Re: Removing and readding bdr nodes

I just realized this also. The node that was removed and recreated shows
only one replication slot once it is recreated. That slot has its own id in
it too. That appears to be the problem. I have no idea why when creating it
the first time the slots for the other two nodes are created correctly, but
when creating it a second time it does this:

======================================================================================================
sql (9.4.1)
Type "help" for help.

ballyhoo=# select * from pg_catalog.pg_replication_slots;
slot_name | plugin | slot_type | datoid |
database | active | xmin | catalog_xmin | restart_lsn
-----------------------------------------+--------+-----------+--------+----------+--------+------+--------------+-------------
bdr_16391_6145097170174308100_1_16391__ | bdr | logical | 16391 |
ballyhoo | f | | 2050 | 0/21BE1798
(1 row)

ballyhoo=#
======================================================================================================

On Mon, May 4, 2015 at 2:26 PM, Mathew Moon <mathew(dot)moon(at)vipaar(dot)com> wrote:

> Hi Craig,
>
> Thank you for your reply. Below you will see the entire process that I
> have used along with relevant log output. Something else that I have
> noticed is that the status of nodes in bdr.bdr_nodes stays 'i' on the first
> server that I initialize (lets call it primary and the others backups for
> sake of clarity). On the backups themselves bdr.bdr_nodes shows themselves
> as status 'r', but other nodes show their status as still initializing even
> though their logs show that the node in question has 'caught up with
> primary'. Anyway here is what I have:
>
> *Here are all three nodes joined to the group and functioning as shown
> from the "primary" server, which is the server that the group is created
> from and the server that the other nodes are joined to the group using:*
>
> ballyhoo=# select bdr.bdr_get_local_nodeid();
>> bdr_get_local_nodeid
>> -------------------------------
>> (6145084718544474516,1,16391)
>> (1 row)
>> ballyhoo=# select bdr.bdr_get_local_node_name();
>> bdr_get_local_node_name
>> -------------------------
>> pg94-test
>> (1 row)
>> ballyhoo=# select * from bdr.bdr_nodes;
>> node_sysid | node_timeline | node_dboid | node_status |
>> node_name | node_local_dsn
>> | node_init_from_dsn
>>
>> ---------------------+---------------+------------+-------------+------------+--------------------------------------------------------------------+-----------------------------------------------
>> --------------------
>> 6145084718544474516 | 1 | 16391 | r |
>> pg94-test | host=pg94-test.vipaar.com port=5432 user=postgres
>> dbname=ballyhoo |
>> 6145085940943605149 | 1 | 16391 | i |
>> pg94-test2 | host=pg94-test2.vipaar.com port=5432 user=postgres
>> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
>> res dbname=ballyhoo
>> 6145083860389238141 | 1 | 16391 | i |
>> pg94-test3 | host=pg94-test3.vipaar.com port=5432 user=postgres
>> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
>> res dbname=ballyhoo
>> (3 rows)
>> ballyhoo=# select * from pg_catalog.pg_replication_slots;
>> slot_name | plugin | slot_type | datoid |
>> database | active | xmin | catalog_xmin | restart_lsn
>>
>> -----------------------------------------+--------+-----------+--------+----------+--------+------+--------------+-------------
>> bdr_16391_6145085940943605149_1_16391__ | bdr | logical | 16391 |
>> ballyhoo | t | | 2362 | 0/21DA7B50
>> bdr_16391_6145083860389238141_1_16391__ | bdr | logical | 16391 |
>> ballyhoo | t | | 2362 | 0/21DA7B50
>> (2 rows)
>
>
> *Now here is the same info from the other two nodes in the group. Notice
> how the "primary" shows status of 'i' on these while they recognize
> themselves as 'r':*
>
> ballyhoo=# select bdr.bdr_get_local_nodeid();
> bdr_get_local_nodeid
> -------------------------------
> (6145085940943605149,1,16391)
> (1 row)
>
> ballyhoo=# select bdr.bdr_get_local_node_name();
> bdr_get_local_node_name
> -------------------------
> pg94-test2
> (1 row)
>
> ballyhoo=# select * from bdr.bdr_nodes;
> node_sysid | node_timeline | node_dboid | node_status |
