repmgr/README.rst

=====================================================
repmgr: Replication Manager for PostgreSQL's clusters
=====================================================

Introduction
============

PostgreSQL 9.0 allow us to have replicated hot standby servers 
which we can query and/or use for high availability.

While the main components of the feature are included with
PostgreSQL, the user is expected to manage the high availability
part of it.

repmgr allows you to monitor and manage your replicated PostgreSQL
databases as a single cluster.

repmgr works in two components:

* repmgr: command program that performs tasks and then exits
* repmgrd: management and monitoring daemon that watches the cluster

Installation Outline
====================

To install and use repmgr and repmgrd follow these steps:

1. Build repmgr programs 

2. Set up trusted copy between postgres accounts, needed for the
   ``STANDBY CLONE`` step

3. Check your primary server is correctly configured

4. Write a suitable repmgr.conf for the node

5. Setup repmgrd to aid in failover transitions

Build repmgr programs
---------------------

Both methods of installation will place the binaries at the same location as your
postgres binaries, such as ``psql``.  There are two ways to build it.  The second
requires a full PostgreSQL source code tree to install the program directly into.
The first instead uses the PostgreSQL Extension System (PGXS) to install.  For
this method to work, you will need the pg_config program available in your PATH.
In some distributions of PostgreSQL, this requires installing a separate
development package in addition to the basic server software.  For example, 
the RPM packages of PostgreSQL put ``pg_config`` into the ``postgresql-devel``
package, not the main server one.

Build repmgr programs - PGXS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you are using a packaged PostgreSQL build and have ``pg_config``
available, the package can be built and installed using PGXS instead::

  tar xvzf repmgr-1.0.tar.gz
  cd repmgr
  make USE_PGXS=1
  make USE_PGXS=1 install

This is preferred to building from the ``contrib`` subdirectory of the main
source code tree.

If you need to remove the source code temporary files from this directory,
that can be done like this::

  make USE_PGXS=1 clean

Using a full source code tree
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In this method, the repmgr distribution is copied into the PostgreSQL source
code tree, assumed to be at the ${postgresql_sources} for this example.
The resulting subdirectory must be named ``contrib/repmgr``, without any
version number::

  cp repmgr.tar.gz ${postgresql_sources}/contrib
  cd ${postgresql_sources}/contrib 
  tar xvzf repmgr-1.0.tar.gz
  cd repmgr
  make
  make install

If you need to remove the source code temporary files from this directory,
that can be done like this::

  make clean

Confirm software was built correctly
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You should now find the repmgr programs available in the subdirectory where
the rest of your PostgreSQL installation is at.  You can confirm the software
is available by checking its version::

  repmgr --version
  repmgrd --version
    
Note that if you have a RPM install of PostgreSQL 9.0, the entire PostgreSQL
binary directory will not be in your PATH by default.  You may need to include
the full path of the binary instead, such as::

  /usr/pgsql-9.0/bin/repmgr --version
  /usr/pgsql-9.0/bin/repmgr --version

Set up trusted copy between postgres accounts
---------------------------------------------

Initial copy between nodes uses the rsync program running over ssh.  For this 
to work, the postgres accounts on each system need to be able to access files 
on their partner node without a password.

First generate a ssh key, using an empty passphrase, and copy the resulting 
keys and a maching authorization file to a privledged user on the other system::

  [postgres@db1]$ ssh-keygen -t rsa
  Generating public/private rsa key pair.
  Enter file in which to save the key (/var/lib/pgsql/.ssh/id_rsa): 
  Enter passphrase (empty for no passphrase): 
  Enter same passphrase again: 
  Your identification has been saved in /var/lib/pgsql/.ssh/id_rsa.
  Your public key has been saved in /var/lib/pgsql/.ssh/id_rsa.pub.
  The key fingerprint is:
  aa:bb:cc:dd:ee:ff:aa:11:22:33:44:55:66:77:88:99 postgres@db1.domain.com
  [postgres@db1]$ cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys
  [postgres@db1]$ chmod go-rwx ~/.ssh/*
  [postgres@db1]$ cd ~/.ssh
  [postgres@db1]$ scp id_rsa.pub id_rsa authorized_keys user@db2:

Login as a user on the other system, and install the files into the postgres 
user's account::

  [user@db2 ~]$ sudo chown postgres.postgres authorized_keys id_rsa.pub id_rsa
  [user@db2 ~]$ sudo mkdir -p ~postgres/.ssh
  [user@db2 ~]$ sudo chown postgres.postgres ~postgres/.ssh
  [user@db2 ~]$ sudo mv authorized_keys id_rsa.pub id_rsa ~postgres/.ssh
  [user@db2 ~]$ sudo chmod -R go-rwx ~postgres/.ssh

Now test that ssh in both directions works.  You may have to accept some new 
known hosts in the process.

