Re: [HACKERS] Slony-I goes BETA (possible bug)
On 6/7/2004 2:33 PM, Jeff Davis wrote: On Mon, 2004-06-07 at 06:20, Jan Wieck wrote: I tend to agree with you that spurious SYNC's aren't the end of the world. The idea of using notify to tell the syncThread somthing happened is probably the right way to do it, but at this time a little invasive. We need more time to investigate how to avoid notice storms during high update activity on the master. There was discussion a while back about improving notify, and one suggestion was to make it use shared memory so no disk writes are involved (I believe the current implementation uses a table somehow). If that was implemented, than we would have no problem with a notice storm, right? It wouldn't use much shared memory since the slon daemon can retrieve the notices just as fast as the backend can send them, right? Keep in mind that for the time being, one of the important features of Slony-I is the ability to replicate from a 7.3.x to anything 7.3.x. You sure don't want to cripple that functionality by heavily depending on features fixed or significantly improved for 7.5. Backtracking a little, I'm still wondering how exactly a replicated sequence is supposed to behave, do you have some comments about that? I don't understand exactly why it's useful. At the moment the origin of a set discovers that there has been update activity, it generates the SYNC event and records all sequences last_values. At that time, a sequence can possibly be incremented again by a not yet committed transaction, so it might be recorded with a higher number than a max() query over the tables would show. When a subscriber applies the SYNC event, it also calls setval() with those recorded values. So on the replica the sequence number is adjusted up just before the SYNC event occupying those numbers commits. This means, that in the case of a failover, the sequences might show a gap. This is absolutely in accordance to PostgreSQL's sequence handling which cannot guarantee gap free sequences in the case of server crashes or other transaction rollback reasons. Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 2: you can get off all lists at once with the unregister command (send unregister YourEmailAddressHere to [EMAIL PROTECTED])
Re: [HACKERS] Slony-I goes BETA (possible bug)
I tend to agree with you that spurious SYNC's aren't the end of the world. The idea of using notify to tell the syncThread somthing happened is probably the right way to do it, but at this time a little invasive. We need more time to investigate how to avoid notice storms during high update activity on the master. Jan On 6/6/2004 2:33 PM, Jeff Davis wrote: On Sun, 2004-06-06 at 10:32, Jan Wieck wrote: You are right. The local slon node checks every -s milliseconds (commandline switch) if the sequence sl_action_seq has changed, and if so generate a SYNC event. Bumping a sequence alone does not cause this, only operations that invoke the log trigger on replicated tables do. Speaking of this, this would also mean that there is a gap between the last sl_action_seq bumping operation and the commit of that transaction. If the local slon will generate the sync right in that gap, the changes done in that transaction will not be replicated until the next transaction triggers another sync. I am not sure how to effectively avoid this problem without blindly creating SYNC events in a maybe less frequent interval. Suggestions? A couple thoughts occur to me: Spurious SYNCs might not be the end of the world, because if someone is using replication, they probably don't mind the unneeded costs of a SYNC when the database is not being used heavily. If it is being used heavily, the SYNCs will have to happen anyway. Also, it might be possibly to make use of NOTIFY somehow, because notifications only occur after a transaction commits. Perhaps you can issue a notify for each transaction that modifies a replicated table and slon could listen for that notification? That way, it wouldn't SYNC before the transaction commits and miss the uncommitted data. Regards, Jeff Davis -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] Slony-I goes BETA (possible bug)
On Mon, 2004-06-07 at 06:20, Jan Wieck wrote: I tend to agree with you that spurious SYNC's aren't the end of the world. The idea of using notify to tell the syncThread somthing happened is probably the right way to do it, but at this time a little invasive. We need more time to investigate how to avoid notice storms during high update activity on the master. There was discussion a while back about improving notify, and one suggestion was to make it use shared memory so no disk writes are involved (I believe the current implementation uses a table somehow). If that was implemented, than we would have no problem with a notice storm, right? It wouldn't use much shared memory since the slon daemon can retrieve the notices just as fast as the backend can send them, right? Backtracking a little, I'm still wondering how exactly a replicated sequence is supposed to behave, do you have some comments about that? I don't understand exactly why it's useful. Regards, Jeff ---(end of broadcast)--- TIP 4: Don't 'kill -9' the postmaster
Re: [HACKERS] Slony-I goes BETA (possible bug)
On 6/6/2004 5:21 AM, Jeff Davis wrote:
I have two nodes, node 1 and node 2.
