Tuesday, May 03, 2011

Pgpool-II Load Balancing

I've been trying, so far unsuccessfully, to get load balancing working in a pgpool-II replication environment with two nodes. This is supposed to spread SELECT queries across multiple nodes, according to the nodes' values of backend_weight. Presumably it does so in a round-robin manner, and if the two nodes had the same weight, query 1 would go to one node, query 2 would go to the other node, query 3 would go back to the first node, and so on...However, my queries are all being sent to one node.

This table, taken from the pgpool-II docs, shows whether a SELECT query will replicate, be sent to master only, or be load balanced based on three conditions:
SELECT is inside a transaction block Y Y Y N N N Y N
replicate_select is true Y Y N N Y Y N N
load_balance_mode is true Y N N N Y N Y Y
results(R:replication, M: send only to master, L: load balance) R R M M R R M L

It's absolutely clear that I have replicate_select set to false and load_balance_mode set to true. I seem to be getting a result of "M", which must mean my SELECT is in a transaction block. It doesn't appear to be though, acc. to my pg logs - at least, I don't see any BEGIN before the SELECT. Hmm!

If I fiddle with my two values of backend_weight and restart pgpool, it will switch to the node with the greater weight. Doesn't help me much, but it shows it's doing...something.

Stay tuned...


MWrynn said...

Found a sugggestion in this presentation - http://www.dalibo.org/_media/pgpool.pdf (page 34) that suggests turning autocommit off if it's not already... Will follow up.

MWrynn said...
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MWrynn said...

Well, that didn't do the job. What it did was, instead of sending standalone SELECTs to only one node, it sent to one node (for each SELECT):


In addition, the other node received (again for each SELECT):

In other words, with autocommit off, I believe I am seeing the Y/N/Y=M case of the table I posted. Prior to this test (when autocommit was on), I believe I SHOULD have been in the N/N/Y=L case. Apparently this is not true however.

Tatsuo Ishii said...

Pgpool-II does not do load balancing using LRU pocily. Rather it sends quries randomly considering "weight". So you need to send enough number of queries(~100 for example) to observe the effect of load balancing. Also note that the node to be sent is determined when client connects to pgpool-II, not each statement. If your client keeps on connecting to pgpool-II, it keeps on sending queries to the same node until it disconnects to pgpool-II.

MWrynn said...

Thank you Tatsuo. I got it working. I mistakenly believed that load balancing worked within a single connection. Now I can see it is balancing connections.