On 11/3/14, 2:36 PM, Alvaro Herrera wrote:
> Jim Nasby wrote:
>> On 11/1/14, 8:41 AM, Petr Jelinek wrote:
>>> Well this is not BDR specific thing, the idea is that with logical replication, commit timestamp is not enough for
conflicthandling, you also need to have additional info in order to identify some types of conflicts conflicts (local
updatevs remote update for example). So the extradata field was meant as something that could be used to add the
additionalinfo to the xid.
>>
>> Related to this... is there any way to deal with 2 transactions that commit in the same microsecond? It seems silly
totry and handle that for every commit since it should be quite rare, but perhaps we could store the LSN as extradata
ifwe detect a conflict?
>
> Well, two things. One, LSN is 8 bytes and extradata (at least in this
> patch when I last saw it) is only 4 bytes. But secondly and more
> important is that detecting a conflict is done in node B *after* node A
> has recorded the transaction's commit time; there is no way to know at
> commit time that there is going to be a conflict caused by that
> transaction in the future. (If there was a way to tell, you could just
> as well not commit the transaction in the first place.)
I'm worried about 2 commits in the same microsecond on the same system, not on 2 different systems. Or, put another
way,if we're going to expose this I think it should also provide a guaranteed unique commit ordering for a single
cluster.Presumably, this shouldn't be that hard since we do know the exact order in which things committed.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com