Gregory Stark wrote:
> TCP is supposed to work even if the network disappears and then reappears
> later, gracefully handling intermittent network failures. The case where you
> want to close connections which doesn't respond quickly is when you have hosts
> (usually clients, which makes it strange to want this in a database client
> driver) which are frequently permanently unplugged or renumbered such as
> dialup users. That's why it's important for ftp servers or web servers.
People seem to like to put stateful firewalls with low connection
activity timeouts between their DB server and client. We get regular
traffic on the list from people who see connections get reset on their
first use after a long idle period. Presumably TCP keepalive would help
in some of these cases.
Also, usually DB connections are local and you don't really want to
plaster over the fact that the other end has gone MIA. Between default
TCP retransmission times and default TCP keepalive settings and the lack
of a uniform setQueryTimeout() within the driver, it's actually really
hard to work out promptly that the DB server is dead which is a royal
pain if you are trying to do failover between servers. Keepalive helps a
little but it's certainly not perfect. I ended up doing all the dead
server detection at a higher level in the end.
> "Kris Jurka" <books@ejurka.com> writes:
>
>> > Java does not support a configurable keepalive interval, it's simply an on/off
>> > flag.
>
> Uhm, well that kind of sucks, though to be honest I suspect most people who
> override the OS default set it to inappropriately short intervals anyways.
The lack of support in Java is why when I did the server-side work to
support configurable TCP keepalive I didn't do the Java side too - the
default OS timeouts are way too large for the case I cared about. Also,
the Java side doesn't sit around waiting on read, so it won't actually
notice the connection is dead until the next time it tries to use it
anyway, at which point you might as well just poll the server with a
dummy query every X seconds.
-O
