PG 12的新特性Connection slots and WAL senders,详细描述如下:

Move max_wal_senders out of max_connections for connection slot handling.Since its introduction, max_wal_senders is counted as part of max_connections when it comes to define how many connection slots can be used for replication connections with a WAL sender context. This can lead to confusion for some users, as it could be possible to block a base backup or replication from happening because other backend sessions are already taken for other purposes by an application, and superuser-only connection slots are not a correct solution to handle that case.This commit makes max_wal_senders independent of max_connections for its handling of PGPROC entries in ProcGlobal, meaning that connection slots for WAL senders are handled using their own free queue, like autovacuum workers and bgworkers.

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在PostgreSQL中,如果客户端连接数超出max_connections参数设定的大小,会拒绝客户端连接.

psql: FATAL:  sorry, too many clients already

为了应对这种情况,PG 11或之前的版本提供了superuser_reserved_connections参数,为超级用户(管理员)提供了连接的后门,但只是面向超级用户而已,并没有为复制用户提供后门,如果超出最大连接数,那么复制需要连接时会出错,导致HA环境的破坏. 
为了确保HA环境的”HA”,在PG 12中,把复制连接所需要的资源单独进行管理,不再与普通的客户端连接(max_connections控制)共享同一个Array(ProcGlobal).同时,这个特性要求备机的max_wal_senders不能小于主机的max_wal_senders,否则可能会导致故障.

参考资料 
Postgres 12 highlight - Connection slots and WAL senders