Transactions through dblink_exec()

Masaru Sugawara wrote:

Hi, all

While trying dblink_exec(), one of dblink()'s functions, I noticed there was an
odd situation: case 1 and case 2 worked well, but case 3 didn't(see below).
I hadn't been aware of it so that I only executed BEGIN and END in
dblink_exec() at first . This time, however, I noticed it by executing ROLLBACK.

I'm hoping that dblink_exec() returns something like warning if those who intend
to do transactions make a declaration of blink_exec('dbname=some', 'begin') by mistake.

for example
WARNING :You should declare dblink_exec('dbname=some', 'BEGIN; some queries;
COMMIT/ROLLBACK/END;') or use dblink_exec('BEGIN/COMMIT/ROLLBACK/END')
around dblink_exec('some queries')s. If not, your transactions won't work.

How can dblink() possibly be used safely for non-readonly
transactions without a full implementation of a two-phase commit
protocol? What happens when the remote server issues the COMMIT
and then the local server crashes?

Mike Mascari
mascarm@mascari.com

Masaru Sugawara wrote:

I'm hoping that dblink_exec() returns something like warning if those who intend
to do transactions make a declaration of blink_exec('dbname=some', 'begin') by mistake.

for example
WARNING :You should declare dblink_exec('dbname=some', 'BEGIN; some queries;
COMMIT/ROLLBACK/END;') or use dblink_exec('BEGIN/COMMIT/ROLLBACK/END')
around dblink_exec('some queries')s. If not, your transactions won't work.

{...snip...]

-- case 3. --
SELECT dblink_exec('dbname=regression_slave', 'BEGIN');
SELECT dblink_exec('dbname=regression_slave',
'INSERT INTO foo VALUES(12,''m'',''{"a12","b12","c12"}'');');
SELECT dblink_exec('dbname=regression_slave', 'ROLLBACK'); -- failure !

Hmmm. No surprise this din't work. Each time you specify the connect string, a
connection is opened, the statement executed, and then the connection is
closed -- i.e. each of the invocations of dblink_exec above stands alone. Are
you suggesting a warning only on something like:
SELECT dblink_exec('dbname=regression_slave', 'BEGIN');
? Seems like maybe a warning in the documentation would be enough. Any other
opinions out there?

What occurs to me though, is that this is one of those "clients affected by
the autocommit setting" situations. (...goes off and tries it out...) Sure
enough. If you have autocommit set to off, you can do:
SELECT dblink_exec('dbname=regression_slave',
'INSERT INTO foo VALUES(12,''m'',''{"a12","b12","c12"}'');');
all day and never get it to succeed.

Given the above, should dblink_exec(CONNSTR, SQL) always wrap SQL in an
explicit transaction? Any thoughts on this?

Joe

Mike Mascari wrote:

How can dblink() possibly be used safely for non-readonly transactions
without a full implementation of a two-phase commit protocol? What
happens when the remote server issues the COMMIT and then the local
server crashes?

It can't be used safely if you're trying to ensure a distributed transaction
either fails or commits. At least I can't think of a way without two-phase
commits implemented.

But depending on your scenario, just being sure that the remote transaction
fails or succeeds as a unit may be all you care about.

Joe

On Sat, 12 Oct 2002 23:37:18 -0700
Joe Conway <mail@joeconway.com> wrote:

Masaru Sugawara wrote:

I'm hoping that dblink_exec() returns something like warning if those
who intend to do transactions make a declaration of
blink_exec('dbname=some', 'begin') by mistake.

for example
WARNING :You should declare dblink_exec('dbname=some', 'BEGIN; some queries;
COMMIT/ROLLBACK/END;') or use dblink_exec('BEGIN/COMMIT/ROLLBACK/END')
around dblink_exec('some queries')s. If not, your transactions won't work.

{...snip...]

-- case 3. --
SELECT dblink_exec('dbname=regression_slave', 'BEGIN');
SELECT dblink_exec('dbname=regression_slave',
'INSERT INTO foo VALUES(12,''m'',''{"a12","b12","c12"}'');');
SELECT dblink_exec('dbname=regression_slave', 'ROLLBACK'); -- failure !

Hmmm. No surprise this din't work. Each time you specify the connect string, a
connection is opened, the statement executed, and then the connection is
closed -- i.e. each of the invocations of dblink_exec above stands alone. Are
you suggesting a warning only on something like:
SELECT dblink_exec('dbname=regression_slave', 'BEGIN');

Yes.

? Seems like maybe a warning in the documentation would be enough.

Yes, certainly. I came to think a warning in the doc is better than in the
command line because that is not a bug.

Any other opinions out there?

What occurs to me though, is that this is one of those "clients affected by
the autocommit setting" situations. (...goes off and tries it out...) Sure
enough. If you have autocommit set to off, you can do:
SELECT dblink_exec('dbname=regression_slave',
'INSERT INTO foo VALUES(12,''m'',''{"a12","b12","c12"}'');');
all day and never get it to succeed.

I didn't think of a situation of autocommit = off. As for me in some
transactions like the following, I haven't deeply worried about behaviors of
dblink_exec(CONNSTR, 'BEGIN') because I would like to use dblink_connect() .
However, I'm not sure whether the following is perfectly safe against every
accident or not .

BEGIN;
SELECT dblink_connect('dbname=regression_slave');
SELECT dblink_exec('BEGIN');
SELECT dblink_exec('INSERT INTO foo VALUES(12, ''m'', ''{"a12","b12","c12"}'');');
INSERT INTO foo VALUES(12, 'm', '{"a12","b12","c12"}');
SELECT dblink_exec('END');
SELECT dblink_disconnect();
END;

or

CREATE OR REPLACE FUNCTION fn_mirror() RETURNS text AS '
DECLARE
ret text;
BEGIN
PERFORM dblink_connect(''dbname=regression_slave'');
PERFORM dblink_exec(''BEGIN'');
-- PERFORM dblink_exec(
-- ''INSERT INTO foo VALUES(12, ''''m'''', ''''{"a12","b12","c12"}'''');'');
SELECT INTO ret * FROM dblink_exec(
''INSERT INTO foo VALUES(12, ''''m'''', ''''{"a12","b12","c12"}'''');'');
RAISE NOTICE ''slave : %'', ret;
INSERT INTO foo VALUES(12, ''m'', ''{"a12","b12","c12"}'');
PERFORM dblink_exec(''END'');
PERFORM dblink_disconnect();
RETURN ''OK'';
END;
' LANGUAGE 'plpgsql';

SELECT fn_mirror();

Given the above, should dblink_exec(CONNSTR, SQL) always wrap SQL in an
explicit transaction? Any thoughts on this?

Joe

Regards,
Masaru Sugawara