Thread: Connection question
I have a two-user point-of-sale application on Windows XP PRO. The DB runs
on the cash register. The second user is a manager's computer. They are
connected through a wired router which is also connected to an internet
cable modem. The manager's computer just checks statistics on the DB and
maintains a connection all the time. The cash register application does not
do any network operations and is not even aware of the network.
About twice per month, it is necessary to reset the modem and router. This,
of course, loses the manager's connection to the DB. The problem is, it also
seems to break the connection at the cash register. The next time it tries to
record a transaction, it gets the error "Unable to write to the backend" or
something very close to that. The DB itself seems fine as we can restart the
cash register and all is well.
Anyone have any idea why this happens? We are running PostgreSQL 8.3.1-1
and the applications are Java 1.6.14.
Bayless
On 01/09/10 16:22, Bayless Kirtley wrote:
It would be a good option to replace your entire system. However, if you don't wish to do that, just simply connect the "manager's pc" and cash register with a network switch, which is separate from the router. So when your router restarts, the switch won't do down, so the 2 machines remain connected.
That is only a work around though, and ultimately you'd want a better solution
Cheers
On Wed, Sep 01, 2010 at 10:22:20AM -0500, Bayless Kirtley wrote: > About twice per month, it is necessary to reset the modem and router. This, > of course, loses the manager's connection to the DB. The problem is, it also > seems to break the connection at the cash register. The next time it tries to > record a transaction, it gets the error "Unable to write to the backend" or > something very close to that. The DB itself seems fine as we can restart the > cash register and all is well. > > Anyone have any idea why this happens? We are running PostgreSQL 8.3.1-1 > and the applications are Java 1.6.14. I'd guess you're connecting to PG using the network and not staying within Windows. What does your connection string look like? Getting it using "localhost" would be my suggestion. -- Sam http://samason.me.uk/
On 09/01/2010 11:22 PM, Bayless Kirtley wrote: > About twice per month, it is necessary to reset the modem and router. This, > of course, loses the manager's connection to the DB. With modern OSes, and many much older ones, it's not "of course" at all. Windows XP Pro is quite odd in that it breaks TCP/IP connections when an interface goes down and comes back up with the same IP address. Windows 7 even retains my SSH connections, made over wifi, when suspended and resumed! They only break if PuTTY tries to send a packet while the interface is still down after resume. Really, resetting a switch, unplugging a network cable and plugging it back in, etc shouldn't break TCP/IP connections, unless it triggers the connected host to do a new DHCP request, and the DHCP server hands out a different IP. No decent DHCP server will do that, but some cheap and nasty modem/router units don't store DHCP leases across a reboot so they "forget" their MAC address to IP address mappings. Anyway, that's a bit of a tangent, since you're on XP Pro and stuck with its rather less than ideal handling of connection loss. > The problem is, it > also > seems to break the connection at the cash register. The next time it > tries to > record a transaction, it gets the error "Unable to write to the backend" or > something very close to that. Is the register application connecting to localhost (127.0.0.1) or to the public IP address assigned by DHCP to the register's ethernet interface? If the latter, you're being bitten by Windows XP tossing out all TCP/IP connections involving that IP. If you're not sure, the easiest way I can think of to find out is to unplug the register from the network, restart the router and see if it can still connect. It should be able to. -- Craig Ringer
Thanks a lot Craig. The register is connecting through localhost so it's not that. I guess I'm not surprised about the Windows thing. I suppose we'll just have to live with it then. Fortunately it doesn't happen too often. But as fate would have it, most often seems when several customers are standing in line trying to pay. I tried initially to get the customer to go Linux instead of Windows but without much luck. Thanks again for the thorough explanation. Bayless ----- Original Message ----- From: "Craig Ringer" <craig@postnewspapers.com.au> To: "Bayless Kirtley" <bkirt@cox.net> Cc: "List, Postgres" <pgsql-general@postgresql.org> Sent: Wednesday, September 01, 2010 10:08 PM Subject: Re: [GENERAL] Connection question > On 09/01/2010 11:22 PM, Bayless Kirtley wrote: > >> About twice per month, it is necessary to reset the modem and router. >> This, >> of course, loses the manager's connection to the DB. > > With modern OSes, and many much older ones, it's not "of course" at all. > Windows XP Pro is quite odd in that it breaks TCP/IP connections when an > interface goes down and comes back up with the same IP address. > > Windows 7 even retains my SSH connections, made over wifi, when suspended > and resumed! They only break if PuTTY tries to send a packet while the > interface is still down after resume. > > Really, resetting a switch, unplugging a network cable and plugging it > back in, etc shouldn't break TCP/IP connections, unless