On 8 November 2015 at 16:59, Konstantin Knizhnik <k.knizh...@postgrespro.ru> wrote:
> On 11/08/2015 02:46 PM, Michael Paquier wrote: > >> On Sun, Nov 8, 2015 at 1:53 AM, Konstantin Knizhnik wrote: >> >>> In tsDTM approach two phase commit is performed by coordinator and >>> currently >>> is using standard PostgreSQL two phase commit: >>> >>> Code in GO performing two phase commit: >>> >>> exec(conn1, "prepare transaction '" + gtid + "'") >>> exec(conn2, "prepare transaction '" + gtid + "'") >>> exec(conn1, "select dtm_begin_prepare($1)", gtid) >>> exec(conn2, "select dtm_begin_prepare($1)", gtid) >>> csn = _execQuery(conn1, "select dtm_prepare($1, 0)", gtid) >>> csn = _execQuery(conn2, "select dtm_prepare($1, $2)", gtid, >>> csn) >>> exec(conn1, "select dtm_end_prepare($1, $2)", gtid, csn) >>> exec(conn2, "select dtm_end_prepare($1, $2)", gtid, csn) >>> exec(conn1, "commit prepared '" + gtid + "'") >>> exec(conn2, "commit prepared '" + gtid + "'") >>> >>> If commit at some of the nodes failed, coordinator should rollback >>> prepared >>> transaction at all nodes. >>> >> Not always. If COMMIT PREPARED fails at some of the nodes but succeeds >> on others, the transaction is already partially acknowledged as >> committed in the cluster. Hence it makes more sense for the >> coordinator to commit transactions on the remaining nodes. Before >> issuing any COMMIT PREPARED queries, I guess that's fine to rollback >> the transactions on all nodes though. >> > We will get inconsistency if transaction is committed on some subset of > nodes involved in transaction. > Assume bank debit-credit example. If some distributed transaction > transfers money from the account at one node to the account and another > node, > then committing transaction just at one node cause incorrect total balance. > The main goal of DTM is to preserve ACID semantic for distributed > transaction, so either transaction is committed at all nodes, either it is > not committed at all. Agreed. 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. -- Simon Riggs http://www.2ndQuadrant.com/ <http://www.2ndquadrant.com/> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
