Making sure that psycopg2 database connection aliv

问题:

I have a python application that opens a database connection that can hang online for an hours, but sometimes the database server reboots and while python still have the connection it won't work with OperationalError exception.

So I'm looking for any reliable method to "ping" the database and know that connection is alive. I've checked a psycopg2 documentation but can't find anything like that. Sure I can issue some simple SQL statement like SELECT 1 and catch the exception, but I hope there is a native method, something like PHP pg_connection_status

Thanks.

回答1:

pg_connection_status is implemented using PQstatus. psycopg doesn't expose that API, so the check is not available. The only two places psycopg calls PQstatus itself is when a new connection is made, and at the beginning of execute. So yes, you will need to issue a simple SQL statement to find out whether the connection is still there.

回答2:

This question is really old, but still pops up on Google searches so I think it's valuable to know that the psycopg2.connection instance now has a closed attribute that will be 0 when the connection is open, and greater than zero when the connection is closed. The following example should demonstrate:

import psycopg2
import subprocess

connection = psycopg2.connect(
    dbname=database,
    user=username,
    password=password,
    host=host,
    port=port
)

print connection.closed # 0

# restart the db externally
subprocess.check_call("sudo /etc/init.d/postgresql restart", shell=True)

# this query will fail because the db is no longer connected
try:
    cur = connection.cursor()
    cur.execute('SELECT 1')
except psycopg2.OperationalError:
    pass

print connection.closed # 2

回答3:

connection.closed does not reflect a connection closed/severed by the server. It only indicates a connection closed by the client using connection.close()

In order to make sure a connection is still valid, read the property connection.isolation_level. This will raise an OperationalError with pgcode == "57P01" in case the connection is dead.

This adds a bit of latency for a roundtrip to the database but should be preferable to a SELECT 1 or similar.

import psycopg2
dsn = "dbname=postgres"
conn = psycopg2.connect(dsn)

# ... some time elapses, e.g. connection within a connection pool

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

c = conn.cursor()
c.execute("SELECT 1")