Check replication from secondary point of view

[Krysztophe]

Following a discussion with @tobixen (#206 (comment)): he expressed the need of a service checking for the replication from the secondary point a view.

Just ideas and questions:

• in which situations would this service be useful? (it was always assumed that the checks where done from the primary)

• there are different cases:

  • pure streaming
  • streaming with log shipping (eg: the service could raise a WARNING if PG had to fall back on log shipping)
  • streaming ok but replay is paused
  • log shipping only, with or without archive_timeout
  • logical replication?

• improve is_hot_standby

• add a new service is_streaming?

[iovcho]

+1