===================================================
repmgr: Replication Manager for PostgreSQL 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.

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
  
See below for building notes specific to RedHat Linux variants.

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

Notes on RedHat Linux, Fedora, and CentOS Builds
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The RPM packages of PostgreSQL put ``pg_config`` into the ``postgresql-devel``
package, not the main server one.  And if you have a RPM install of PostgreSQL
9.0, the entire PostgreSQL binary directory will not be in your PATH by default
either.  Individual utilities are made available via the ``alternatives``
mechanism, but not all commands will be wrapped that way.  The files installed
by repmgr will certainly not be in the default PATH for the postgres user
on such a system.  They will instead be in /usr/pgsql-9.0/bin/ on this
type of system.

When building repmgr against a RPM packaged build, you may discover that some
development packages are needed as well.  The following build errors can
occur::

  /usr/bin/ld: cannot find -lxslt
  /usr/bin/ld: cannot find -lpam
  
Install the following packages to correct those::

  yum install libxslt-devel
  yum install pam-devel

If building repmgr as a regular user, then doing the install into the system
directories using sudo, the syntax is hard.  ``pg_config`` won't be in root's
path either.  The following recipe should work::

  sudo PATH="/usr/pgsql-9.0/bin:$PATH" make USE_PGXS=1 install

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
    
You may need to include
the full path of the binary instead, such as this RHEL example::

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

Below this binary installation base directory is referred to as PGDIR.

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.

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 does 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.

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

Command line syntax
===================

The current supported syntax for the program can be seen using::

  repmgr --help
  
The output from this program looks like this::

  repmgr: Replicator manager 
  Usage:
   repmgr [OPTIONS] master  {register}
   repmgr [OPTIONS] standby {register|clone|promote|follow}

  General options:
    --help                     show this help, then exit
    --version                  output version information, then exit
    --verbose                  output verbose activity information

  Connection options:
    -d, --dbname=DBNAME        database to connect to
    -h, --host=HOSTNAME        database server host or socket directory
    -p, --port=PORT            database server port
    -U, --username=USERNAME    database user name to connect as

  Configuration options:
    -D, --data-dir=DIR         local directory where the files will be copied to
    -f, --config_file=PATH     path to the configuration file
    -R, --remote-user=USERNAME database server username for rsync
    -w, --wal-keep-segments=VALUE  minimum value for the GUC wal_keep_segments (default: 5000)

  repmgr performs some tasks like clone a node, promote it or making follow another node and then exits.
  COMMANDS:
   master register       - registers the master in a cluster
   standby register      - registers a standby in a cluster
   standby clone [node]  - allows creation of a new standby
   standby promote       - allows manual promotion of a specific standby into a new master in the event of a failover
   standby follow        - allows the standby to re-point itself to a new master

The ``--verbose`` option can be useful in troubleshooting issues with
the program.

Commands
========

Not all of these commands need the ``repmgr.conf`` file, but they need to be able to
connect to the remote and local databases.

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

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: you 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
=============

Command line syntax
-------------------

The current supported syntax for the program can be seen using::

  repmgrd --help
  
The output from this program looks like this::

  repmgrd: Replicator manager daemon 
  Usage:
   repmgrd [OPTIONS]
  
  Options:
    --help                    show this help, then exit
    --version                 output version information, then exit
    --verbose                 output verbose activity information
    -f, --config_file=PATH    database to connect to
  
  repmgrd monitors a cluster of servers.

The ``--verbose`` option can be useful in troubleshooting issues with
the program.

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.  This work
may be done for you by the daemon itself, as described below.

Lag monitoring
--------------

To look at the current lag between primary and each node listed
in ``repl_node``, consult the repl_status view::

  psql -d postgres -c "SELECT * FROM repl_status"

This view shows the latest monitor info from every node.
 
* replication_lag: in bytes.  This is how far the latest xlog record 
  we have received is from master.

* apply_lag: in bytes.  This is how far the latest xlog record
  we have applied is from the latest record we have received.

* time_lag: in seconds.  How many seconds behind the master is this node.

Usage
-----

repmgrd 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.

