如何教SQLAlchemy从断开连接中恢复？

Question

根据http://docs.sqlalchemy.org/en/rel_0_9/core/pooling.html#disconnect-handling-pessimistic,如果连接池中的条目不再有效,可以检测SQLAlchemy重新连接.我创建了以下测试用例来测试它:

import subprocess
from sqlalchemy import create_engine, event
from sqlalchemy import exc
from sqlalchemy.pool import Pool

@event.listens_for(Pool, "checkout")
def ping_connection(dbapi_connection, connection_record, connection_proxy):
    cursor = dbapi_connection.cursor()
    try:
        print "pinging server"
        cursor.execute("SELECT 1")
    except:
        print "raising disconnect error"
        raise exc.DisconnectionError()
    cursor.close()

engine = create_engine('postgresql://postgres@localhost/test')

connection = engine.connect()

subprocess.check_call(['psql', str(engine.url), '-c',
    "select pg_terminate_backend(pid) from pg_stat_activity " +
    "where pid <> pg_backend_pid() " +
    "and datname='%s';" % engine.url.database],
    stdout=subprocess.PIPE)

result = connection.execute("select 'OK'")
for row in result:
    print "Success!", " ".join(row)

但是没有恢复,我收到了这个例外:

sqlalchemy.exc.OperationalError: (OperationalError) terminating connection due to administrator command
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.

由于"ping服务器"打印在终端上,因此可以安全地得出事件监听器已附加的结论.如何教SQLAlchemy从断开连接中恢复？

Answer 1

看起来只有当您第一次从池中获取连接时才会调用checkout方法(例如,您的线路)connection = engine.connect()

如果您随后丢失了连接,则必须明确替换它,因此您可以只获取一个新连接,然后重试您的sql:

try:
    result = connection.execute("select 'OK'")
except sqlalchemy.exc.OperationalError:  # may need more exceptions here
    connection = engine.connect()  # grab a new connection
    result = connection.execute("select 'OK'")  # and retry

这对于每一个sql都是很痛苦的,所以你可以使用类似的东西来包装数据库查询:

def db_execute(conn, query):
    try:
        result = conn.execute(query)
    except sqlalchemy.exc.OperationalError:  # may need more exceptions here (or trap all)
        conn = engine.connect()  # replace your connection
        result = conn.execute(query)  # and retry
    return result

下列:

result = db_execute(connection, "select 'OK'")

现在应该成功.

另一种选择是监听invalidate方法,并在那时采取一些措施来替换你的连接.