> COMMIT PREPARED is a pretty thin layer; the work is all done in the PREPARE. > With a DTM, the main commit itself is done once only in the DTM, so all the > COMMIT PREPARED would do is release local node resources.
Sure. Now imagine that the pg_twophase entry is corrupted for this transaction on one node. This would trigger a PANIC on it, and transaction would not be committed everywhere. I am aware of the fact that by definition PREPARE TRANSACTION ensures that a transaction will be committed with COMMIT PREPARED, just trying to see any corner cases with the approach proposed. The DTM approach is actually rather close to what a GTM in Postgres-XC does :)
This is wrong.
There is no "approach proposed", this is just normal 2PC feature that PostgreSQL has supported since 8.1. All that is proposed here is that we have an API that allows this to be exposed.
The whole point of PREPARE is that it allows a database to crash after that step and it can still be recovered. That has nothing to do with Xanything.
In this case, the role of the GTM is to record the commit. Other nodes consult the GTM, not local state to see whether the transaction has committed, acting as the transaction manager in an XA sense.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services