This post is SEO bait for people trying to scale MySQL’s write
capacity by writing to both servers in master-master replication.
The short answer: you can’t do it. It’s impossible.
I keep hearing this line of reasoning: “if I make a MySQL
replication ‘cluster’ and move half the writes to machine A and
half of them to machine B, I can increase my overall write
capacity.” It’s a fallacy. All writes are repeated on both
machines: the writes you do on machine A are repeated via
replication on machine B, and vice versa. You don’t shield either
machine from any of the load.
In addition, doing this introduces a very dangerous side effect:
in case of a problem, neither machine has the authoritative data.
Neither machine’s data can be trusted, but neither machine’s data
can be discarded either. This is a very difficult situation to
recover from. Save yourself grief, work, and money. Never
write to both …
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