Re: [PERFORM] Is DBLINK transactional

2010-03-16 Thread Chris Browne
cr...@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...
-- 
let name=cbbrowne and tld=gmail.com in name ^ @ ^ tld;;
http://linuxdatabases.info/info/nonrdbms.html
Barf, what is all   this  prissy pedantry?  Groups,  modules,  rings,
ufds, patent-office algebra.  Barf!  -- R. William Gosper

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Re: [PERFORM] Is DBLINK transactional

2010-03-15 Thread Hannu Krosing
On Sat, 2010-03-13 at 20:10 +0800, Craig Ringer wrote:
 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.

And you still have the problem of committing one 2PC transaction and
then crashing before committing the other and then crashing the
transaction monitor before being able to record what crashed :P, though
this possibility is even more remote than just crashing between the 2
original commits (dblink and local).

To get around this fundamental problem, you can actually do async queues
and remember, what got replayed on the remote side, so if you have
crashes on either side, you can simply replay again.

  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.
 
 --
 Craig Ringer
 


-- 
Hannu Krosing   http://www.2ndQuadrant.com
PostgreSQL Scalability and Availability 
   Services, Consulting and Training



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Re: [PERFORM] Is DBLINK transactional

2010-03-13 Thread Craig Ringer

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.



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.


--
Craig Ringer

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[PERFORM] Is DBLINK transactional

2010-03-12 Thread elias ghanem
Hi,

I am using dblink to read data from a remote data base, insert these data in
the local database, update the red data in the remote database then continue
to do some other work on the local database in the same transaction.

My question is : Is db link transactional; If the local transaction failed,
would the update in the remote data base roll back or if the update in the
remote data base failed, would the insert in the local data base roll back. 

If not, is there a way to make db link transactional?

Thanks

 



Re: [PERFORM] Is DBLINK transactional

2010-03-12 Thread Merlin Moncure
On Fri, Mar 12, 2010 at 10:27 AM, elias ghanem e.gha...@acteos.com wrote:
 Hi,

 I am using dblink to read data from a remote data base, insert these data in
 the local database, update the red data in the remote database then continue
 to do some other work on the local database in the same transaction.

 My question is : Is db link transactional; If the local transaction failed,
 would the update in the remote data base roll back or if the update in the
 remote data base failed, would the insert in the local data base roll back.

 If not, is there a way to make db link “transactional”?

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.

merlin

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Re: [PERFORM] Is DBLINK transactional

2010-03-12 Thread Jeff Davis
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.

The problem with things that are extremely remote possibilities are
that they tend to be less remote than we expect ;)

Regards,
Jeff Davis


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