[pgpool-hackers: 3886] Re: Proposal: If replication delay exceeds delay_threshold, elect a new load balance node with less delay

[Bo Peng]

Hi,

Thank you for your patch.
I have reviewed your patch and below is the feedback.

1. Pgpool-II follows the PostgreSQL coding conventions. 
For example, the following code

  -                       pool_set_node_to_be_sent(query_context, PRIMARY_NODE_ID);
  +                       if (prefer_lower_delay_standby){
  +                           session_context->load_balance_node_id = select_load_balancing_node();
  +                           pool_set_node_to_be_sent(query_context, session_context->load_balance_node_id);
  +                       } else {
  +                           pool_set_node_to_be_sent(query_context, PRIMARY_NODE_ID);
  +                       }

should be in the following style:

  if (prefer_lower_delay_standby)
  {
      session_context->load_balance_node_id = select_load_balancing_node();
      pool_set_node_to_be_sent(query_context, session_context->load_balance_node_id);
  } 
  else 
  {
      pool_set_node_to_be_sent(query_context, PRIMARY_NODE_ID);
  }

2. The patch should include a regression test which is used to test the new feature.

3. The patch should include documentation update.

4. I want to confirm the feature.
   According the follwoing code, it seems that if the replication_delay exceeds delay_threshold, 
   you are electing the new load balancing node which has less delay than pool_config->delay_threshold.
   The new load balancing load should be the backend with the least delay, isn't it?
 
  +				if ( VALID_BACKEND(i) &&
  +					 !IS_PRIMARY_NODE_ID(i) &&
  +				 	(bkinfo->standby_delay < pool_config->delay_threshold) &&
  +				 	bkinfo->backend_weight > 0 )
  +				{
  +					selected_slot = i;
  +					break;
  +				}


On Tue, 20 Apr 2021 17:24:35 +0900
KAWAMOTO Masaya <kawamoto at sraoss.co.jp> wrote:

> Hi hackers.
> 
> On streaming replication mode, pgpool check the streaming delay on selected load
> balance node if delay_threshold paraeter is larger than 0. When the delay exceeds
> delay_theshold, pgpool send the query to primary regardless other standbies not
> delayed. I want the user to be able to choose whether pgpool always send to primary
> when there are other standbys with less delay by adding a boolean parameter
> "prefer_lower_delay_standby" to pgpool.conf.
> 
> I improve select_load_balancing_node() function.
> If "prefer_lower_delay" is true, pgpool selects another standby node that is lower delayed
> than the suggested node even if the suggested node name matches
> database_redirect_preference_list or app_redirect_preference_list.
> 
> Best Regards.
> 
> -- 
> KAWAMOTO Masaya <kawamoto at sraoss.co.jp>
> SRA OSS, Inc. Japan


-- 
Bo Peng <pengbo at sraoss.co.jp>
SRA OSS, Inc. Japan