How to solve the problem that Sqlalchemy database connection cannot be closed in Python?

Python SQLAlchemy database connection leak problem and solution

When using the Python SQLAlchemy library for database operations, you often encounter the problem that the database connection cannot be closed normally, resulting in connection leakage. This article analyzes a typical code example and provides an effective solution.

The following code snippet shows a database class that may have a connection leak:

 from sqlalchemy import create_engine, url, delete, update, select, exists
from sqlalchemy.orm import sessionmaker, scoped_session
from core.database.base import base # Assume this module exists from lib.type import type # Assume this module exists from typing import Any
from flask import g, current_app
import importlib
import re

class database:
    env = None

    # ... (Some code is omitted, it has nothing to do with connection closing) ...

    def __create_session(self, **config):
        engine = self.create_engine(**config)
        session = scoped_session(sessionmaker(bind=engine, autoflush=True))
        return type.database(engine=engine, session=session())

    # ... (Some code is omitted, it has nothing to do with connection closing) ...

    def table_data_query_all(self, model: Any, condition: list = None, order: list = None, limit: int = 500,
                             fields: list = None) -> list[dict]:
        query = select(model)
        # ... (Query logic omitted) ...
        asdasdas = [row.dict() for row in self.database.execute(query.limit(limit)).scalars()]
        self.database.get_bind().dispose() # Here try to close the connection return asdasdas

    # ... (Other methods are omitted) ...

    def close(self):
        if self.database is not None:
            self.database.close()
            # Consider a more thorough shutdown: self.database.get_bind().dispose()

The table_data_query_all method in the code tries to close the connection using self.database.get_bind().dispose() , but this may not always work, as the presence of scoped_session may cause the connection to be closed delayed, or not to be released correctly in some exceptional situations. self.database.close() may also be incomplete.

Solution:

1. Manage sessions using with statements: This is the most efficient way to solve SQLAlchemy connection leaks. Although scoped_session is convenient, it is not as efficient as context managers in resource management. It is recommended to refactor the code and use the with statement to manage the database session:

 def table_data_query_all(self, model: Any, condition: list = None, order: list = None, limit: int = 500,
                             fields: list = None) -> list[dict]:
        with self.__create_session(**self.database_conf).session as session: # Use the with statement query = select(model)
            # ... (Query logic omitted) ...
            asdasdas = [row.dict() for row in session.execute(query.limit(limit)).scalars()]
            return asdasdas

    # ...Remove the close method because the with statement automatically handles resource release...

2. Avoid scoped_session : If possible, try to avoid scoped_session . Although it simplifies the code, it increases the complexity of managing connections, which can easily lead to connection leakage. Create a new session directly where you need it and close it immediately after use.

3. Use teardown_appcontext in Flask app: If you use SQLAlchemy in Flask app, you can use teardown_appcontext decorator to ensure that the connection is closed after the request is over:

 from flask import Flask, g, current_app
from flask import teardown_appcontext

app = Flask(__name__)

# ...Other codes...

@app.teardown_appcontext
def close_connection(exception):
    db = getattr(g, 'db', None)
    if db is not None:
        db.close() # or db.get_bind().dispose()

Through the above methods, especially using the with statement to manage sessions, SQLAlchemy database connection leakage problem can be effectively avoided, ensuring that the database connection is correctly released, and improving the stability and performance of the application. Remember to choose the solution that best suits your application architecture. If a connection pool is used, it is necessary to make corresponding adjustments according to the characteristics of the connection pool.

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