> node_name | node_local_dsn
> | node_init_from_dsn
>
>
> ---------------------+---------------+------------+-------------+------------+--------------------------------------------------------------------+-----------------------------------------------
> --------------------
> 6145084718544474516 | 1 | 16391 | r |
> pg94-test | host=pg94-test.vipaar.com port=5432 user=postgres
> dbname=ballyhoo |
> 6145085940943605149 | 1 | 16391 | r |
> pg94-test2 | host=pg94-test2.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> 6145083860389238141 | 1 | 16391 | i |
> pg94-test3 | host=pg94-test3.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> (3 rows)
>
> *And the third node:*
>
> ballyhoo=# select bdr.bdr_get_local_nodeid();
> bdr_get_local_nodeid
> -------------------------------
> (6145083860389238141,1,16391)
> (1 row)
>
> ballyhoo=# select bdr.bdr_get_local_node_name();
> bdr_get_local_node_name
> -------------------------
> pg94-test3
> (1 row)
>
> ballyhoo=# select * from bdr.bdr_nodes;
> node_sysid | node_timeline | node_dboid | node_status |
> node_name | node_local_dsn
> | node_init_from_dsn
>
>
> ---------------------+---------------+------------+-------------+------------+--------------------------------------------------------------------+-----------------------------------------------
> --------------------
> 6145084718544474516 | 1 | 16391 | r |
> pg94-test | host=pg94-test.vipaar.com port=5432 user=postgres
> dbname=ballyhoo |
> 6145085940943605149 | 1 | 16391 | i |
> pg94-test2 | host=pg94-test2.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> 6145083860389238141 | 1 | 16391 | r |
> pg94-test3 | host=pg94-test3.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> (3 rows)
>
> ballyhoo=# select * from pg_catalog.pg_replication_slots;
> slot_name | plugin | slot_type | datoid |
> database | active | xmin | catalog_xmin | restart_lsn
>
> -----------------------------------------+--------+-----------+--------+----------+--------+------+--------------+-------------
> bdr_16391_6145085940943605149_1_16391__ | bdr | logical | 16391 |
> ballyhoo | t | | 2071 | 0/21C30AC8
> bdr_16391_6145084718544474516_1_16391__ | bdr | logical | 16391 |
> ballyhoo | t | | 2071 | 0/21C30AC8
> (2 rows)
>
> ballyhoo=#
>
> *Now I remove a node by stopping postgres on that node and executing
> "select bdr.bdr_part_by_node_names('{node_name}') on the "primary" node:*
>
> ballyhoo=# select bdr.bdr_get_local_node_name();
> bdr_get_local_node_name
> -------------------------
> pg94-test2
> (1 row)
>
> ballyhoo=# \q
> [root(at)pg94-test2 pgsql]# service postgres stop
> Stopping postgres service: [ OK ]
> [root(at)pg94-test2 pgsql]#
>
> *Now here is the primary:*
>
> ballyhoo=# select bdr.bdr_part_by_node_names('{pg94-test2}');
> bdr_part_by_node_names
> ------------------------
>
> (1 row)
>
> ballyhoo=# select * from bdr.bdr_nodes;
> node_sysid | node_timeline | node_dboid | node_status |
> node_name | node_local_dsn
> | node_init_from_dsn
>
>
> ---------------------+---------------+------------+-------------+------------+--------------------------------------------------------------------+-----------------------------------------------
> --------------------
> 6145084718544474516 | 1 | 16391 | r |
> pg94-test | host=pg94-test.vipaar.com port=5432 user=postgres
> dbname=ballyhoo |
> 6145083860389238141 | 1 | 16391 | i |
> pg94-test3 | host=pg94-test3.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> 6145085940943605149 | 1 | 16391 | k |
> pg94-test2 | host=pg94-test2.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> (3 rows)
>
> ballyhoo=# select * from pg_catalog.pg_replication_slots;
> slot_name | plugin | slot_type | datoid |
> database | active | xmin | catalog_xmin | restart_lsn
>
> -----------------------------------------+--------+-----------+--------+----------+--------+------+--------------+-------------
> bdr_16391_6145085940943605149_1_16391__ | bdr | logical | 16391 |
> ballyhoo | f | | 2362 | 0/21DA7C28
> bdr_16391_6145083860389238141_1_16391__ | bdr | logical | 16391 |
> ballyhoo | t | | 2363 | 0/21DA8238
> (2 rows)
>
> ballyhoo=# select bdr.bdr_get_local_node_name();
> bdr_get_local_node_name
