Hi Serg, Earlier this year we discussed MDEV-6429, which is to implement a proper storage engine API for a feature used in parallel replication. This is where InnoDB reports to the replication layer whenever transaction T1 needs to wait for T2; parallel replication uses this to detect and resolve deadlocks caused by enforced commit order. In 10.0, we decided to not do a "proper" API, instead just modify InnoDB to call into the server layer directly. In 10.0, the need for this API/feature is somewhat InnoDB-specific, it is not known whether any of the other transactional storage engines would need to do anything similar for parallel replication to work. If we implement MDEV-6676, however, then the need for the API becomes more critical. Because MDEV-6676 absolutely only works if we have a mechanism to detect conflicts that cause a deadlock with commit order, as was implemented for InnoDB. If we were to attempt to enable speculative parallel replication for eg. Tokutek transactions, without implementing the necessary check in Tokutek, then the slave would quickly end up hanging. Thinking more on this, I suppose what we really need is what you requested also during the original discussion: A true optional feature for storage engines to implement this additional check. And a way for the server layer to see if a storage engine has the implementation. If it does, then we can try to run its transactions in parallel as per MDEV-6676. If not, we will not try, and things will still work correctly (just without the parallelisation optimisation). Maybe for the first version, the best thing to do is to just hard-code a check for the InnoDB storage engine. This will allow us to get real-world testing to see if the new speculative parallel replication feature is interesting to any users. Then if it is, and there turns out to be interest from other storage engine venders (such as Tokutek) to also support speculative parallel replication, we can work with them to implement a proper API. This approach would also allow to consider the needs of a different storage engine when designing the API, which makes it much more likely that the API will be of general use and not too tied to InnoDB internals. What do you think? - Kristian.