone,

I’m hitting a wall with a deployment issue on Vercel for my Python FastAPI application, and I’m hoping someone in the community might have experienced something similar or can offer a fresh perspective.

Project Setup:

  • Backend: Python FastAPI

  • Database: PostgreSQL (currently Neon, but similar issues were seen trying to connect to Supabase before)

  • Database Driver: asyncpg

  • Hosting: Vercel (Serverless Function)

  • Issue: 503 Service Unavailable on API endpoints because the database connection fails during application startup.

The Problem - A Logging Black Hole:

My startup_event in FastAPI is where I try to establish the asyncpg connection pool to my Neon PostgreSQL database.

  • Locally (Python 3.13): The application connects perfectly fine to Neon, and all database operations work as expected.

  • Vercel Deployment (Python 3.11/3.12 - tried both):

    • The Vercel build phase completes successfully every time.

    • However, when the application attempts to start running, the startup_event clearly fails. All API calls relying on the database return a 503 Service Unavailable error to the frontend.

    • The critical and most frustrating part: My logger.info and logger.error statements, specifically added to log the POSTGRES_URL (masked, and even hardcoded for a test) and any exceptions during asyncpg.create_pool, do not appear anywhere in the Vercel Runtime Logs. The logs are silent during the application’s startup phase, other than showing the incoming HTTP requests and the 503 responses.

What I’ve Tried & Verified (All failed to solve the core logging/startup issue):

  • Database Connection String (POSTGRES_URL):

    • Confirmed postgresql:// scheme, correct credentials, host, port, database name, and ?sslmode=require on Vercel’s environment variables (triple-checked against Neon’s dashboard).

    • Even hardcoded the exact POSTGRES_URL directly into main.py to bypass os.getenv entirely (for diagnostic purposes, then immediately removed). Still no logs, still 503.

  • SSL Configuration:

    • Tried ssl=ssl.create_default_context() with asyncpg.create_pool.

    • Tried ssl=True.

    • Tried statement_cache_size=0 for asyncpg.

    • (Previously with Supabase, battled CERTIFICATE_VERIFY_FAILED and key usage extension errors, which were solved locally with specific CA bundles, but ultimately decided to switch to Neon for simplicity – this current problem is not that same SSL specific error, or it’s masked by the log failure).

  • Python Version: Explicitly set PYTHON_VERSION to 3.10 and 3.11 in vercel.json (as Vercel’s default was 3.12). No change.

  • Vercel Deployment Type: Forced new deployments without cache (Redeploy with existing Build Cache = No).

  • Database Provider: The issue persists across attempts to connect to both Supabase and Neon from Vercel, strongly pointing away from the database providers themselves.

  • Network (Basic Check): Neon is publicly accessible and doesn’t have IP whitelisting blocking Vercel.

My Hypothesis:

It seems like the Python application itself isn’t fully initializing or is crashing before its logging can be captured by Vercel’s runtime environment, preventing any meaningful diagnosis. The problem appears to be a deeper interaction or limitation within Vercel’s serverless Python runtime for my application’s specific startup pattern.

Seeking Community Insights:

  • Has anyone experienced Python application startup logs being completely missing on Vercel despite explicit logging calls?

  • Are there any obscure Vercel configurations or Python runtime behaviors I might be missing that could cause a “silent” startup failure like this?

  • Any suggestions on how to force more verbose output from a Vercel Python serverless function that isn’t even logging its own startup events?

Any ideas or shared experiences would be greatly appreciated. Thanks in advance!