> -------------------------
> pg94-test
> (1 row)
>
> *And the removal is confirmed on the third node:*
>
> ballyhoo=# select * from bdr.bdr_nodes;
> node_sysid | node_timeline | node_dboid | node_status |
> node_name | node_local_dsn
> | node_init_from_dsn
>
>
> ---------------------+---------------+------------+-------------+------------+--------------------------------------------------------------------+-----------------------------------------------
> --------------------
> 6145084718544474516 | 1 | 16391 | r |
> pg94-test | host=pg94-test.vipaar.com port=5432 user=postgres
> dbname=ballyhoo |
> 6145083860389238141 | 1 | 16391 | r |
> pg94-test3 | host=pg94-test3.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> 6145085940943605149 | 1 | 16391 | k |
> pg94-test2 | host=pg94-test2.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> (3 rows)
>
> ballyhoo=# select bdr.bdr_get_local_node_name();
> bdr_get_local_node_name
> -------------------------
> pg94-test3
> (1 row)
>
> ballyhoo=#
>
> *So this is done to simulate an instance where one of the nodes is down
> for so long that the accumulated WAL on the other nodes is causing them to
> run out of disk space and/or we need for the other nodes to be able to get
> a concensus for DDL, a quorum for global sequence allocation, etc. so we
> remove the node all together. Now we want to simulate adding that node back
> to the group. Since this cannot actually be done we remove all of the
> postgres data all together and initialize the node as if it were a totally
> new server an expect the other nodes to view it as such.*
>
> *So here we remove all traces of the old node from the group on the
> "primary" first:*
>
> ballyhoo=# delete from bdr.bdr_nodes where node_status='k';
> DELETE 1
> ballyhoo=# select * from bdr.bdr_nodes;
> node_sysid | node_timeline | node_dboid | node_status |
> node_name | node_local_dsn
> | node_init_from_dsn
>
>
> ---------------------+---------------+------------+-------------+------------+--------------------------------------------------------------------+-----------------------------------------------
> --------------------
> 6145084718544474516 | 1 | 16391 | r |
> pg94-test | host=pg94-test.vipaar.com port=5432 user=postgres
> dbname=ballyhoo |
> 6145083860389238141 | 1 | 16391 | i |
> pg94-test3 | host=pg94-test3.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> (2 rows)
>
> ballyhoo=# select * from pg_replication_slots;
> slot_name | plugin | slot_type | datoid |
> database | active | xmin | catalog_xmin | restart_lsn
>
> -----------------------------------------+--------+-----------+--------+----------+--------+------+--------------+-------------
> bdr_16391_6145085940943605149_1_16391__ | bdr | logical | 16391 |
> ballyhoo | f | | 2362 | 0/21DA7C28
> bdr_16391_6145083860389238141_1_16391__ | bdr | logical | 16391 |
> ballyhoo | t | | 2364 | 0/21DA8868
> (2 rows)
>
> ballyhoo=# select
> pg_drop_replication_slot('bdr_16391_6145085940943605149_1_16391__');
> pg_drop_replication_slot
> --------------------------
>
> (1 row)
>
> ballyhoo=# select * from pg_replication_slots;
> slot_name | plugin | slot_type | datoid |
> database | active | xmin | catalog_xmin | restart_lsn
>
> -----------------------------------------+--------+-----------+--------+----------+--------+------+--------------+-------------
> bdr_16391_6145083860389238141_1_16391__ | bdr | logical | 16391 |
> ballyhoo | t | | 2364 | 0/21DA8868
> (1 row)
>
> ballyhoo=# select bdr.bdr_get_local_node_name();
> bdr_get_local_node_name
> -------------------------
> pg94-test
> (1 row)
>
> *Now we check up on the other still active node:*
>
> ballyhoo=# select bdr.bdr_get_local_node_name();
> bdr_get_local_node_name
> -------------------------
> pg94-test3
> (1 row)
>
> ballyhoo=# select * from bdr.bdr_nodes;
> node_sysid | node_timeline | node_dboid | node_status |
> node_name | node_local_dsn
> | node_init_from_dsn
>
>
> ---------------------+---------------+------------+-------------+------------+--------------------------------------------------------------------+-----------------------------------------------
> --------------------
> 6145084718544474516 | 1 | 16391 | r |
