PG 12的新特性Connection slots and WAL senders,詳細描述如下:

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.

在PostgreSQL中,如果客戶端連接數超出max_connections參數設定的大小,會拒絕客戶端連接.

psql: FATAL:  sorry, too many clients already

為了應對這種情況,PG 11或之前的版本提供了superuser_reserved_connections參數,為超級用戶(管理員)提供了連接的后門,但只是面向超級用戶而已,并沒有為復制用戶提供后門,如果超出最大連接數,那么復制需要連接時會出錯,導致HA環境的破壞.
為了確保HA環境的”HA”,在PG 12中,把復制連接所需要的資源單獨進行管理,不再與普通的客戶端連接(max_connections控制)共享同一個Array(ProcGlobal).同時,這個特性要求備機的max_wal_senders不能小于主機的max_wal_senders,否則可能會導致故障.

參考資料
Postgres 12 highlight - Connection slots and WAL senders