504 Error in Azure Web App (Fastapi & Postgres) with structure like Microsoft Fastapi/Postgres Demo App

Question

504 Error in Azure Web App (Fastapi & Postgres) with structure like Microsoft Fastapi/Postgres Demo App

Nicolas Joye 0

I keep getting 504 errors when trying to connect to the Postgres database from an app that I built using the microsoft fastapi/postgres demo app. I followed these instructors to the T. https://learn.microsoft.com/en-us/azure/app-service/tutorial-python-postgresql-app-fastapi?tabs=windows&pivots=azure-portal and even cloned the repo here https://github.com/Azure-Samples/msdocs-fastapi-postgresql-sample-app. I have seemingly tried everything. This happens not just on a cold start, but even after the cold start. I am using the $250 premium tier for the web app. I am using the General Purpose 2-64 vCores Azure Database for PostgresSQL flexible Server with Standard_D2s_v3 (2 vCores, 8 GiB memory, 3200 max iops) with 128 GiB storage and P10 (500 iops) performance tar. With this type of setup, i would assume a speedy response from any call that is within a limited character length. This is currently happening with small or large calls, seemingly agnostic to the size of call. Since I used the sample project, I assumed the setup that connects to the database would be solid. Thank you for your support.

Siva Nair 2,250 Reputation points Microsoft External Staff Moderator

2025-05-29T19:30:28.6233333+00:00
Hi Nicolas Joye,

Reaching out to you in private chat, Please check , meanwhile please check the below troubleshooting steps.

A 504 error usually indicates a timeout, which could be caused by multiple factors. Here are a few suggestions to help you troubleshoot the issue:

Make sure that your FastAPI application can establish a connection to the PostgreSQL database. You can test this by logging into your database from a local machine using the same credentials and connection details.

Double-check the application settings in Azure. Ensure that the AZURE_POSTGRESQL_CONNECTIONSTRING is correctly set up and points to your PostgreSQL server. If this setting is missing or incorrect, your app won't be able to connect to the database.

Since you're using a General Purpose 2-64 vCores Azure Database for PostgreSQL, ensure that it is not under heavy load or hitting any IOPS limits. Monitor the performance metrics in the Azure portal to see if there are any spikes or throttles in usage.

Ensure there are no firewall rules or network security group settings that could be blocking access to your database from the app service. You might need to allow the IP range of your app service in the PostgreSQL firewall settings.

Sometimes, a simple restart of your Azure web app can resolve transient issues.

Enable verbose logging in your FastAPI app to get more insight into the error. This will help you trace where the connection is failing.

here are a few follow-up questions that might help narrow down the problem:

Check if the PostgreSQL server is running and accessible from other environments (like your local machine)?

Are there any specific error messages or logs from the FastAPI application that you can share?

If you have any further assistant, do let me know.
Sampath 3,750 Reputation points Microsoft External Staff Moderator

2025-06-04T14:55:22.9433333+00:00
Hi Nicolas Joye,
In the Azure portal or via the CLI, create a new private endpoint and specify the Table service.

Verify that the DNS name (<your-storage-account-name>.table.core.windows.net) resolves to the private IP address of your new Table private endpoint. If you’re using Azure DNS Private Zones, make sure it includes an entry for this FQDN.

For example, you can verify with:

nslookup <your-storage-account-name>.table.core.windows.net

In the App Service’s Configuration → Environment Variables, ensure that the connection string AZURE_POSTGRESQL_CONNECTIONSTRING is correct.

Use the App Service’s SSH console to manually connect to the database:
psql "<connection-string-from-env-var>"

Or run: python3 src/fastapi_app/seed_data.py ```- Confirm that the database **VNet rules** allow traffic from the App Service’s **subnet**.

In the Azure portal:

Go to PostgreSQL Flexible Server → Networking.

Confirm the VNet/subnet integration is configured for the App Service.

In your App Service SSH console, test database resolution:
nslookup <db-hostname> ping <db-hostname>
If these fail, there’s likely a DNS resolution issue within the VNet or Private DNS zone misconfiguration.

Check PostgreSQL logs for long-running queries, locks, or connection limits.

Confirm that your Startup Command is set correctly in the App Service:
src/entrypoint.sh

In entrypoint.sh, ensure it’s not stuck on migrations (like running seed_data.py) repeatedly on each app start, which could cause delays.

In App Service → App Service logs, enable and stream application logs to see real-time logs:

File system logging enabled.

Use logger.info() calls in your FastAPI app to capture slow points.

Check logs for repeated errors or delays in DB connectivity.

Open App Service → SSH console:

Run: python3 src/fastapi_app/seed_data.py

Check PostgreSQL connection logs for suspicious entries:

Failed authentication

Rejected IP addresses

Connection pool exhaustion

Validate Private DNS zone:

In the Azure portal, go to Private DNS zones linked to the VNet.

Confirm it has a record for your PostgreSQL server (<servername>.postgres.database.azure.com).

Use psql in the SSH session to run a direct DB query:

psql "<AZURE_POSTGRESQL_CONNECTIONSTRING>" SELECT now();

If this is fast, DB is reachable.

In App Service logs, watch for lines like:

KeyError: 'AZURE_POSTGRESQL_CONNECTIONSTRING'

or

psycopg2.OperationalError

This would confirm if environment variables are missing.

If everything checks out, review FastAPI code:

Confirm DB sessions are closed properly (use FastAPI’s dependency lifecycle to avoid long-lived connections).

Monitor for connection leaks in get_db_session().

If you have any further assistant, do let me know.

Your answer

Siva Nair 2,250 Reputation points Microsoft External Staff Moderator

2025-05-29T19:30:28.6233333+00:00

Hi Nicolas Joye,

Reaching out to you in private chat, Please check , meanwhile please check the below troubleshooting steps.

A 504 error usually indicates a timeout, which could be caused by multiple factors. Here are a few suggestions to help you troubleshoot the issue:

Make sure that your FastAPI application can establish a connection to the PostgreSQL database. You can test this by logging into your database from a local machine using the same credentials and connection details.

Double-check the application settings in Azure. Ensure that the AZURE_POSTGRESQL_CONNECTIONSTRING is correctly set up and points to your PostgreSQL server. If this setting is missing or incorrect, your app won't be able to connect to the database.

Since you're using a General Purpose 2-64 vCores Azure Database for PostgreSQL, ensure that it is not under heavy load or hitting any IOPS limits. Monitor the performance metrics in the Azure portal to see if there are any spikes or throttles in usage.

Ensure there are no firewall rules or network security group settings that could be blocking access to your database from the app service. You might need to allow the IP range of your app service in the PostgreSQL firewall settings.

Sometimes, a simple restart of your Azure web app can resolve transient issues.

Enable verbose logging in your FastAPI app to get more insight into the error. This will help you trace where the connection is failing.

here are a few follow-up questions that might help narrow down the problem:

Check if the PostgreSQL server is running and accessible from other environments (like your local machine)?

Are there any specific error messages or logs from the FastAPI application that you can share?

If you have any further assistant, do let me know.

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504 Error in Azure Web App (Fastapi & Postgres) with structure like Microsoft Fastapi/Postgres Demo App

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