Afternoon tutorial at MySQL Users Conference (2006) is Replication for Scaling and High Availaility by Jeremy Cole (not this jeremycole) from Yahoo!.
Jeremy starts with a general overview of replication and provides some detail on general things replication is and is not. There is no limit to the number of slaves, but there is a limit on the bandwidth that a server can serve so with too many slaves the master could be overloaded. If you need massive numbers of replicated machines it's better to not have them all pointed at one master. It's better to have a relay slave who's job it is to stand in the middle and serve data out to groups of machines.
Master to master replication is possible, but there are lots of problems. Primary keys can be carefully disignated but the bottom line is that when it fails it's hard to know what …
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