>> My concern is, FDW+2PC without atomic visibility could lead to data >> inconsistency among servers in some cases. If my understanding is >> correct, FDW+2PC (without atomic visibility) cannot prevent data >> inconsistency in the case below. Initially table t1 has only one row >> with i = 0 on both N1 and N2. By executing S1 and S2 concurrently, t1 >> now has different value of i, 0 and 1. > > IIUC the following sequence won't happen because COMMIT PREPARED > 's1n1' cannot be executed before PREPARE TRANSACTION 's1n2'.
You are right. > But as > you mentioned, we cannot prevent data inconsistency even with FDW+2PC > e.g., when S2 starts a transaction between COMMIT PREPARED on N1 and > COMMIT PREPARED on N2 by S1. Ok, example updated. S1/N1: DROP TABLE t1; DROP TABLE S1/N1: CREATE TABLE t1(i int); CREATE TABLE S1/N1: INSERT INTO t1 VALUES(0); INSERT 0 1 S1/N2: DROP TABLE t1; DROP TABLE S1/N2: CREATE TABLE t1(i int); CREATE TABLE S1/N2: INSERT INTO t1 VALUES(0); INSERT 0 1 S1/N1: BEGIN; BEGIN S1/N2: BEGIN; BEGIN S1/N1: UPDATE t1 SET i = i + 1; -- i = 1 UPDATE 1 S1/N2: UPDATE t1 SET i = i + 1; -- i = 1 UPDATE 1 S2/N1: BEGIN; BEGIN S2/N2: BEGIN; BEGIN S1/N1: PREPARE TRANSACTION 's1n1'; PREPARE TRANSACTION S1/N2: PREPARE TRANSACTION 's1n2'; PREPARE TRANSACTION S2/N1: PREPARE TRANSACTION 's2n1'; PREPARE TRANSACTION S2/N2: PREPARE TRANSACTION 's2n2'; PREPARE TRANSACTION S1/N1: COMMIT PREPARED 's1n1'; COMMIT PREPARED S2/N1: DELETE FROM t1 WHERE i = 1; DELETE 1 S2/N2: DELETE FROM t1 WHERE i = 1; DELETE 0 S1/N2: COMMIT PREPARED 's1n2'; COMMIT PREPARED S2/N1: COMMIT PREPARED 's2n1'; COMMIT PREPARED S2/N2: COMMIT PREPARED 's2n2'; COMMIT PREPARED S2/N1: SELECT * FROM t1; i --- (0 rows) S2/N2: SELECT * FROM t1; i --- 1 (1 row) > The point is this data inconsistency is > lead by an inconsistent read but not by an inconsistent commit > results. I think there are kinds of possibilities causing data > inconsistency but atomic commit and atomic visibility eliminate > different possibilities. We can eliminate all possibilities of data > inconsistency only after we support 2PC and globally MVCC. IMO any permanent data inconsistency is a serious problem for users no matter what the technical reasons are. Best regards, -- Tatsuo Ishii SRA OSS, Inc. Japan English: http://www.sraoss.co.jp/index_en.php Japanese:http://www.sraoss.co.jp
