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Re: Group commit id in mysqlbinlog output


Robert Hodges <robert.hodges@xxxxxxxxxxxxxx> writes:

> a.) It seems logical that transactions within a group commit should appear
> together in the binlog and should be serialized before and after other
> transactions in the binlog.  Is there *any* way this ordering could be
> violated, for example to mix in a non-grouped transaction?

No the ordering is completely fixed. All transactions that group-commit
together will be written into the binlog as a unit, without any other
transactions in-between being possible. The ordering will also be identical in
the binlogs on any slaves (if using multiple replication domains, then the
ordering is identical within a single domain).

> b.) Is there any ordering of the transactions within the group commit in
> the binlog for example sorted based on the resources each uses?  Or is it
> more or less random based on time locks are acquired, etc.?

There is no ordering. It is just the order in which each thread happens to
reach the point in the code where it obtains the necessary lock and adds
itself to the list of transactions waiting for group commit.

> c.) How do you handle commit timestamps on group-committed transactions?
>  Are they identical? In past MySQL releases I have found instances where
> timestamps can walk backwards across succeeding transactions.  Such
> anomalies can be very troublesome for downstream consumers like data
> warehouses that want to create materialized, point-in-time views or
> partition data based on time of commit.   (Ask me how I know.)

I am not very familiar with timestamps in binlog events. However, all the
transactions in the group commit are written out one after the other, in a
single thread holding the lock on the binlog. And the GTID events are
generated during that loop. So I think that at least the timestamps of the
GTID events in the group can never walk backwards, nor can they from one group
to the next. But I am not sure if the time stamps of other events in the
transaction can be earlier (maybe they were generated when the query was run,
not when it was committed?

> Any clarifications you can offer would be most welcome.

I hope the above helps, else please ask again.

 - Kristian.

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