Re: Feature freeze date for 8.1

On E, 2005-05-02 at 18:47 +0300, Heikki Linnakangas wrote:
> On Mon, 2 May 2005, Hannu Krosing wrote:

> > It would be nice if I coud st up some timeut using keepalives (like ssh-
> > s ProtocoKeepalives") and use similar timeouts on client and server.
> 
> FWIW, I've been bitten by this problem twice with other applications.
> 
> 1. We had a DB2 database with clients running in other computers in the 
> network. A faulty switch caused random network outages. If the connection 
> timed out and the client was unable to send it's request to the server, 
> the client would notice that the connection was down, and open a new one. 
> But the server never noticed that the connection was dead. Eventually, 
> the maximum number of connections was reached, and the administrator had 
> to kill all the connections manually.
> 
> 2. We had a custom client-server application using TCP across a network. 
> There was stateful firewall between the server and the clients that 
> dropped the connection at night when there was no activity. After a 
> couple of days, the server reached the maximum number of threads on the 
> platform and stopped accepting new connections.
> 
> In case 1, the switch was fixed. If another switch fails, the same will 
> happen again. In case 2, we added an application-level heartbeat that 
> sends a dummy message from server to client every 10 minutes.
> 
> TCP keep-alive with a small interval would have saved the day in both 
> cases. Unfortunately the default interval must be >= 2 hours, according 
> to RFC1122.
> 
> On most platforms, including Windows and Linux, the TCP keep-alive 
> interval can't be set on a per-connection basis. The ideal solution would 
> be to modify the operating system to support it.

Yep. I think this could be done for (our instance of) linux, but getting
it into mainstream kernel, and then into all popular distros is a lot of
effort.

Going the ssh way (protocol level keepalives) might be way simpler.

> What we can do in PostgreSQL is to introduce an application-level 
> heartbeat. A simple "Hello world" message sent from server to client that 
> the client would ignore would do the trick.

Actually we would need a round-trip indicator (some there-and-back
message: A: do you copy 42 --> B: yes I copy 42), and not just send. The
difficult part is what to do when one side happens to send the keepalive
in the middle of actual data transfer ? 

move to packet oriented connections (UDP) and make different packet
types independant of each other? 

-- 
Hannu Krosing <hannu@skype.net>