Re: Is DBLINK transactional

craig@postnewspapers.com.au (Craig Ringer) writes:

> On 13/03/2010 5:54 AM, Jeff Davis wrote:
>> On Fri, 2010-03-12 at 12:07 -0500, Merlin Moncure wrote:
>>> of course.  You can always explicitly open a transaction on the remote
>>> side over dblink, do work, and commit it at the last possible moment.
>>> Your transactions aren't perfectly synchronized...if you crash in the
>>> precise moment between committing the remote and the local you can get
>>> in trouble.  The chances of this are extremely remote though.
>>
>> If you want a better guarantee than that, consider using 2PC.
>
> Translation in case you don't know: 2PC = two phase commit.
>
> Note that you have to monitor "lost" transactions that were prepared
> for commit then abandoned by the controlling app and periodically get
> rid of them or you'll start having issues.

There can be issues even if they're not abandoned...

Note that prepared transactions establish, and maintain, until removed,
all the appropriate locks on the underlying tables and tuples.

As a consequence, maintenance-related activities may be somewhat
surprisingly affected.

foo=# begin; set transaction isolation level serializable;
BEGIN
SET
foo=# insert into my_table (date_time, hostname, duration, diag) values (now(), 'foo', 1, 2);
INSERT 0 1
foo=# prepare transaction 'foo';
PREPARE TRANSACTION

[then, I quit the psql session...]

foo=# select * from pg_locks where relation = (select oid from pg_class where relname = 'my_table');
-[ RECORD 1 ]------+-----------------
locktype           | relation
database           | 308021
relation           | 308380
page               |
tuple              |
virtualxid         |
transactionid      |
classid            |
objid              |
objsubid           |
virtualtransaction | -1/433653
pid                |
mode               | RowExclusiveLock
granted            | t

If I try to truncate the table...

foo=# truncate my_table;
[hangs, waiting on the lock...]

[looking at another session...]

foo=#  select * from pg_locks where relation = (select oid from pg_class where relname = 'my_table');
-[ RECORD 1 ]------+--------------------
locktype           | relation
database           | 308021
relation           | 308380
page               |
tuple              |
virtualxid         |
transactionid      |
classid            |
objid              |
objsubid           |
virtualtransaction | -1/433653
pid                |
mode               | RowExclusiveLock
granted            | t
-[ RECORD 2 ]------+--------------------
locktype           | relation
database           | 308021
relation           | 308380
page               |
tuple              |
virtualxid         |
transactionid      |
classid            |
objid              |
objsubid           |
virtualtransaction | 2/13
pid                | 3749
mode               | AccessExclusiveLock
granted            | f

Immediately upon submitting "commit prepared 'foo';", both locks are
resolved quite quickly.

>> The problem with things that are "extremely remote" possibilities are
>> that they tend to be less remote than we expect ;)
>
> ... and they know just when they can happen despite all the odds to
> maximise the pain and chaos caused.

A lot of these kinds of things only come up as race conditions.  The
trouble is that a lot of races do wind up synchronizing themselves.

In sporting events, this is intended and desired; an official fires the
starter pistol or activates the horn, or what have you, with the
intended result that athletes begin very nearly simultaneously.  And at
the end of Olympic races, their times frequently differ only by
miniscule intervals.

In my example up above, there's a possibly unexpected synchronization
point; the interweaving of the PREPARE TRANSACTION and TRUNCATE requests
lead to a complete lock against the table.  Supposing 15 processes then
try accessing that table, they'll be blocked until the existing locks
get closed out.  Which takes place the very instant after the COMMIT
PREPARED request comes in.  At that moment, 15 "racers" are released
very nearly simultaneously.

If there is any further mischief to be had in the race, well, they're
set up to tickle it...
--
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