CONFIGURATION FILE
==================

``repmgr.conf`` is looked for in the directory repmgrd or repmgr exists.
The configuration file should have 3 lines:

It should have these three parameters:

1. cluster: A string (single quoted) that identify the cluster we are on 

2. node: An integer that identify our node in the cluster

3. conninfo: A string (single quoted) specifying how we can connect to this node's PostgreSQL service

Commands
========

None of this commands need the ``repmgr.conf`` file but they need to be able to
connect to the remote and local database.

You can teach it which is the remote database by using the -h parameter or 
as a last parameter in standby clone and standby follow. If you need to specify
a port different then the default 5432 you can specify a -p parameter.
Standby is always considered as localhost and a second -p parameter will indicate
its port if is different from the default one.

* master register

  * Registers a master in a cluster, it needs to be executed before any node is 
    registered

* standby register

  * Registers a standby in a cluster, it needs to be executed before any repmgrd 
    is executed

* standby clone [node to be cloned] 

  * Backup via rsync the data directory of the primary. And creates the recovery file
    we need to start a new hot standby server.
    It doesn't need the repmgr.conf so it can be executed anywhere in the new node.
    So, you can step where you want your new data directory and execute::

      ./repmgr standby clone 10.68.1.161

    or from wherever you are::

     ./repmgr -D /path/to/new/data/directory standby clone 10.68.1.161

    That will make a backup of the primary then you only need to execute::

      pg_ctl -D /your_data_directory_path start


* standby promote 

  * Allows manual promotion of a specific standby into a new primary in the event of a failover
    This needs to be executed on the same directory where the repmgr.conf is in the standby or 
    then use the -f option to indicate where the repmgr.conf is.
    Doesn't need any additional arguments::

      ./repmgr standby promote

    That will restart your standby postgresql service

* standby follow 

    * Allows the standby to re-point itself to a new primary indicated as a parameter.
      This needs to be executed on the same directory where the repmgr.conf is in the standby or 
      then use the -f option to indicate where the repmgr.conf is.  Example::

        ./repmgr standby follow

Primary server configuration
============================

PostgreSQL should have been previously built and installed on the system.  Here
is a sample of changes to the postgresql.conf file::

  listen_addresses='*'
  wal_level = 'hot_standby'
  archive_mode = on
  archive_command = 'cd .'	 # we can also use exit 0, anything that just do 
                             # nothing
  max_wal_senders = 10
  wal_keep_segments = 5000     # 80 GB required on pg_xlog
  hot_standby = on

Also you need to add the machines that will participate in the cluster in 
``pg_hba.conf`` file.  One possibility is to trust all connections from the
replication users from all internal addresses, such as::

  host     all              all         192.168.1.0/24         trust
  host     replication      all         192.168.1.0/24         trust

A more secure setup adds a repmgr user and database, just giving
access to that user:

  host     repmgr           repmgr      192.168.1.0/24         trust
  host     replication      all         192.168.1.0/24         trust

If you give a password to the user, you need to create a ``.pgpass`` file for
them as well to allow automatic login.  In this case you might use the
``md5`` authentication method instead of ``trust`` for the repmgr user.

Examples
========

Suppose we have 3 nodes: node1 (the master), node2 and node3

To make node2 and node3 be standbys of node1, execute this on both nodes
(node2 and node3)::

  repmgr -D /var/lib/postgresql/9.0 standby clone node1

If we lose node1 we can run on node2::

  repmgr -f /home/postgres/repmgr.conf standby promote 

Which makes node2 the new master.  We then run on node3::

  repmgr standby follow

To make node3 follow node2 (rather than node1)

If now we want to add a new node we can a prepare a new server (node4) and run::

  repmgr -D /var/lib/postgresql/9.0 standby clone node2

NOTE: yu need to have PGDIR/bin in your path, if you don't want that as a 
permanent setting you can do it this way::

  PATH=$PGDIR/bin:$PATH repmgr standby promote

repmgr Daemon
=============

Setup
-----

To use the repmgrd (repmgr daemon) to monitor standby so we know how is going 
the replication and how far they are from primary, you need to execute the 
``repmgr.sql`` script in the postgres database.

You also need to add a row for every node in the repl_node table

Usage
-----

It reads the repmgr.conf file in current directory or as indicated with -f 
parameter looks if the standby is in repl_nodes and if it is not add it.

Before you can run the repmgr daemon (repmgrd) you need to register a master
and at least a standby in a cluster, for that you need to use the MASTER 
REGISTER and STANDBY REGISTER commands.

For example, following last example and assuming that repmgr.conf is in postgres
home directory you will run this on the master::

  repmgr -f /home/postgres/repmgr.conf master register

and the same in the standby.

The repmgr daemon creates 2 connections: one to master and other to standby.