While using Flask-SQLAlchemy I get the error 'QueuePool limit of size 10 overflow 10 reached, connection timed out' consistently, after some time. I tried to increase connection pool size, but it only deferred the problem.

def create_app(config_name):
    app = Flask(__name__)
    app.config.from_object(config[config_name])
    config[config_name].init_app(app)
    initialize_db(app)

db = SQLAlchemy()

def initialize_db(app):
    db.init_app(app)

SQLALCHEMY_POOL_SIZE = 100

solution

I figured out the problem. The issue was sometimes database connection was going to lost state, which is causing pool size to be Exhausted after some interval.
To fix the issue I made the MySQL server configuration change for query timeout and made it 1 second.
After 1 second if the query didn't respond it will throw Exception and I added except block in code where that query was invoked(In my case it was GET query). In the Except block, I issued rollback command.