Principles of PgaaS role determination

Every host except master is in recovery state from PostgreSQL's POV. This means the check 'select pg_is_in_recovery()' returns false for the master and true for every other host type.
Some hosts are in sync slave mode. This may be determined by executing 'show synchronous_standby_names' on the master. See https://www.postgresql.org/docs/current/runtime-config-replication.html#GUC-SYNCHRONOUS-STANDBY-NAMES for more information.

PgaaS sync slaves lag: By default, PgaaS synchronous slaves are working with 'synchronous_commit' set to 'remote_apply'. Therefore, sync slave may be lagging behind the master and thus is not truly 'synchronous' from the reader's POV, but things may change with time.

Implementation: Topology update runs every second.

Every host is assigned a connection with special ID (4100200300). Using this connection we check for host availability, writability (master detection) and perform RTT measurements.

After the initial check we know about master presence and RTT for each host. Master host is queried about synchronous replication status. We use this info to identify synchronous slaves and to detect "quorum commit" presence.

⇦ uPg: Postgres errors | uPg: Automatic connection limit ⇨