>> 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.
I have incorporated "Pangea" algorithm into Pgpool-II to implement the
atomic visibility. In a test below I have two PostgreSQL servers
(stock v12), server0 (port 11002) and server1 (port
11003). default_transaction_isolation was set to 'repeatable read' on
both PostgreSQL, this is required by Pangea. Pgpool-II replicates
write queries and send them to both server0 and server1. There are two
tables "t1" (having only 1 integer column "i") and "log" (having only
1 integer c column "i"). I have run following script
(inconsistency1.sql) via pgbench:
BEGIN;
UPDATE t1 SET i = i + 1;
END;
like: pgbench -n -c 1 -T 30 -f inconsistency1.sql
In the moment I have run another session from pgbench concurrently:
BEGIN;
INSERT INTO log SELECT * FROM t1;
END;
pgbench -n -c 1 -T 30 -f inconsistency2.sql
After finishing those two pgbench runs, I ran following COPY to see if
contents of table "log" are identical in server0 and server1:
psql -p 11002 -c "\copy log to '11002.txt'"
psql -p 11003 -c "\copy log to '11003.txt'"
cmp 11002.txt 11003.txt
The new Pgpool-II incorporating Pangea showed that 11002.txt and
11003.txt are identical as expected. This indicates that the atomic
visibility are kept.
On the other hand Pgpool-II which does not implement Pangea showed
differences in those files.
Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp