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Re: Group commit id in mysqlbinlog output


Hi Kristian,

First of all thanks for the great on-list explanations of your parallel
replication features.  It looks as if you are making good progress on a
very hard problem.

Second, this is slightly off-topic but can you expand somewhat on the
semantics of group-committed transactions in the binlog?  Here are a few

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?

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.?

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.)

Any clarifications you can offer would be most welcome.

Cheers, Robert

On Fri, Mar 14, 2014 at 5:51 AM, Kristian Nielsen

> There was a question on the output of mysqlbinlog related to group commit.
> In 10.0, if two transactions group commit together on the master, their
> event contains a "commit id" - a 64-bit number. This is used by parallel
> replication; if two transactions have the same commit id, they can be
> executed
> in parallel.
> In mysqlbinlog output, this looks like this:
> #140314 13:42:56 server id 1  end_log_pos 772   GTID 0-1-12 cid=180
> ...
> #140314 13:42:56 server id 1  end_log_pos 1027  GTID 0-1-13 cid=180
> But if a transaction commits alone on the master (no other transactions
> participate in the group commit), the mysqlbinlog outpus has no commit id:
> #140314 13:42:56 server id 1  end_log_pos 437   GTID 0-1-10
> The question was why there is no cid=X in the second case, as it makes
> scripting/grepping the output harder. The reason is that there is no
> commit id
> in the event in the binlog in this case (to reduce the size of the binlog).
> So there is no valid number to put in there.
> One option could be to make the output something like this:
> #140314 13:42:56 server id 1  end_log_pos 437   GTID 0-1-10 cid=<none>
> This would allow to grep for "cid=" and catch everything. I do not have a
> strong opinion one way or the other, if there are people who find this
> useful,
> then let me know and I can change it.
>  - Kristian.
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