Robert Hodges 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.

AFAIK, timestamp for a binlog event is the time when corresponding statement started to execute. The order of statements in the binlog is according to the time when statements were committed, which of course doesn't have anything to do with the time statement started to execute. That's why timestamps in the binlog can jump back and forth randomly. But I'd think that if one looks at timestamps only on GTID events then he should expect to see monotonically increasing time within each domain. Pavel On Fri, Mar 14, 2014 at 8:54 AM, Robert Hodges

wrote:

Hi Kristian,

Thanks for the prompt and detailed response!

I’m glad you clarified that GTIDs cannot ever walk backwards. It’s really bad design if they are not monotonically increasing and comparable. This will really help restart logic in a number of places, not just for your own replication.

The timestamp issue is mysterious one. I also don’t fully understand how the timestamp is generated for the event header. All I know is that it sometimes walks backwards, possibly as a result of large transactions or load data infile commands. If I can find a reproducible case I will post it, as it can cause inscrutable problems downstream when loading to other systems. (Ask me how I know…)

Meanwhile, good luck on your replication work. It seems to be proceeding in a good direction.

Cheers, Robert

On March 14, 2014 at 8:01:04 AM, Kristian Nielsen ( knielsen@knielsen-hq.org) wrote:

Robert Hodges 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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"Kristian" == Kristian Nielsen writes:

<cut> Kristian> I am not very familiar with timestamps in binlog events. However, all the Kristian> transactions in the group commit are written out one after the other, in a Kristian> single thread holding the lock on the binlog. And the GTID events are Kristian> generated during that loop. So I think that at least the timestamps of the Kristian> GTID events in the group can never walk backwards, nor can they from one group Kristian> to the next. But I am not sure if the time stamps of other events in the Kristian> transaction can be earlier (maybe they were generated when the query was run, Kristian> not when it was committed? Time stamps in a statement is based on when the statement started, not when it committed. This is needed to ensure that all rows has the same value f you do: update big_table set timpestamp=now(); So it's normal that in a group commit, the transactions in the same commit can have totally different timestamps. Longer running transactions will typically have older timestamps. Regards, Monty