I have three servers. One is running pgpool, another two in master-slave mode streaming replication. When installing pgpool, I was suggested to install the pgpool_regclass on my database servers as well. There's no problem installing it in the master node, but when I tried to do the same in the slave, I got error ERROR: cannot execute CREATE EXTENSION in a read-only transaction.
I think it's because the slave is a hot standby, and SELECT pg_is_in_recovery(); returns true. So I wonder am I supposed to install pgpool_regclass on the slave or not. It seems not, but pgpool doc says I should install it on every database pgpool is going to access.
I found the cause. Delete the recovery.conf file in the slave database, and then run pgpool_regclass. Otherwise, the slave is in recovery mode and cannot execute write commands.
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I am in the process of writing a bash script to upgrade Postgres servers in our org from 9.6 to 10.4, under Ubuntu trusty. I've got the master upgrade done, but we have instances in dev, staging and production with and without standby servers. I know how to upgrade a standby server, but I am struggling with a generic way to determine if a master has standby's, and if so, their hostnames. Anybody done this already and can share some light on this?
On the master, you can see the listening standby servers in pg_stat_replication:
select client_addr, client_hostname from pg_stat_replication
On the standby, you can see the master in pg_stat_wal_receiver:
select conninfo from pg_stat_wal_receiver
Just wanted to know if i'm patching my postgres database and I should not have any downtime whatsoever what should i do ? (I'm patching both my Master and Slave Databases)
If you don't mind running modified PostgreSQL servers, you can run a cluster solution like BDR or Postgres-XL.
With standard PostgreSQL, you can use streaming replication. First upgrade the standby, then manually fail over to the standby, upgrade the primary and use it as new standby server.
All these solutions require that you have a connection pooler like pgBouncer that allows you to redirect client connections between the servers.
I have tried to do database replication in linux 7.0 red hat using postgresql.
I refered this URL for Confuring http://blog.terminal.com/postgresql-replication-and-backup-methods/ I completed the the steps upto this step
Configuring the slave server
but the step
Initial replication
when I executed this command in Master
-bash-4.2$ psql -c "select pg_start_backup('initial_backup');"
I got Error Like this
ERROR: WAL level not sufficient for making an online backup
HINT: wal_level must be set to "archive" or "hot_standby" at server start.
So kindly let me know where we are wrong.
On your master PostgreSQL server's configuration file <PG_DATA>/postgresql.conf there is parameter called wal_level it should be set to either "archive" or "hot_standby"
Both said to postgres to keep WAL segmetnts for replication server. The difference between the two is rather simple:
"hot_standby" means that your replication server will allow connections for SELECT statements
"archive" means you don't want that possibility.
More to read in this tutorial on streaming replication
Also keep in mind that change of wal_level property needs server restart to take an effect.
I am very new to postgres and being new I got stuck at a point and need some help, please pardon if you find it silly.
I am doing a pgpool HA and at postgres level i have streaming replication between 3 nodes of postgresql-9.5 - 1 master and 2 slaves
I was trying to configure auto failover but when i switched back to my original master, and restarted the postgres service, I am getting the following error:
slave 1-highest timeline 1 of the primary is behind recovery timeline 11
slave 2-highest timeline 1 of the primary is behind recovery timeline 10
slave 3-highest timeline 1 of the primary is behind recovery timeline 3
I tried deleting pg_xlog files in slaves and copying all the files from master pg_xlog into the slaves and then did a rsync.
i also did a pg_rewind but it says:
target server needs to use either data checksums or wal_log_hints = on
(I have wal_log_hints = on set in postgresql.conf already)
I've tried doing a pg_basebackup but since the data base server in slaves are still starting up its not able to connect to the server
Is there any way to bring the master and the slave at a same timeline?
In my case, it happened because ( experimentally ), I updated the standby database tables and again when I simulate the master-standby streaming replication I got the same errors.
So once again I cleaned the whole standby database directory and migrate the master database using cmd like
"pg_basebackup -P -R -X stream -c fast -h 10.10.40.105 -U postgres -D standby/"
I think something is wrong in your pgpool configuration. What tool you have been using for manement of replication and master-slave control? Is it post master or repmgr?
I was trying to configure pgpool with 3 data nodes using a tutorial from http://jensd.be/591/linux/setup-a-redundant-postgresql-database-with-repmgr-and-pgpool and have done it correctly.
Also you can lean auto failover here.
(These question is obviously duplicate of this one, so I'll repeat the answer also.)
I'm not sure what you exactly mean by "when i switched back to my original master", but it looks that you are doing the wrongest possible thing in PostgreSQL streaming replication - introducing the second master.
The most important thing you should know about PostgreSQL replication is that once the failover is performed, you cannot simply "switch back to original master" - there's now a new master in cluster, and existence of two masters will make damage.
After a slave is promoted to master, the only way for you to re-join the old master is to:
Destroy it (delete the data directory);
Join it as a slave.
If you want it to be master again you'll continue with the following:
Let it run for awhile as a slave so that it can sync the data;
Kill temporary master and failover to old master;
Rejoin temporary master again as a slave.
You cannot simply switch master servers! Master can be created ONLY by failover (promoting a slave)
You should also know that whenever you are performing failover (whenever the master is changed), all slaves (except for the one that is promoted) need to be reconfigured to target the new master.
I suggest you reading this tutorial - it'll help.
I am planning to use a pgpool instance with 2 Postgresql databases only to run parallel query without replication.
Can I add a new node to pgpool without blocking the whole system?
Thanks a lot
Yes sure you can add another postgresql server. Prepare your database and the open your pgpool.conf file and add another server on it and reload your pgpool.
Reload command is: service pgpool reload