> pg94-test | host=pg94-test.vipaar.com port=5432 user=postgres
> dbname=ballyhoo |
> 6145083860389238141 | 1 | 16391 | r |
> pg94-test3 | host=pg94-test3.vipaar.com port=5432 user=postgres
> dbname=ballyhoo | host=pg94-test.vipaar.com port=5432 user=postg
> res dbname=ballyhoo
> (2 rows)
>
> ballyhoo=# select * from pg_catalog.pg_replication_slots;
> slot_name | plugin | slot_type | datoid |
> database | active | xmin | catalog_xmin | restart_lsn
>
> -----------------------------------------+--------+-----------+--------+----------+--------+------+--------------+-------------
> bdr_16391_6145085940943605149_1_16391__ | bdr | logical | 16391 |
> ballyhoo | f | | 2071 | 0/21C31E30
> bdr_16391_6145084718544474516_1_16391__ | bdr | logical | 16391 |
> ballyhoo | t | | 2073 | 0/21C333A8
> (2 rows)
>
> ballyhoo=# select
> pg_drop_replication_slot('bdr_16391_6145085940943605149_1_16391__');
> pg_drop_replication_slot
> --------------------------
>
> (1 row)
>
> *As you can see it did require a little bit of clean up. The replication
> slot did have to be manually removed, but I expected this. Now we totally
> remove the data directory on the dropped node and initialize it again from
> the "primary". I use the following script for this:*
>
> #!/bin/bash
>
> SOURCE=$1
> DATADIR=/var/lib/pgsql/9.4-bdr/data
> DATABASE=$2
> NAME=$(hostname -s)
>
> if [ -z "$2" ] || [ -z "$1" ]; then
> echo "You must provide the source server, and database name."
> exit
> fi
>
> #stop postgres
> service postgres stop
>
> #Back up configs
> su - postgres -c "cp ${DATADIR}/{postgresql.conf,pg_hba.conf} ~/"
>
> #clear data dir
> rm -rf ${DATADIR}/*
>
> #init a new database dir
> su - postgres -c "initdb -D $DATADIR"
>
> #get new configs
> su - postgres -c "scp -P22299
> ${SOURCE}:${DATADIR}/{postgresql.conf,pg_hba.conf,ca.crt,vipaar.wc.cert,vipaar.wc.key}
> ${DATADIR}/"
>
> #start postgres
> service postgres start
>
> #Get dump of roles from source
> su - postgres -c "ssh -p22299 $SOURCE 'pg_dumpall --globals-only'| psql -U
> postgres"
>
> #Create extensions and group for each database we want to replicate
> psql -U postgres -h 127.0.0.1<<EOF
> create database $DATABASE;
> \c $DATABASE;
> create extension btree_gist;
> create extension bdr;
> select bdr.bdr_group_join(
> local_node_name := '$NAME',
> node_external_dsn := 'host=$(hostname) port=5432 user=postgres
> dbname=$DATABASE',
> join_using_dsn := 'host=$SOURCE port=5432 user=postgres
> dbname=$DATABASE'
> );
> SELECT bdr.bdr_node_join_wait_for_ready();
> EOF
>
> *Everything goes fine as far as importing the database. Once the import is
> finished and the newly initialized node tries to connect for replication
> the errors start. This is from the log on the server that I was recreating:*
>
> d= p=20358 a=DEBUG: 00000: syncing bdr_nodes and bdr_connections
> d= p=20358 a=LOCATION: bdr_init_replica, bdr_init_replica.c:964
> d= p=20358 a=DEBUG: 00000: dump and apply finished, preparing for catchup
> replay
> d= p=20358 a=LOCATION: bdr_init_replica, bdr_init_replica.c:970
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot DEBUG:
> 00000: received replication command: IDENTIFY_SYSTEM
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> exec_replication_command, walsender.c:1291
> d= p=20358 a=DEBUG: 00000: Creating new slot
> bdr_16391_6145097170174308100_1_16391__
> d= p=20358 a=LOCATION: bdr_establish_connection_and_slot, bdr.c:583
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot DEBUG:
> 00000: received replication command: CREATE_REPLICATION_SLOT
> "bdr_16391_6145097170174308100_1_16391__" LOGICAL bdr
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> exec_replication_command, walsender.c:1291
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot DEBUG:
> 00000: searching for logical decoding starting point, starting at
> 0/21BE1798
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> DecodingContextFindStartpoint, logical.c:469
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOG:
> 00000: logical decoding found consistent point at 0/21BE1798
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot DETAIL:
> There are no running transactions.
