Hi, Aleksey!
On Apr 04, Aleksey Midenkov wrote:
> Hello, Sergei!
>
> In unversioned -> versioned scenario in the code below it first gets to Set
> time 4, creates some records (on slave) and then seconds on slave increased
> (X+1) while on master seconds are not yet increased (X). Then we get to Set
> time 3 and reset time on slave to X.0 therefore we get back in time and all
> stored records with timestamp X.n will be in future. 'n' came from
> ++system_time.sec_part in Set time 4.
That's not how it is supposed to work.
As long as the master sends events with seconds=X,
the slave will generate microsecons, X.0, X.1, X.2, etc.
When the master sends an event with a new timestamp, Y,
the slave goes back to Y.0, and continues with Y.1, etc.
> Why did you decided to use such logic of getting seconds from master
> and microseconds from slave? Since microseconds sooner or later reset
> to 0 it's not better than just assigning some random number. What
> sending microseconds from master conditionally only is good for?
Because the master was sending microseconds conditionally, since 5.3.
The slave had to cope with that somehow anyway.
If there is an installation from unversioned 5.3 to versioned 10.3 we can warn user about lost microseconds. This is minor issue since such setups are rare, I guess. But for what microseconds are sent in 5.3?
And I didn't want to force the master to include microseconds in every
single event for every single user just in case someone would decide to
do unversioned->versioned replication.
If it's critical, this can be configured. But is it really performance issue?
Also, I thought that processing of 1000000 Query_log_event's in a second
is not realistic.
Now it fails just with several events. I guess because system_time.sec_part is not reset to 0 initially.
But now I see some issues with that. One can freeze the time on the
master with 'SET TIMESTAMP' and send an arbitrary number of events with
the same timestamp.
Or one can generate a query event that includes microseconds, to force
the slave to count not from X.0, but from, say, X.999998.
So, a wraparound is possible and we need some fix for it.
Looks like a lots of complications for a minor issue. Other DBMS don't use microseconds for System Versioning at all. I guess user should cope either with sent microseconds unconditionally (for both SBR and RBR) or he should not use microseconds-precise System Versioning on slave.
Regards,
Sergei
Chief Architect MariaDB
and security@mariadb.org