Shulou(Shulou.com)06/01 Report--

Connection slots and WAL senders, a new feature of PG 12, is described in detail as follows:

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.

In PostgreSQL, client connections are rejected if the number of client connections exceeds the size set by the max_connections parameter.

Psql: FATAL: sorry, too many clients already

In order to deal with this situation, PG 11 or previous versions provide superuser_reserved_connections parameters, which provide a back door for super users (administrators), but only for super users, but not for replication users. If the maximum number of connections is exceeded, then the replication will make an error when it needs to connect, resulting in the destruction of the HA environment.

In order to ensure the "HA" of the HA environment, in PG 12, the resources needed for replication connections are managed separately, and the same Array (ProcGlobal) is no longer shared with ordinary client connections (max_connections control). At the same time, this feature requires that the max_wal_senders of the standby cannot be less than the max_wal_senders of the host, otherwise it may lead to failure.

references

Postgres 12 highlight-Connection slots and WAL senders

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