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> SnapBuildFindSnapshot, snapbuild.c:1271
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOG:
> 00000: exported logical decoding snapshot: "00000802-1" with 0 transaction
> IDs
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> SnapBuildExportSnapshot, snapbuild.c:604
> d= p=20358 a=DEBUG: 00000: created replication identifier 2
> d= p=20358 a=LOCATION: bdr_create_slot, bdr.c:389
> d= p=20358 a=ERROR: 55000: System identification mismatch between
> connection and slot
> d= p=20358 a=DETAIL: Connection for bdr (6145085940943605149,1,16391,)
> resulted in slot on node bdr (6145097170174308100,1,16391,) instead of
> expected node
> d= p=20358 a=LOCATION: bdr_init_make_other_slots, bdr_init_replica.c:579
> d= p=20314 a=LOG: 00000: worker process: bdr db: ballyhoo (PID 20358)
> exited with exit code 1
> d= p=20314 a=LOCATION: LogChildExit, postmaster.c:3325
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOG:
> 08006: could not receive data from client: Connection reset by peer
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> pq_recvbuf, pqcomm.c:871
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOG:
> 08006: unexpected EOF on client connection with an open transaction
> d=ballyhoo p=20371 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> SocketBackend, postgres.c:344
> d= p=20314 a=LOG: 00000: starting background worker process "bdr db:
> ballyhoo"
> d= p=20314 a=LOCATION: do_start_bgworker, postmaster.c:5403
> d= p=20372 a=NOTICE: 00000: version "1.0" of extension "btree_gist" is
> already installed
> d= p=20372 a=LOCATION: ExecAlterExtensionStmt, extension.c:2700
> d= p=20372 a=NOTICE: 00000: version "0.9.0.3" of extension "bdr" is
> already installed
> d= p=20372 a=LOCATION: ExecAlterExtensionStmt, extension.c:2700
> d= p=20372 a=DEBUG: 00000: per-db worker for node bdr
> (6145097170174308100,1,16391,) starting
> d= p=20372 a=LOCATION: bdr_perdb_worker_main, bdr_perdb.c:653
> d= p=20372 a=DEBUG: 00000: init_replica init from remote host=
> pg94-test.vipaar.com port=5432 user=postgres dbname=ballyhoo
> d= p=20372 a=LOCATION: bdr_init_replica, bdr_init_replica.c:808
> d=ballyhoo p=20373 a=bdr (6145097170174308100,1,16391,):mkslot DEBUG:
> 00000: received replication command: IDENTIFY_SYSTEM
> d=ballyhoo p=20373 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> exec_replication_command, walsender.c:1291
> d= p=20372 a=DEBUG: 00000: found valid replication identifier 2
> d= p=20372 a=LOCATION: bdr_establish_connection_and_slot, bdr.c:568
> d= p=20372 a=ERROR: 55000: System identification mismatch between
> connection and slot
> d= p=20372 a=DETAIL: Connection for bdr (6145085940943605149,1,16391,)
> resulted in slot on node bdr (6145097170174308100,1,16391,) instead of
> expected node
> d= p=20372 a=LOCATION: bdr_init_make_other_slots, bdr_init_replica.c:579
> d=ballyhoo p=20373 a=bdr (6145097170174308100,1,16391,):mkslot LOG:
> 08006: could not receive data from client: Connection reset by peer
> d=ballyhoo p=20373 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> pq_recvbuf, pqcomm.c:871
> d= p=20314 a=LOG: 00000: worker process: bdr db: ballyhoo (PID 20372)
> exited with exit code 1
> d= p=20314 a=LOCATION: LogChildExit, postmaster.c:3325
> d=ballyhoo p=20373 a=bdr (6145097170174308100,1,16391,):mkslot DEBUG:
> 08003: unexpected EOF on client connection
> d=ballyhoo p=20373 a=bdr (6145097170174308100,1,16391,):mkslot LOCATION:
> SocketBackend, postgres.c:355
> ^C
> [root(at)pg94-test2 data]#
>
> I am at a loss. What do you make of this Craig?
