SQLAlchemy 在查询期间丢失与MySQL服务器的连接

遇到问题

pymysql.err.OperationalError: (2013, 'Lost connection to MySQL server during query')

建立的 pymysql 的连接

# mysql connect
engine = create_engine("mysql+{driver}://{username}:{password}@{server}/{database}?charset={charset}"
                       .format(driver=MYSQL_DRIVER,
                               username=MYSQL_USERNAME,
                               password=MYSQL_PASSWORD,
                               server=MYSQL_SERVER,
                               database=DB_NAME,
                               charset=DB_CHARSET),
                       pool_size=100,
                       max_overflow=100,
                       # pool_recycle=7200,
                       pool_recycle=10,
                       echo=False)
engine.execute("SET NAMES {charset};".format(charset=DB_CHARSET))
MapBase = declarative_base(bind=engine)
DBSession = sessionmaker(bind=engine)

运行具体报错

Connected to pydev debugger (build 191.6183.50)
init mysql_db success
ai access running...
{'recordId': 'e2d432da51214c54aa2bb4d43b513835', 'carImg1Data': '/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAI
{'recordId': 'e2d432da51214c54aa2bb4d43b513835', 'carImg1Data': '/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAI
exception occurs when get data! Traceback (most recent call last):
  File "/home/ubuntu/.virtualenvs/access_py3/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 1244, in _execute_context
    cursor, statement, parameters, context
  File "/home/ubuntu/.virtualenvs/access_py3/lib/python3.6/site-packages/sqlalchemy/engine/default.py", line 552, in do_execute
    cursor.execute(statement, parameters)
  File "/home/ubuntu/Desktop/data_access_py3/camera-coll-ai/packages/pymysql/cursors.py", line 170, in execute
    result = self._query(query)
  File "/home/ubuntu/Desktop/data_access_py3/camera-coll-ai/packages/pymysql/cursors.py", line 328, in _query
    conn.query(q)
  File "/home/ubuntu/Desktop/data_access_py3/camera-coll-ai/packages/pymysql/connections.py", line 517, in query
    self._affected_rows = self._read_query_result(unbuffered=unbuffered)
  File "/home/ubuntu/Desktop/data_access_py3/camera-coll-ai/packages/pymysql/connections.py", line 732, in _read_query_result
    result.read()
  File "/home/ubuntu/Desktop/data_access_py3/camera-coll-ai/packages/pymysql/connections.py", line 1075, in read
    first_packet = self.connection._read_packet()
  File "/home/ubuntu/Desktop/data_access_py3/camera-coll-ai/packages/pymysql/connections.py", line 657, in _read_packet
    packet_header = self._read_bytes(4)
  File "/home/ubuntu/Desktop/data_access_py3/camera-coll-ai/packages/pymysql/connections.py", line 707, in _read_bytes
    CR.CR_SERVER_LOST, "Lost connection to MySQL server during query")
pymysql.err.OperationalError: (2013, 'Lost connection to MySQL server during query')

分析

配置了 pool_recycle = 10，导致池在给定的秒数过后再循环连接，连接池 100 连接不够用了，导致上述错误

解决方案

配置 pool_recycle = 1，或者自动回收（设置为 True 是可以的，貌似与设置为 1 一样），而默认等于 -1，表示不回收

实践例子

设置 pool_recycle=3，然后设置 wait_timeout = 2，在程序执行了第二次的过程中，就会触发异常

import time

from sqlalchemy.engine import create_engine

url = 'mysql+pymysql://user:pass@127.0.0.1:3306/db'
engine = create_engine(url, pool_recycle=3).connect()

query = 'SELECT NOW();'

while True:
    print('Q1', engine.execute(query).fetchall())
    # time.sleep(2)
    print('Q2', engine.execute(query).fetchall())

设置 wait_timeout = 2

mysql> set global wait_timeout = 2;
Query OK, 0 rows affected (0.00 sec)

mysql> show global variables like '%timeout%';
+-----------------------------+----------+
| Variable_name               | Value    |
+-----------------------------+----------+
| connect_timeout             | 10       |
| delayed_insert_timeout      | 300      |
| have_statement_timeout      | YES      |
| innodb_flush_log_at_timeout | 1        |
| innodb_lock_wait_timeout    | 50       |
| innodb_rollback_on_timeout  | OFF      |
| interactive_timeout         | 28800    |
| lock_wait_timeout           | 31536000 |
| net_read_timeout            | 30       |
| net_write_timeout           | 60       |
| rpl_stop_slave_timeout      | 31536000 |
| slave_net_timeout           | 60       |
| wait_timeout                | 2        |
+-----------------------------+----------+
13 rows in set (0.04 sec)

好了，更多 create_engine() 函数的连接参数说明，可以去官网查看，以便更好的理解