Actually, Kristian's description is pretty accurate. When last semi-sync slave disconnects from master rpl_semi_sync_master_clients becomes 0. When after that waiting for semi-sync ack times out rpl_semi_sync_master_status becomes OFF and master continues to commit transactions without waiting for semi-sync acks. Note that it's pretty much impossible to fail and rollback in this case because the transaction is already written to the binlog (no matter whether waiting for semi-sync ack happens after_sync or after_commit) and possibly other transactions are written to binlog after it too. So the only cure against this at Google was to set rpl_semi_sync_master_timeout to a very big value. Although that doesn't help much in case of crashes, shutdowns and forced failovers. To help with those Jonas implemented recently additional code that: a) Never stops waiting for semi-sync ack, even if client is disconnected or connection is killed. b) Forcefully crashes when rpl_semi_sync_master_enabled is switching to OFF and there are transactions in progress waiting for semi-sync ack. c) Rollbacks all prepared and uncommitted transactions in InnoDB at startup, and truncates those transactions from binlog. We believe that this new code together with rpl_semi_sync_master_wait_point=after_sync will guarantee no unacked transactions on any server. On Mon, Dec 28, 2015 at 4:38 AM, wrote:

Hi Kristian, Your description is not accurate. If there are no slaves ready to acknowledge a transaction, after a timeout the status variable Rpl_semi_sync_master_clients in the master becomes 0. The master itself, though, remains able to perform semi-synch operations. As soon as one of the slaves is enabled, the variable Rpl_semi_sync_master_clients is incremented and semi-synch replication resumes. HTH

Giuseppe

On 28 Dec 2015 at 13:17:10 , Kristian Nielsen (knielsen@knielsen-hq.org) wrote:

Hi Jonas (or anyone else who may know),

If I understand correctly, semisync replication has a timeout for how long it will wait for a slave to acknowledge transactions. If the timeout is exceeded, semisync is turned off on the master.

Do you know if there is a facility to avoid semisync being turned off automatically in this case, maybe instead fail and roll back the master transaction?

I was thinking at Google you might already have needed and implemented this (merged or not merged into MariaDB yet), or you may be aware of a patch by someone else for this?

Thanks, and merry Christmas!

- Kristian.

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