Re: BDR: cannot drop database even after parting the node

On 17 September 2015 at 06:15, Florin Andrei <florin(at)andrei(dot)myip(dot)org> wrote:

> Then, from node1, I've parted node2 like this:
>
> SELECT bdr.bdr_part_by_node_names('{node2}');
>
> And then also on node1 I've parted node1 like this:
>
> SELECT bdr.bdr_part_by_node_names('{node1}');

The second step is not necessary. In fact we should detect that case
and ERROR, since it doesn't make sense to issue bdr_part_by_node_names
from a node that's already left, it can't have any effect.

> Now I want to start over with a clean slate, so I want to drop the bdrdemo
> database on node1.

That's because it's still a live BDR instance, just with one node.

> But I can't:
>
> postgres=# DROP DATABASE bdrdemo;
> ERROR: database "bdrdemo" is being accessed by other users
> DETAIL: There is 1 other session using the database.
> postgres=# SELECT pid FROM pg_stat_activity where pid <> pg_backend_pid();
> pid
> -------
> 10259
> 10260
> (2 rows)
>
> # ps ax | grep -e 10259 -e 10260 | grep -v grep
> 10259 ? Ss 0:00 postgres: bgworker: bdr supervisor
> 10260 ? Ss 0:00 postgres: bgworker: bdr db: bdrdemo
>
> If I kill those workers and then drop the database, the workers get
> respawned, and then the logs fill up with complaints from the workers that
> they can't find the bdrdemo database.
>
> Is there a way to stop BDR completely, so that those workers are laid to
> rest and never respawn?

You've made a good point. We address shutdown and removal on some
nodes, but not the case where you want to shut down and remove BDR on
a single remaining node.

I've been meaning to write a helper function for this for some time,
but other priorities keep intervening.

Here's the manual process:

BEGIN;
SET LOCAL bdr.skip_ddl_locking = on;
SET LOCAL bdr.permit_unsafe_ddl_commands = on;
SET LOCAL bdr.skip_ddl_replication = on;
SECURITY LABEL FOR bdr ON DATABASE mydb IS NULL;
DELETE FROM bdr.bdr_connections;
DELETE FROM bdr.bdr_nodes;
SELECT bdr.bdr_connections_changed();
COMMIT;

SELECT pg_terminate_backend(pid)
FROM pg_stat_activity
WHERE datname = current_database() AND application_name LIKE '%): perdb';

DROP EXTENSION bdr;

... then remove 'bdr' from shared_preload_libraries .

> Basically, how do I reset BDR completely? It seems to retain the memory of
> the bdrdemo database somewhere.

Sort-of. What happens in your example is that when you part the nodes,
they're separated and stop communicating. So your second part command
never reaches the remaining node. That's expected and normal, but we
should issue an error when it's attempted to make it clearer to the
user what's going on.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services