Support for postgresql cluster using pgpool

• Support for Postgresql cluster using pgpool
  • Currently there are a lot of postgresql users who are already using pgpool, but at the moment it is not supported because it has not been tested. 
  • Using pgpool is to achieve availability and load balancing for the application side, not Debezium side.
    • If it's about restarting after connection losses, that's handled by Debezium itself.
• pgpool support and propagate logical replication streams in case of logical replication mode(1)
  • if my understanding is correct, Debezium only relies on logical decoding on the primary server, and not related to pgpool directly.
  • I think it would be a combination of the pgpool logical replication mode and Debezium. So if we're really lucky, I think Debezium could work with pgpool without modifying Debezium itself and just be a test.

(1) 4.3.2. Running mode of Pgpool-II
    https://www.pgpool.net/docs/pgpool-II-3.7.1/en/html/configuring-pgpool.html
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The logical replication mode can be used with PostgreSQL servers operating logical replication. In this mode, PostgreSQL is responsible for synchronizing tables. Load balancing is possible in the mode. Since logical replication does not replicate all tables, it's user's responsibility to replicate the table which could be load balanced. Pgpool-II load balaces all tables. This means that if a table is not replicated, Pgpool-II may lookup outdated tables in the subscriber side. The sample configuration file is $prefix/etc/pgpool.conf.sample-logical.
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