[pgpool-online-recovery] / README.md

pgpool-online-recovery
======================

This simple project aims to automate and make easy the online recovery process of a failed pgpool's backend node in master/slave mode.

This version is work-in-progress using Centos7 and upstream packages. It doesn't require psmisc package, making Centos7 minimal installation sufficient for scripts to run, since it uses systemd to manage postgresql-9.6 installed in /var/lib/pgsql/9.6/data/

Hardware configuration is 2 nodes:

10.200.1.61 edozvola-db-01
10.200.1.62 edozvola-db-02

Deployment script ./t/1-init-cluster.sh assumes that management machine from which it's run is 10.200.1.1
which is added in pg_hba.conf as authorized to be able to deploy cluster. It also assumes that management machine
has ssh access to nodes of clustetr using ssh keys or you will need to type passwords multiple times.

You can run cluster creation it with:

make init

This will destroy all databases on all nodes, archive logs, etc, so don't do this if you need your old data later.

On the other hand this will also create setup whole cluster, and you can examine it's status using:

make

If you edited local files, push changes to all nodes using:

make push

To restart pgpool (and cleanup it's state) do:

make restart

If you want to see systemd status of services just type:

make status


Now you can start './t/80-insert-test.sh' in one terminal to create insert and select load on cluster and
kill one of nodes with 'echo b > /proc/sysrq-trigger'

For example, kill slave:

ssh root@10.200.1.62 'echo b > /proc/sysrq-trigger'

pgpool should detect broken back-end and remove it.
You can verify that using just 'make' and see that one node is down.
To issue online recovery, you can use:

make fix

now, try to kill master:

ssh root@10.200.1.61 'echo b > /proc/sysrq-trigger'

FIXME: pgpool is stuck and needs to be restarted


If installing on existing streaming replication you will need to tell pgpool where current master is with:

echo 0 > /tmp/postgres_master

You can also force re-check of nodes by removing status file and restarting pgool:

rm /var/log/pgpool_status
systemctl restart pgpool



Requirements
============

There are two requirements to these scripts to work.

* The first one is [pgpool-II](http://www.pgpool.net) (v3.6.5) available for [Centos7 from upstream](http://www.pgpool.net/yum/rpms/3.6/redhat/rhel-7-x86_64/pgpool-II-pg96-3.6.5-1pgdg.rhel7.x86_64.rpm).
* The second one is obviously Postgres server (v9.6) also for [Centos7 from upstream](https://yum.postgresql.org/9.6/redhat/rhel-7-x86_64/pgdg-redhat96-9.6-3.noarch.rpm)

There are several tutorials about setting up pgpool2 and postgres servers with [Streaming Replication](http://wiki.postgresql.org/wiki/Streaming_Replication) and this readme is far to be a howto for configuring both of them.

Installation and configuration
==============================
What about the given scripts and config files ?

**pgpool.conf** : This is a sample config file for pgpool that activates master/slave mode, loadbalancing, backends health check, failover, ...

**postgresql.conf.master** : A config file for postgres master node.

**postgresql.conf.slave** : A config file for postgres slave node.

**recovery.conf** : A config file used by postgres slave for streaming replication process.

**failover.sh** : This script will be executed automatically when a pgpool's backend node (postgres node) fails down. It'll switch the standby node (slave) to master (new master).

**online-recovery.sh** : This is the bash script which you'll execute manually in order to :
* Reboot, sync and reattach slave node to pgpool if it fails.
* Setup new master and new slave, sync and reattach them to pgpool if current master fails.
This script will invoque remotely the script streaming-replication.sh (in the new slave node) to start the [online recovery process](http://www.postgresql.org/docs/8.1/static/backup-online.html) within the standby node.
PS : When a node (master or slave) fails, pgpool still running and DBs remain available. Otherwise, pgpool will detach this node for data consistancy reasons.

**streaming-replication.sh** : This script can be executed manually to synchronize a slave node with a given master node (master name/ip must be passed as argument to streaming-replication.sh). Otherwise, this same script is triggred be online-recovery.sh via ssh during failback process.

Installation
------------

The installation steps are simple. You just need to copy provided bash scripts and config files as follow.

**In pgpool node** :
* Copy pgpool.conf to /etc/pgpool-II/. This is an optional operation and in this case you have to edit the default pgpool.conf file in order to looks like the config file we provided.
* Copy failover.sh into /etc/pgpool-II/ and online-recovery.sh to same directory or another directory that will be easily accessible.

**In the master and slave postgres nodes** :
* Copy streaming-replication.sh script into /var/lib/pgsql/ (postgres homedir).
* Copy postgresql.conf.master and postgresql.conf.slave files to /var/lib/pgsql/9.6/data/.
* Finally copy recovery.conf into /var/lib/postgresql/9.1/main/.

