On 5 January 2017 at 20:43, Stas Kelvich <s.kelv...@postgrespro.ru> wrote:
>> On 5 Jan 2017, at 13:49, Simon Riggs <si...@2ndquadrant.com> wrote:
>> Surely in this case the master server is acting as the Transaction
>> Manager, and it knows the mapping, so we are good?
>> I guess if you are using >2 nodes then you need to use full 2PC on each node.
>> Please explain precisely how you expect to use this, to check that GID
>> is required.
> For example if we are using logical replication just for failover/HA and 
> allowing user
> to be transaction manager itself. Then suppose that user prepared tx on 
> server A and server A
> crashed. After that client may want to reconnect to server B and commit/abort 
> that tx.
> But user only have GID that was used during prepare.
>> But even then, if you adopt the naming convention that all in-progress
>> xacts will be called RepOriginId-EPOCH-XID, so they have a fully
>> unique GID on all of the child nodes then we don't need to add the
>> GID.
> Yes, that’s also possible but seems to be less flexible restricting us to some
> specific GID format.
> Anyway, I can measure WAL space overhead introduced by the GID’s inside 
> commit records
> to know exactly what will be the cost of such approach.


Have you had a chance to look at this further?

I think the approach of storing just the xid and fetching the GID
during logical decoding of the PREPARE TRANSACTION is probably the
best way forward, per my prior mail. That should eliminate Simon's
objection re the cost of tracking GIDs and still let us have access to
them when we want them, which is the best of both worlds really.

 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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