Difference between revisions of "Hot Standby Host"

Latest revision as of 15:11, 21 February 2018

Configuring Rivendell Hot Standby Hosts on SuSE

Originally based on APPNOTE 0003 by Fred Gleason

SCOPE

This application note details the procedures for configuring one or more host systems to act as a 'hot standby' Rivendell server. Such a 'hot standby' system can be used to quickly restore a Rivendell network in the event of failure in the primary server.

DEFINITION OF TERMS

Master

The normal mode of operation for a Rivendell host--client or server--configured with the command 'rd_config master'.

Standby

The Hot Standby mode of operation for a Rivendell client configured with the command 'rd_config standby'. Operating in standby mode, all audio and MySQL data is located locally.

Hot Standby

A Rivendell host operating as a client which has been configured to backup both audio and MySQL data on a Rivendell server.

PREREQUISITES

This procedure assumes that Rivendell is already properly installed on both the master and any standby host(s). This includes proper mySQL installation and configuration. For further information on basic Rivendell installation, see SRL Application Note #1.

Ideally, the versions of mySQL being employed on the master and any standby system(s) should be identical, as data replication between different versions is not supported for all combinations. For more information regarding cross-version compatibility, consult the mySQL documentation.

OVERVIEW

It is possible to configure Rivendell so as to have two alternative configurations, called 'Master' and 'Standby' Each configuration can use a different mySQL server and audio storage location (either local or connected via the network), thus allowing a backup configuration that can be quickly activated in the event of a problem with the Master setup. It is also possible to configure a standby system so that it is automatically kept up to date with any changes (configuration changes, new recordings, etc) made to the Master system. These capabilities are collectively referred to a 'hot standby' capability.

Configuring hot standby for one or more Rivendell hosts consists of three basic steps:

1. Enabling database replication
2. Enabling audio replication
3. Setting up the master and standby configurations.

Package Requirements In addition to the standard Rivendell dependencies, both the Master Server and any standby hosts will need the 'rsync' package installed. The package is included in SuSE, or can be downloaded from http://rsync.samba.org/

CONFIGURING DATABASE REPLICATION

Preparing the Master Server

On the master server, copy the 'my.cnf-master' file (from the Rivendell distribution) to '/etc/my.cnf'. This file can be found in the 'conf/' directory of the Rivendell sources, or in the package documentation directory ('/usr/share/doc/packages/rivendell') if an RPM distribution is being used.

Next, create a mySQL user account called 'repl' with rights to connect from each hot standby host. This account should have RELOAD, REPLICATION SLAVE, REPLICATION CLIENT and SUPER rights enabled, and no others.

  shell> mysql -u root -p
  mysql> GRANT RELOAD, REPLICATION SLAVE, REPLICATION CLIENT, SUPER
     ->     ON *.*
     ->     TO 'repl'@'localhost'
     ->     IDENTIFIED BY '<password>';
  mysql> GRANT RELOAD, REPLICATION SLAVE, REPLICATION CLIENT, SUPER
     ->     ON *.*
     ->     TO 'repl'@'%.my.domain'
     ->     IDENTIFIED BY '<password>';
  mysql> quit

Finally, shutdown and then restart the mySQL server by doing (as root):

/etc/init.d/mysql stop
/etc/init.d/mysql start

Generating The Initial Data Snapshot

Before starting replication, it is necessary to generate an initial “snapshot” of data on the master server for importation into the slave. (IMPORTANT NOTE: this procedure will lock the Rivendell database while it is being performed, effectively taking it off-line. Plan accordingly!) To generate the snapshot, proceed as follows:

On the master mySQL server , use the mySQL monitor program to log into mySQL with an administrator account, using a command such as:

mysql -u root -p

Once logged in, enter the following command:

FLUSH TABLES WITH READ LOCK;

Next, open up a root shell and make a copy of all the files in the '/var/lib/mysql' directory, by doing:

cd /var/lib
tar -zcf snapshot.tar.gz mysql

You should now have a file called 'snapshot.tar.gz', containing a snapshot of the server data. Next, enter the following command in the mySQL monitor:

SHOW MASTER STATUS;

You should see output similar to the following:

+-------------------+-----------+--------------+------------------+
| File              | Position  | Binlog_Do_DB | Binlog_Ignore_DB |
+-------------------+-----------+--------------+------------------+
| rdserv-bin.000001 | 269414518 |              |                  |
+-------------------+-----------+--------------+------------------+

Note down the values of the 'File' and 'Position' fields. You will need these when starting replication on the slave system(s).

Finally, enter the following in the mySQL monitor:

UNLOCK TABLES;

Enter 'quit' to exit the monitor, and you're done creating the snapshot.

Preparing the Standby Host

(IMPORTANT NOTE: this procedure will completely destroy any previously existing mySQL data on the slave system!). On the standby host, copy the 'my.cnf-standby' file (from the Rivendell distribution) to '/etc/my.cnf'. This file can be found in the 'conf/' directory of the Rivendell sources, or in the package documentation directory ('/usr/share/doc/packages/rivendell') if an RPM distribution is being used. Open the '/etc/my.cnf' in a text editor and set the following values appropriately for your site:

server-id = <id-num>

where <id-num> is a site unique number that identifies this mySQL server. By convention, the master Rivendell server gets a server-id of '1', and standby hosts are sequentially numbered, starting with '100'.

master-host = <hostname> 

where <hostname> is the hostname of the master Rivendell server.

master-user = <username>

where <username> is the name of the account on the master server to use for replication. Normally, this is the repl account.

master-password = <password>

where <password> is the password for accessing the replication account on the master server.