>
>
>
> On Sun, May 3, 2015 at 8:39 PM, Craig Ringer <craig(at)2ndquadrant(dot)com>
> wrote:
>
>>
>>
>> On 1 May 2015 at 12:40, Mathew Moon <mathew(dot)moon(at)vipaar(dot)com> wrote:
>>
>>> Hi,
>>>
>>> I have a group of 5 bdr nodes and before we can move to production with
>>> them we must demonstrate that we can remove a node from the group and add
>>> that node back later. When I remove a node it stays in the bdr.bdr_nodes
>>> view with status 'k'. If I try to add that node back the node itself errors
>>> saying that it is already part of a bdr group.
>>>
>>
>> That's intended, though the error message needs to be improved.
>>
>> You can't remove a node then add it back later. When you remove a node,
>> the remaining nodes are still generating change streams, but they aren't
>> saving them up for the removed node anymore. So if you remove a node, make
>> some changes, and add the node back then the node will have a "gap" in its
>> history, putting it out of sync with all its peers. Changes on the re-added
>> node could replicate old data to new nodes, changes from new nodes might
>> not apply on the old re-added node, etc. Worse, if any table structures
>> have changed then the node can't possibly apply changes or send changes
>> that can be applied by other nodes.
>>
>> Once a node is removed you must add a new node to replace it, you can't
>> re-add a removed node.
>>
>> There's room for improvement here, but the fundamental limitations mean
>> we're never going to support simply removing and re-adding nodes. We may be
>> able to provide a way to clean and resync a node later, but it'll be much
>> the same thing as dropdb; createdb; and rejoin.
>>
>> Note that short of removing a node, you can (a) just shut it down for a
>> while or (b) pause replay on that node using bdr.bdr_apply_pause() and
>> bdr.bdr_apply_resume(). While a node is down, other nodes will function
>> mostly normally, but will be unable to purge WAL required for replaying to
>> the down/paused node so they'll eventually run out of space in pg_xlog.
>> They will also be unable to perform DDL, because that requires consensus.
>>
>> If I totally remove the entire database from the node, deleting all of
>>> the data directory, and run initdb on the data directory again and try to
>>> add the node to the group I get errors saying that the other nodes expect
>>> this one to use its old sysid and connect to its old replication slot.
>>>
>>
>> That doesn't make sense. Odd. Can you please show the step-by-step
>> process you used to get that effect, with exact commands run,
>> exact text of error messages, etc?
>>
>>
>>> I don't understand how the other nodes are identifying this one by its
>>> old information since I removed the entire data directory and started over.
>>>
>>
>> Nor do I. When you remove the datadir you remove the only place the sysid
>> for that node is stored. Are you certain you ran the join query on the
>> newly created not-yet-joined node?
>>
>>
>>> I saw in another thread that support for removing nodes is not complete
>>> but surely there must be some way to do this even manually.
>>>
>>
>> Node remove by SQL function calls is supported in 0.9.0. The remaining
>> work centers mainly around making it more robust under load and handling
>> unexpected node loss better.
>>
>>
>>> How would one go about removing ALL traces of an existing node from all
>>> of the others so it was like it never existed before?
>>>
>>
>> Once it's confirmed removed, delete the bdr.bdr_nodes entry with status =
>> 'k'. All replication slots (pg_catalog.pg_replication_slots) should already
>> be gone.
>>
>> There should never be any reason to do this though. If you need to do it,
>> then something is already wrong. A database oid shouldn't get reused, so if
>> you dropdb and createdb you get a new node identity. The same is true if
>> you re-initdb. Since re-adding a removed node won't work, there's no reason
>> to ever remove the record of the node's existence and removal.
>>
>>
>>> Any help would be greatly appreciated. BDR is the perfect solution for
>>> our infrastructure's needs for backup and availability
>>>
>>
>> You might want to consider BDR's single-master UDR mode too, or tools
>> like Londiste. Don't add multi-master unless you really need it.
>> Significant limitations are introduced around how and when you can do DDL,
>> etc, when doing multi-master BDR, per the manual.
>>
>> --
>> Craig Ringer http://www.2ndQuadrant.com/
>> PostgreSQL Development, 24x7 Support, Training & Services
>>
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