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，则可以检测 SQLAlchemy 以重新连接。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)

但是我收到了这个异常，而不是恢复：

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.

由于终端上打印了“pinging server”，因此似乎可以安全地得出结论，事件侦听器已附加。如何教 SQLAlchemy 从断开连接中恢复？

Answer 1

看起来checkout 方法仅在您第一次从池中获得连接时调用（例如您的connection = engine.connect() 行）

如果您随后失去连接，则必须明确替换它，因此您可以获取一个新的，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

这将是一个痛苦的每一个 sql，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

以下内容：

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

现在应该成功了。

另一种选择是同时侦听invalidate 方法，并在那时采取一些措施来替换您的连接。