On Mar 22, 2016, at 11:14 AM, Petr Jelinek wrote:
> And each slot means connection with logical decoding attached to it so you 
> don't really want to have thousands of those anyway. I think you'll hit other 
> problems faster than loop over slots becomes problem if you plan to keep all 
> of them active.

Assume that cluster have thousands of nodes and we use sharding to scatter data 
through cluster nodes.
But to provide HA we want to perform sharding with some level of redundancy, 
for example save the same record at 3 different nodes.
Once possible approach (pg_shard) is to execute the same query at three 
different shards.
But there is no warranty that  result of execution will be the same at all 
Alternative approach is to execute transaction at one node and then replicate 
it using logical replication to replicas.
So we do not perform logical replication to all 1000 nodes. Just to 2 of them. 
But each time it will be different pair of nodes. So we still need to have 1000 
active replication slots.

May be logical replication can not be used at all in such scenario - I have not 
thought much about it yet. Our first step will be multimaster without sharding.

> -- 
>  Petr Jelinek                  http://www.2ndQuadrant.com/
>  PostgreSQL Development, 24x7 Support, Training & Services
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