App Engine Instance times out while connecting to the Cloud SQL Database

[himanshuy]

I have a bare minimum Flask App which talks to the Cloud SQL Postgres Database. This application works fine when run it from local server and connect to the cloud postgres instance. But I receive `502 Bad Gateway` if I run the same code from App Engine.

```app = Flask(__name__)
app.config['TIMEOUT'] = 300 # set timeout to 300 seconds (5 minutes)

# Replace with your actual PostgreSQL database credentials
DB_USER = os.environ.get('CLOUD_SQL_USERNAME')
DB_PASSWORD = os.environ.get('CLOUD_SQL_PASSWORD')
DB_NAME = os.environ.get('CLOUD_SQL_DATABASE_NAME')
DB_HOST = os.environ.get('CLOUD_SQL_CONNECTION_NAME')

# Initialize SQLAlchemy
app.config['SQLALCHEMY_DATABASE_URI'] = f"postgresql://{DB_USER}:{DB_PASSWORD}@{DB_HOST}/{DB_NAME}"
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)

def init_db():
db.create_all()

@App.route('/')
def index():
return '<a href="{}">Login with GCP</a>'.format(get_login_url())

@App.route('/test-db-connection')
def test_db_connection():
try:
db.session.execute(text('SELECT 1'))
return 'Database connection successful.'
except Exception as e:
return f'Database connection error: {e}'```

[christianpaula]

Hi @himanshuy,

Welcome to Google Cloud Community!

The 502 Bad Gateway error when connecting to a Cloud SQL database from an App Engine instance can be caused by a number of issues, including firewall rules, authorized networks, connection string, connection pool, timeout settings, database load, and more.

To troubleshoot this issue, you can:

1. Check the firewall rules for your Cloud SQL instance and ensure that they allow incoming connections from the App Engine instance.

2. Make sure that the IP address of the App Engine instance is added to the list of authorized networks for the Cloud SQL instance.

3. Verify that the connection string in your Flask app is correct and points to the correct Cloud SQL instance.

4. Adjust the connection pool settings if you're using a connection pool in your Flask app.

5. Increase the timeout limit using the app.config['TIMEOUT'] parameter in your Flask app.

6. Check the database load in the Cloud SQL console to see if it's causing the issue.

If none of these solutions work, you may contact Google Cloud Support to further check your project.

Thanks