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Re: Group commit id in mysqlbinlog output


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@xxxxxxxxxxxxxxx) wrote:

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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