I had this great idea to setup a couple slaves off a master-master replication set something like this:
MASTER A <--------------> MASTER B
SLAVE C SLAVE D
Makes sense, huh?
Then I'll just point our read / write app servers at MASTER A, and point half of our read / only app servers at SLAVE C, and half of our read only servers at SLAVE D. Sweet. Works like a charm.
Oops... Replication shows current everywhere, no alerts going off. But app servers pointed at SLAVE C show higher row counts than app servers on SLAVE D...
Yea, I never bothered to test that. That's what I get for making assumptions - if you could even call it that. When I thought things through, I quickly realized that there was no reason for MASTER B to send any of it's replicated updates on to SLAVE D, because I had told it not to write any of it's updates it received via replication to it's binary log... :)
All you replication experts out there could have probably told me that in your sleep, but of course, I never bothered to ask...
Just a couple lines in the config file could have saved me a few headaches, and a couple embarrassing apologies...
replicate-same-server-id = 0