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And how about another killer case: what is the maximum number of parallel updates per second that you can make to a single row?
But of course, it is now well known, 1/RTT.
I guess this is true for Galera Cluster. MySQL with semi-sync can accept much more than that (not with the new default semi-sync mode from 5.7.2 though).
That's very curious. And semi-sync can do so while at the same time satisfying your requirement that the client finds the data even in the event of immediate master crash following OK?
What you are saying also implies that semi-sync can do more than 1/RTT transactions per second (if by "accept" we shall understand actual row modification, not mere queueing for lock). That refutes the findings of the tests I referred to, which to my knowledge so far have not been disputed by anybody. That makes it even more curious.
However your remark about 5.7.2 seems to smear this whole claim, and Oracle press release confirms that:
"MySQL 5.7.2 DMR also delivers lossless semi-synchronous replication, enabling transactions to only be committed to the storage engine and externalized on the master after the slave has acknowledged receipt."
This sounds like prior to 5.7.2 semi-sync isn't even that "semi-sync" as one would expect. So I'm not exactly sure how it can fit your requirements.
Answering to myself: indeed this can be done if transactions are committed on master asynchronously and only OK is sent to client after ACK from slave. But then semi-sync should be capable of more than 1/RTT transactions per second, yet somehow it is not what was found in any benchmark that I know of. So this is still a controversy I'm begging you to resolve. -- Alexey Yurchenko, Codership Oy, www.codership.com Skype: alexey.yurchenko, Phone: +358-400-516-011