BDR/HAProxy server pool management
==================================

The ``tpaexec pool-disable-server`` and ``pool-enable-server`` commands
allow a PGD instance in a :ref:`BDR-Always-ON cluster <BDR-Always-ON>`  to be temporarily removed from
the HAProxy active server pool for maintenance, and then added back
afterwards.

These commands follow the same process as :ref:`rolling updates <Upgrading your cluster>`  by default, so
``pool-disable-server`` will wait for active transactions against a PGD
instance to complete and for pgbouncer to direct new connections to
another instance before completing. Use the ``--nowait`` option if you
don’t want to wait for active sessions to end.

Running ``pool-disable-server`` immediately followed by
``pool-enable-server`` on an instance will have the effect of moving all
active traffic to a different instance (in essence, a switchover). This
allows you to run online maintenace tasks such as long-running VACUUM
commands, while maintaining instance availability.

If there are multiple HAProxy servers configured with the same set of
``haproxy_backend_servers`` , this command will remove or add the given
server to the pool of every relevant proxy in parallel.

Example
-------

.. code:: bash

   $ tpaexec pool-disable-server ~/clusters/clockwork orange # --nowait

   #  HAProxy will no longer direct any traffic to the PGD instance named
   #  orange, so you can perform maintenance on it (e.g., run `tpaexec
   #  rehydrate`).

   $ tpaexec pool-enable-server ~/clusters/clockwork orange