Detailed walkthrough
====================

The following scenario involves two PostgreSQL installations on the same server hardware:

* A primary (master) server called “prime,” with a user as “prime,” who is also the owner of the files. This server is operating on port 5433.  This server will be known as “node1” in the cluster “test”

* A standby server called “standby,” with a user of “standby,” who is the owner of the files.  This server is operating on port 5434.  This server will be known and “node2” on the cluster “test.”
* A database exists on “prime” called “testdb.”
* The Postgress installation in each of the above is defined as $PGDATA, which represents /stripe/prime on the “prime” server and /stripe/standby on the “standby server.

Clearing the PostgreSQL installation on the Standby
---------------------------------------------------

To reenact a “new” build of a streaming replica, I take my existing test environment as described above and strip away the PostgreSQL installation on my existing replica

* Stop both servers.

* Assuming I have a terminal window open for each server, I go to “standby” and remove the PostgreSQL installation by typing::

    cd $PGDATA  (the equivalent in my installation, of typing cd /stripe/standby)
    rm -rf *

  To delete the entire PostgreSQL installation.

At this point, I have my original “prime” server with its PostgreSQL installation and repmgr installed in the ~/repmgr directory – in this specific installation that is /home /prime/repmgr

Building the standby
--------------------

We assume that repmgr is installed on “prime” in the directory: /home/prime/repmgr
There needs to be a repmgr.conf file on each server as follows:
On “prime” it should contain::

  cluster=test
  node=1
  conninfo='host=127.0.0.1 dbname=dbtest'

On “standby” it should contain::

  cluster=test
  node=2
  conninfo='host=127.0.0.1 dbname=dbtest'

Next, with “prime” server running, we want to use the clone standby command in repmgr to copy over the entire PostgreSQL installation and the database onto the “standby” server.  On the “standby” server, type::

  repmgr -D $PGDATA -p 5433 -U prime -R prime --verbose standby clone localhost

Next, we need a recovery.conf file on “standby” in the $PGDATA directory that reads as follows::

  standby_mode = 'on'
  primary_conninfo = 'host=127.0.0.1 port=5433'

Make sure that standby has a qualifying role in the database, “testdb” in this case and can login. Start psql on the testdb database on “prime” and type at the testdb#  prompt::

  CREATE ROLE standby SUPERUSER LOGIN


Registering the master and standby
----------------------------------

First, register the master by typing on “prime”::

  repmgr -f /home/prime/repmgr/repmgr.conf --verbose master register

On “standby,” edit the postgresql.conf file and change the port to 5434.

Start the “standby” server.

Then register the standby by typing on “standby”::

  repmgr -f /home/standby/repmgr/repmgr.conf --verbose standby register

Verify that one can make queries against the standby server and cannot make insertions into the standby database.

At this point, we have a functioning primary on “prime” and a functioning standby server running on “standby.”


Simulating the failure of the primary server
--------------------------------------------

To simulate the loss of the primary server, we simply stop the “prime” server.  At this point, the standby contains the database as it existed at the time of the “failure” of the primary server.


Promoting the Standby to be the Primary
---------------------------------------

We now need to promote the standby server to be the primary to allow our applications to read and write to the database by typing::

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

The server stops and restarts and now has read/write ability.

Bringing the former Primary up as a Standby
-------------------------------------------

To make the former primary act as a standby, which is necessary before restoring the original roles, type::

  repmgr -U standby -R prime -h 127.0.0.1 -p 5433 -d dbtest --force --verbose standby clone

Stop and restart the “prime” server, which is now acting as a standby server.

Make sure the record(s) inserted in step four now appear on the now standby (prime).
Make sure the database on “prime” is read-only.


Restoring the original roles of prime to primary and standby to standby
-----------------------------------------------------------------------

We restore the configuration to its original configuration by stopping the “standby” (now acting as a primary) and promoting “prime” back to be the primary server, then bringing up “standby” as a standby with a valid recovery.conf file on “standby.”

Stop the “standby” server.

Type::

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

Now the original primary, “prime” is acting again as primary.

Start the “standby” server and type this on “prime”::

  repmgr standby clone --force -h 127.0.0.1 -p 5434 -U prime -R standby --verbose

Stop the “standby” and change the port to be 5434 in the postgresql.conf file.

Verify the roles have reversed by attempting to insert a record on “standby” and on “prime.”

The servers are now again acting as primary on “prime” and standby on “standby.”