Next, populate the snapshot data from the master system. Copy the snapshot file you created on the master to the standby system, then open a root shell on the standby system and do the following:

cd /var/lib
tar -zvxf /path/to/snapshot/file/snapshot.tar.gz

Finally, restart the mySQL server by doing:

/etc/init.d/mysql start

Starting Replication

To actually start replication, log into a mySQL account with administrative rights on the standby host (using the mysql -u root -p command) and issue the following commands:

STOP SLAVE;
CHANGE MASTER TO
MASTER_HOST=”<hostname>”,
MASTER_USER=”repl”,
MASTER_PASSWORD=”<password>”,
MASTER_LOG_FILE=”<filename>”,
MASTER_LOG_POS=<position>;

where <hostname> and <password> are the hostname and mySQL password of the master system and <filename> and <position> are the values of the File and Position fields reported by the SHOW_MASTER_STATUS command on the master.

Next, enter the following command:

START SLAVE;

Each of these commands should return 'Query OK'. If errors occur, consult the mySQL log (at '/var/lib/mysql/mysql.log') for clues as what could be going wrong. Replication should now be operational.

For more information about MySQL data replication along with instructions for converting a slave to a master, go here http://dev.mysql.com/doc/refman/5.0/en/replication-faq.html

CONFIGURING AUDIO REPLICATION

Preparing the Master Server Configure an rsync module for the Rivendell audio archive by inserting the following into the '/etc/rsyncd.conf' file:

[rivendell]
 path = /var/snd
 comment = Rivendell Audio Archive
 read only = true

In addition, make sure that the 'hosts allow=' directive in '/etc/rsyncd.conf' is set so as to allow the standby host(s) to connect. See the rsyncd.conf(5) man page for more details. Next, open Yast2, click on System and then click on Runlevel Editor. Select the rsyncd service and the click Enable, then Finish.

Next, create the replication check file by doing the following as the Rivendell user:

touch /var/snd/repl.chk

The audio replication code will check to see that this file exists each time before replicating audio. This is to prevent catastrophes with mirror deletes in the event of a failure of the storage device containing the primary archive.

Preparing the Standby Host Create a '/var/snd.standby' directory with enough free space to be able to contain all of the audio data from the master server, and make sure that it is owned as well as readable and writable by the Rivendell user and group (normally, user 'rd' and group 'users').

Run the initial audio synchronization by entering the following command as the Rivendell user:

rsync -av --delete --progress <hostname>::rivendell/*.wav /var/snd.standby

where <hostname> is the name of the master server. Since this is the initial synchronization, this command may take a significant amount of time to complete, as the entire audio archive must be transferred.

Enabling Audio Replication Add the following cron job for the Rivendell user (using the crontab -e command):

0 * * * * /usr/local/bin/rd_audio_sync <hostname>

where <hostname> is the name of the master server. This will cause the archive to be synchronized once per hour. Some sites may want synchronization to occur more frequently. This can be accomplished by adjusting the parameters of the cron job – see the crontab(5) man page for details.

SETTING UP CONFIGURATIONS

Stop Daemons Before proceeding with the rest of this procedure, stop the Rivendell daemons on the standby host by doing (as root):

/etc/init.d/rivendell stop

Database Configurations Prepare two 'rd.conf' files, one each for your 'Master' and 'Standby' setup. The master file should be saved as '/etc/rd.conf.master', while the standby file should be saved as '/etc/rd.conf.standby'. Be sure to back up anything in the original '/etc/rd.conf' file, as it's contents will be overwritten.

Audio Directories If you haven't already, create two directories, called '/var/snd.master' and '/var/snd.standby'. They should both be owned by the Rivendell user, as well as readable and writtable by that user. Move the contents of the original '/var/snd' directory to '/var/snd.master', and then delete '/var/snd'.

Start It Up

To get around the script limitations, you need to either edit the script before the following or manually set a couple of things up so rivendell will start and be running before the following. The rd_config script assumes (and requires) the rivendell deamons to be running before it will run).

I edited the /usr/local/bin/rd_config script and commented out the following lines for the first run:

   if [ $? -ne 0 ] ; then
     echo "rd_config: can't stop current config, aborting"
     exit 1
   fi

To generate the initial master configuration, enter the following (as root):

rd_config master

This will generate the appropriate symbolic links and then start up the Rivendell daemons.

After this is running the first time, you can un-comment the lines above to reactivate the check in the script.

SWITCHING BETWEEN CONFIGURATIONS

To switch between the Master and Standby configurations, all you need to do is:

Close any open Rivendell programs.

Do the following command (as root):

rd_config <config-name>

where <config-name> is either master or standby. This command will shut down the Rivendell daemons, regenerate the appropriate sym links for the selected configuration and then restart the daemons. IMPORTANT NOTE: running this command will cause any active audio operations (playout or recording) to be interrupted.