it triggers the > connected host to do a new DHCP request, and the DHCP server hands out a > different IP. No decent DHCP server will do that, but some cheap and > nasty modem/router units don't store DHCP leases across a reboot so they > "forget" their MAC address to IP address mappings. > > Anyway, that's a bit of a tangent, since you're on XP Pro and stuck with > its rather less than ideal handling of connection loss. > >> The problem is, it >> also >> seems to break the connection at the cash register. The next time it >> tries to >> record a transaction, it gets the error "Unable to write to the backend" >> or >> something very close to that. > > Is the register application connecting to localhost (127.0.0.1) or to the > public IP address assigned by DHCP to the register's ethernet interface? > If the latter, you're being bitten by Windows XP tossing out all TCP/IP > connections involving that IP. > > If you're not sure, the easiest way I can think of to find out is to > unplug the register from the network, restart the router and see if it can > still connect. It should be able to. > > -- > Craig Ringer
On 2/09/2010 11:59 PM, Bayless Kirtley wrote: > Thanks a lot Craig. The register is connecting through localhost so it's > not that. I > guess I'm not surprised about the Windows thing. I suppose we'll just > have to > live with it then. I wouldn't be too sure yet. Did you run the test where you reboot the POS system with the ethernet unplugged? Does it still connect to the database OK? I didn't think XP broke TCP/IP connections to *localhost* when an ethernet interface went up or down, unless the connection was to a local IP associated with that interface. If you're connected to 127.0.0.1 from 127.0.0.1 it shouldn't, AFAIK, matter if you lose your wifi/ethernet. Come to think of it, you might want to check to make sure the local Ethernet interface is set to use DHCP. If it's using a static IP, Windows might permit that IP to be used to talk to the local host even when there's no link on the interface. So - to be sure, you can disable the Ethernet interface in the network control panel before your reboot for testing. Anyway, if you really can't make it work properly, there's a simple if mildly clumsy workaround. You can prevent XP from realizing it's lost its connection to the router by placing another device between the XP machine(s) and the router, like a plain old cheap switch. That way Ethernet link on the XP machines is never lost when you reboot the router - only between the router and the switch, where the XP machines can't tell. When the router goes down they lose their DHCP server, but that doesn't matter if it comes up again before they try to renew their DHCP leases, and for local-to-local traffic that's about all they'll be relying on the router for. You could even avoid that by setting static IP addresses. -- Craig Ringer Tech-related writing at http://soapyfrogs.blogspot.com/
----- Original Message ----- From: "Craig Ringer" <craig@postnewspapers.com.au> To: "Bayless Kirtley" <bkirt@cox.net> Cc: "List, Postgres" <pgsql-general@postgresql.org> Sent: Thursday, September 02, 2010 10:15 PM Subject: Re: [GENERAL] Connection question > On 2/09/2010 11:59 PM, Bayless Kirtley wrote: >> Thanks a lot Craig. The register is connecting through localhost so it's >> not that. I >> guess I'm not surprised about the Windows thing. I suppose we'll just >> have to >> live with it then. > > I wouldn't be too sure yet. > > Did you run the test where you reboot the POS system with the ethernet > unplugged? Does it still connect to the database OK? > > I didn't think XP broke TCP/IP connections to *localhost* when an ethernet > interface went up or down, unless the connection was to a local IP > associated with that interface. If you're connected to 127.0.0.1 from > 127.0.0.1 it shouldn't, AFAIK, matter if you lose your wifi/ethernet. > > Come to think of it, you might want to check to make sure the local > Ethernet interface is set to use DHCP. If it's using a static IP, Windows > might permit that IP to be used to talk to the local host even when > there's no link on the interface. So - to be sure, you can disable the > Ethernet interface in the network control panel before your reboot for > testing. > > > Anyway, if you really can't make it work properly, there's a simple if > mildly clumsy workaround. You can prevent XP from realizing it's lost its > connection to the router by placing another device between the XP > machine(s) and the router, like a plain old cheap switch. That way > Ethernet link on the XP machines is never lost when you reboot the > router - only between the router and the switch, where the XP machines > can't tell. When the router goes down they lose their DHCP server, but > that doesn't matter if it comes up again before they try to renew their > DHCP leases, and for local-to-local traffic that's about all they'll be > relying on the router for. You could even avoid that by setting static IP > addresses. > > -- > Craig Ringer > > Tech-related writing at http://soapyfrogs.blogspot.com/ Hmmm, more to consider then. I have not done the test you suggested. Guess I pounced on the Windows losing TCP/IP prematurely. It has been a while but I think I set them up for static IP addresses initially. I did check to be certain it is using localhost though. I will catch a slow day soon and try your ideas. I'll report the results back soon. Thanks Craig, Bayless