Both are working with node 1 as the master, and data from subscribed
tables is being properly replicated to node 2.
However, it looks like there's a possible bug with sequences. First let
me explain that I don't entirely understand how a replicated sequence is
expected to behave, but as far as this report is concerned, I assume
that if you do a nextval() on node 1, than SELECT last_value FROM
test_seq on node 2 will return the updated value.
It looks like the sequence value is not updated on node 2, until some
other event happens, like doing an UPDATE on a replicated table on node
1.
You are right. The local slon node checks every -s milliseconds
(commandline switch) if the sequence sl_action_seq has changed, and if
so generate a SYNC event. Bumping a sequence alone does not cause this,
only operations that invoke the log trigger on replicated tables do.
Speaking of this, this would also mean that there is a gap between the
last sl_action_seq bumping operation and the commit of that transaction.
If the local slon will generate the sync right in that gap, the changes
done in that transaction will not be replicated until the next
transaction triggers another sync.
I am not sure how to effectively avoid this problem without blindly
creating SYNC events in a maybe less frequent interval. Suggestions?
Jan
I already have a table t2 which is properly replicating.
So, here's what I give to slonik to add the sequence to set 1:
slonik _EOF_
cluster name = $CLUSTER;
node 1 admin conninfo = 'dbname=$DBNAME1 host=$HOST1 user=$SLONY_USER';
node 2 admin conninfo = 'dbname=$DBNAME2 host=$HOST2 user=$SLONY_USER';
create set (id=34, origin=1, comment='set 34');
set add sequence (set id = 34, origin = 1, id = 35, full qualified
name='public.test_seq', comment = 'sequence test');
subscribe set (id=34,provider=1,receiver=2,forward=no);
merge set (id=1,add id = 34, origin=1);
subscribe set (id=1,provider=1,receiver=2,forward=no);
_EOF_
Note: results of the query are put after the -- following the query
for easier readability.
node1= SELECT last_value FROM test_seq; -- 1
node2= SELECT last_value FROM test_seq; -- 1
node1= SELECT nextval('test_seq'); -- 1
node1= SELECT nextval('test_seq'); -- 2
node1= SELECT nextval('test_seq'); -- 3
node1= SELECT last_value FROM test_seq; -- 3
node2= SELECT last_value FROM test_seq; -- 1
node2= -- wait for a long time, still doesn't update
node2= SELECT last_value FROM test_seq; -- 1
node1= INSERT INTO t2(a) VALUES('string');
node2= SELECT last_value FROM test_seq; -- 3
node2= -- now it's updated!
So, that looks like a possible bug where a nextval() call doesn't
trigger the replication. But it does appear to replicate after an
unrelated event triggers the replication (in this case an update to t2,
an unrelated table).
If not, what is the expected behavior of replicated sequences anyway? It
seems you couldn't call nextval() from a slave node, and because of that
you also can't make use of currval(). It looks like the slaves can
really only get the SELECT last_value FROM test_seq. So is there a
particular use case someone had in mind when implementing the SET ADD
SEQUENCE for slonik?
Regards,
Jeff Davis
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Re: [HACKERS] Slony-I goes BETA (possible bug)
On Sun, 2004-06-06 at 10:32, Jan Wieck wrote: You are right. The local slon node checks every -s milliseconds (commandline switch) if the sequence sl_action_seq has changed, and if so generate a SYNC event. Bumping a sequence alone does not cause this, only operations that invoke the log trigger on replicated tables do. Speaking of this, this would also mean that there is a gap between the last sl_action_seq bumping operation and the commit of that transaction. If the local slon will generate the sync right in that gap, the changes done in that transaction will not be replicated until the next transaction triggers another sync. I am not sure how to effectively avoid this problem without blindly creating SYNC events in a maybe less frequent interval. Suggestions? A couple thoughts occur to me: Spurious SYNCs might not be the end of the world, because if someone is using replication, they probably don't mind the unneeded costs of a SYNC when the database is not being used heavily. If it is being used heavily, the SYNCs will have to happen anyway. Also, it might be possibly to make use of NOTIFY somehow, because notifications only occur after a transaction commits. Perhaps you can issue a notify for each transaction that modifies a replicated table and slon could listen for that notification? That way, it wouldn't SYNC before the transaction commits and miss the uncommitted data. Regards, Jeff Davis ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
