MySQL disconnection reports an error MySQL ldb_ exceptions. OperationalError 4031, The client was disconnected by the server

Problem description

Flask + SQLAlchemy An error is reported after the interface service of (MySQLdb._exceptions.OperationalError) (4031, ‘The client was disconnected by the server because of inactivity. See wait_timeout and interactive_timeout for configuring this behavior.’)↵[SQL: xxx]↵(Background on this error at: https://sqlalche.me/e/14/e3q8)

OperationalError Not necessarily caused by programmers , For example, accidental disconnection 、 Data source not found 、 Unable to process transaction 、 Memory allocation error, etc . This error is caused by the database driver （DBAPI）, instead of SQLAlchemy.

OperationalError The most common reason is that the database connection is disconnected , Read in detail ：Dealing with Disconnects

The connection pool can refresh the connections and make the connections in the old pool invalid , There are two options ：

1. Pessimism

Every time you use the connection pool, execute the test statement to test whether the database connection is normal , Such as SELECT 1. This method adds a little overhead but is simple and reliable . however , If the loss of connection occurs in a transaction or other SQL In operation , This pre ping The method fails .

This method can pass the parameter pre_ping=True Realization

engine = create_engine('mysql+pymysql://root:[email protected]:3306/test', pool_pre_ping=True)

2. Optimism

Refresh connection when disconnected .

Parameters pool_recycle , Prevent the pool from using timed out connections

engine = create_engine('mysql+pymysql://root:[email protected]:3306/test', pool_recycle=3600)

Solution

1. View related variables

SHOW VARIABLES WHERE `Variable_name` IN ('wait_timeout', 'interactive_timeout');

Default free 8 Hour disconnect

2. Code

SQLAlchemy

from sqlalchemy import create_engine

engine = create_engine(
    'mysql+pymysql://root:[email protected]:3306/test',
    pool_pre_ping=True,
    pool_size=10,
    pool_recycle=7200
)

Flask_SQLAlchemy

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql+pymysql://root:[email protected]:3306/test'
app.config['SQLALCHEMY_ENGINE_OPTIONS'] = {
    'pool_pre_ping': True, 'pool_size': 10, 'pool_recycle': 7200}
db = SQLAlchemy(app)

pool_timeout It may also be useful , To be tested


according to 《SQLAlchemy Python Database practice 》, Just set up pool_recycle=3600

reference

1. The client was disconnected by the server because of inactivity Solution
2. MySQL SHOW VARIABLES Statement
3. airflow Report errors mysql Connection timeout disconnected problem solved
4. Connection Pooling — SQLAlchemy Documentation
5. solve mysql The server actively disconnects when there is no operation timeout
6. Further study of sqlalchemy Connection pool
7. flask_sqlalchemy pool_pre_ping only working sometimes