PS : All similar old files must be backed up to be able to rollback in case of risk (e.g: cp -p /etc/pgpool-II/pgpool.conf /etc/pgpool-II/pgpool.conf.backup).
Make sure that :
- All scripts are executable and owned by the proper users. 
- /var/lib/pgsql/9.6/archive directory is created (used to archive WAL files). This folder must be owned by postgres user !
- Do not forge to edit pg_hba.conf in each postgres server to allow access to cluster's nodes.

Not enough ! It remains only the configuration steps and we'll be done :)

Configuration
-------------

To do, just follow these steps :

1- First of all make sure you have created a postgres user in pgpool node with SSH access to all Postgres nodes. All cluster's nodes have to be able to ssh each other. You can put "config" file with "StrictHostKeyChecking=no" option under .ssh/ directory of postgres user. This is a best practice (essencially when automating a bunch of operations) that allows postgres to ssh remote machine for the first time without prompting and validating Yes/No authorization question.

2- In Pgpool node set up pgpool.conf file for instance the parameters :

        # Controls various backend behavior for instance master and slave(s).
        backend_hostname0='master.foo.bar'
        backend_port0 = 5432
        backend_weight0 = 1
        backend_data_directory0 = '/var/lib/postgres/9.1/main/'
        backend_flag0 = 'ALLOW_TO_FAILOVER'
        backend_hostname1='slave.foo.bar'
        backend_port1 = 5432
        backend_weight1 = 1
        backend_data_directory1 = '/var/lib/postgres/9.1/main/'
        backend_flag1 = 'ALLOW_TO_FAILOVER'
        # Pool size
        num_init_children = 32
        max_pool = 4
        # Master/Slave and load balancing (replication mode must be off)
        load_balance_mode = on
        master_slave_mode = on
        master_slave_sub_mode = 'stream'
        #Health check (must be set up to detecte postgres server status up/down)
        health_check_period = 30
        health_check_user = 'postgres'
        health_check_password = 'postgrespass'
        # - Special commands -
        follow_master_command = 'echo %M > /tmp/postgres_master'
        # Failover command
        failover_command = '/path/to/failover.sh %d %H %P /tmp/trigger_file'

3- In failover.sh script, specify the proper ssh private key to postgres user to access new master  node via SSH.

        ssh -i /var/lib/postgresql/.ssh/id_rsa postgres@$new_master "touch $trigger_file"

4- Idem for online-recovery.sh you have juste to change if needed the postgres's private key, the rest of params is set automatically when the script runs. Magic hein ! :)

5- Change the primary_conninfo access parameters (to master) in recovery.conf file in slave side :

        primary_conninfo = 'host=master-or-slave.foo.bar port=5432 user=postgres password=nopass'

6- Rename recovery.conf to recovery.done in master side.

7- Setup postgres master node (after backup of postgresql.conf) :

        cp -p postgresql.conf.master postgresql.conf
        /etc/init.d/postgresql restart

8- Setup postgres slave node (after backup of postgresql.conf) :

        cp -p postgresql.conf.slave postgresql.conf

9- Start first slave synchronisation with master by executing streaming-replication.sh as postgres user :

        su postgres
        cd ~
        ./streaming-replication.sh master.foo.bar

10- Restart pgpool :

        /etc/init.d/pgpool2 restart

At his stage slave node is connected to master and both of them are connected to pgpool. If the master fails down, pgpool detach it from the pool and perform failover process (slave become master) automatically.

Tests
=====

Test PCP interface (as root) :

        #retrieves the node information
        pcp_node_info 10 localhost 9898 postgres "postgres-pass" "postgres-id"
        #detaches a node from pgpool
        pcp_detach_node 10 localhost 9898 postgres "postgres-pass" "postgres-id"
        #attaches a node to pgpool
        pcp_attach_node 10 localhost 9898 postgres "postgres-pass" "postgres-id"

After starting pgpool, try to test this two scenarios :

**1. When a slave fails down** :

Open pgpool log file 'journalctl -u pgpool -f'

Stop slave node 'sudo systemctl stop postgresql-9.6'

After exceeding health_check_period, you should see this log message :

        [INFO] Slave node is down. Failover not triggred !

Now, start slave failback process (as root) :

        # ./online-recovery.sh

**2. When a master fails down** :

Idem, open pgpool log file.

Stop master node '/etc/init.d/postgres stop'.

After exceeding health_check_period, you should see this log message :

        [INFO] Master node is down. Performing failover...

Start failback process (as root) to switch master(new slave) and slave(new master) roles :

        # ./online-recovery.sh