Psycopg2 db连接在丢失的网络连接上挂起

Question

Problem description

我正在使用psycopg2连接到远程主机上的PostgreSQL数据库。我打开一个连接并等待请求，然后对于每个请求，我在连接上运行查询并返回数据。

但是当连接已经打开后网络连接丢失时，下一个数据库查询会挂起，我必须手动终止该程序。

细节：

• 它挂了至少2个小时（我等不了多久）
• “网络故障”情况实际上是VPN下降（数据库主机只能通过VPN访问）
• 我不能使用异步连接，因为我需要事务
• python 2.6
• psycopg 2.2.1
• debian linux 6和7,64bit
• postgresql 8.4和9.1

What I want/need

我需要一些可靠的方法来在运行查询之前检测失败的连接，因此我的程序不会挂起，或者让cursor.execute(..)在失败的连接上引发异常的方法。

Example:

import psycopg2
import time

conn = psycopg2.connect("host='dbs' dbname='foo' user='joe' password='x'")
time.sleep(10) # I manually turn VPN off during this sleep..
cu = conn.cursor()
cu.execute('SELECT 1') # <- hangs here
print cu.fetchone()
cu.commit()

What have I tried (and what didn't work):

• 设置TCP超时“全局” - 在psycopg2导入之前，我添加了：

  import socket
  socket.setdefaulttimeout(10)

• 在psycopg.connection的套接字上设置TCP超时：

  ..
  conn = psycopg2.connect(...
  s = socket.fromfd(conn.fileno(), socket.AF_INET, socket.SOCK_STREAM)
  s.settimeout(5)
  ..

• 为psycopg.connection的套接字启用keepalive：

  ...
  conn = psycopg2.connect(...
  s = socket.fromfd(conn.fileno(), socket.AF_INET, socket.SOCK_STREAM)
  s.settimeout(5)
  s.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
  s.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPIDLE, 1)
  s.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPINTVL, 3)
  s.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPCNT, 5)
  ...

Answer 1

看看套接字超时，在阅读this和this后，这些设置对我有用

s = socket.fromfd(connection.fileno(),
                  socket.AF_INET, socket.SOCK_STREAM)
# Enable sending of keep-alive messages
s.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
# Time the connection needs to remain idle before start sending
# keepalive probes
s.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPIDLE, int(ceil(time)))
# Time between individual keepalive probes
s.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPINTVL, 1)
# The maximum number of keepalive probes should send before dropping
# the connection
s.setsockopt(socket.IPPROTO_TCP, socket.TCP_KEEPCNT, 3)

Answer 2

为了确保连接仍然有效，请阅读属性connection.isolation_level。如果连接已经死亡，这将用OperationalError提高pgcode == "57P01"。

try:
    connection.isolation_level
except OperationalError as oe:
    conn = psycopg2.connect(dsn)