![](https://secure.gravatar.com/avatar/ca4ece3ff34709052ec708895f9b8887.jpg?s=120&d=mm&r=g)
Hi. I have the following setup: Two data centres which I shall label North and South for the following explanation. Four virtual machines, Whiskey and Xray in data centre North, and Yankee and Zulu in data centre South, which have the Debian package unixodbc installed, together with libmaodbc. Four other virtual machines, Alfa and Bravo in North and Charlie and Delta in South, all running MariaDB with Galera replication. A single hostname DB.example.com which resolves to the four IP addresses of the four database servers Alfa, Bravo, Charlie and Delta. Every instance of /etc/odbc.ini on machines Whiskey, Xray, Yankee and Zulu has the setting "SERVER=DB.example.com" in it. When I start an application which uses ODBC (in my case this is Asterisk), I find that each application server manages to connect to a database server in its own data centre. Given a 50% chance of this happening for machine Whiskey, a 50% chance for machine Xray, etc., this comes out as a 1 in 16 combined probability of happening by chance. Therefore I think it is not by chance :) I've done some packet captures, and I see each machine Whiskey, Xray, Yankee and Zulu doing a single DNS query for DB.example.com and getting the four IP addresses of Alfa, Bravo, Chalie and Delta returned. As expected, the order the addresses is returned in is randomised for different queries. Each machine then makes a single connection to MariaDB on port 3306 on one of the IP addresses in the list. This address can be the first in the list returned from the DNS resolver, or it can the the last one, or it can be on in the middle. There is no attempt to ping the four addresses and compare round-trip times, or to connect to MariaDB on more than one server and measure the query response times. Each applicaton server connects to one database server, and they always manage to connect to a machine in their own data centre. My question is: how do they do this? How does machine Whiskey manage to select one of the two IP addresses for database servers Alfa and Bravo, and connect to it, ignoring the addresses for Charlie and Delta? If I can supply any further information to help with answering this question, please let me know. Thanks, Antony. -- "Have you been drinking brake fluid again? I think you're addicted to the stuff." "No, no, it's alright - I can stop any time I want to." Please reply to the list; please *don't* CC me.