Previously, we discussed how to set up MariaDB Galera cluster on Ubuntu and how to encrypt replication traffic in Galera cluster. In this tutorial, we will be learning how to add an asynchronous replication slave to Galera cluster, which means the Galera cluster will be acting as a master.

Galera Cluster As a Master

Keep in mind that replication is not a replacement for backup. Although there are multiple nodes in Galera cluster holding a copy of your database, if a DROP DATABASE command is accidentally run on one of the nodes in the cluster, all other nodes will drop the database. We are going to set up master-slave replication and the Galera cluster will be acting a master. You will need another server acting as slave. Once the setup is done, you can take backups on the slave using mysqldump and the workload in Galera cluster won’t be interrupted when a backup is created.

Hint: Galera cluster uses synchronous replication between the nodes, whereas the traditional MariaDB master-slave replication uses asynchronous replication between master and slave.

Setting up master-slave replication with Galera cluster can be done in 5 steps:

1. Configure binary log and replication on Galera master
2. Enable relay log and replication on the slave
3. Dump databases on the Galera master and import them into the slave
4. (optional) Enable TLS encryption
5. Connect the slave to Galera master

The overall steps are similar to setting up a traditional master-slave replication, but you need to adjust some settings for the Galera master.

Prerequisites

You should have already set up a Galera cluster with at least 3 nodes and you need to prepare another server as slave.

Ideally, the Galera master and slave should use the same MariaDB version. Replication between different MariaDB versions may cause problems. If you run MariaDB database server on Ubuntu, then you should either install MariaDB from Ubuntu repository on all the servers, or install the latest MariaDB version from MariaDB.org repository on all the servers.

When following the steps below, you should apply master configuraions on all nodes in Galera cluster, so if one node goes down, you can quickly configure the slave to replicate from another node.

Step 1: Configure Binary Log and Replication on Galera Master

Master-slave replication is based on the binary log. You must enable binary logging on the master server in order for replication to work. The Galera nodes have already enabled binary logging in row format. (Galera only supports row-based binary log format.) We need to edit some other parameters, so open the MariaDB main configuration file.

sudo nano /etc/mysql/mariadb.conf.d/50-server.cnf

or

sudo nano /etc/mysql/my.cnf

Add the following lines in the [mysqld] unit.

# Galera node as master
wsrep_gtid_mode      = on
wsrep_gtid_domain_id = 0
server-id            = 01
log_slave_updates    = on
log-bin              = /var/log/mysql/master-bin 
log-bin-index        = /var/log/mysql/master-bin.index
gtid_domain_id       = 1

In the above configuration,

• we enabled GTID mode for write-set replication.
• wsrep_gtid_domain_id and server-id need to set to the same value on all nodes in the cluster.
• By default, write-set replication in Galera are not written to binary log. In order for a node in Galera cluster to replicate write-sets to an asynchronous slave, log-slave-updates must be enabled on the Galera master. If this isn’t enabled, then changes replicated from another node in the cluster won’t be replicated to the asynchronous slave. It’s recommended that you enable log-slave-updates on all nodes in the cluster. If one node in the cluster goes offline, you can configure the slave to replicate from another node in the cluster.
• The binary log files need to be set to the same path on all nodes in the cluster.
• gitd_domain_id should be set to a different value on each node in Galera cluster and the value should be different from the value of wsrep_gtid_domain_id.

Save and close the file. After you apply this configuration on all nodes in Galera cluster, you need to restart the whole cluster for the changes to take effect. To restart the cluster, you need to shut down all MariaDB server one at a time.

sudo systemctl stop mariadb

Then run the following command on the last node that leaves the cluster to bootstrap the cluster.

sudo galera_new_cluster

Next, you need to start MariaDB server on other nodes one at a time.

sudo systemctl start mariadb

When the master node sits on the public Internet, it’s recommended to restrict access to port 3306 (default MariaDB port). For example, you can use UFW to create an IP address whitelist, allowing only the slave’s IP addresses to connect to port 3306.

sudo ufw insert 1 allow in from IP_address_of_slave to any port 3306

For how to use UFW, please see the following article:

• Getting started with UFW firwall on Debian, Ubuntu, Linux Mint server

After that, we need to add an replication user on the master server. The slave server will use this user to remotely log into master server and request binary logs. Log into MariaDB monitor.

sudo mysql -u root

Then create a user and grant replication slave privilege to this user. Replace the red text with your preferred username and password.

create user 'replicant'@'%' identified by 'replicant_password';

grant replication slave on *.* to replicant;

If the slave is going to connect to the master over the public Internet, it’s a good practice to enforce TLS encryption.

grant replication slave on *.* to replicant require ssl;

Then flush the privilege table.

flush privileges;

This database user will be replicated to other nodes in the cluster, so you don’t need to create this user again on the other nodes.

Step 2: Enable Relay Log and Replication on the Slave

Open the main MariaDB configuration file on the slave.

sudo nano /etc/mysql/mariadb.conf.d/50-server.cnf

or

sudo nano /etc/mysql/my.cnf

Find the Logging and Replication section in [mysqld] unit and Add the following lines.

#Replication slave to Galera Cluster
server-id       = 02
relay-log-index = /var/log/mysql/slave-relay-bin.index
relay-log       = /var/log/mysql/slave-relay-bin
gtid_domain_id  = 99

log-bin         = /var/log/mysql/slave-bin
log-bin-index   = /var/log/mysql/slave-bin.index
binlog_format   = mixed

In the above configuration:

• We need to set the server_id to a value different than the one on Galera master.
• The relay log needs to be enabled for master-slave replication.
• The value of gtid_domain_id on slave should be different than the value of wsrep_gtid_domain_id and gtid_domain_id on Galera master.
• Master-slave replication does not require binary logging on the slave, but if you are going to take backups on the slave using mysqldump, then you need to enable binary logging. The format of the binary log can be different than the format used on Galera master.

You can also use replication filters like below to replicate specific database. (Note that replication filter should be used on the slave, not on the Gelara master.)

replicate-do-db=db1_name
replicate-do-db=db2_name

If the slave is going to act a master of another slave, add the following line as well.

log_slave_updates = ON

Save and close the file. Then restart the slave MariaDB server for the changes to take effect.

sudo systemctl restart mariadb

Sometimes MariaDB may fail to restart. Run systemctl status mariadb to check the status.

Step 3: Copy Database From the Galera Master to the Slave

Choose one Galera node as the master and log into the master MariaDB monitor. Run the following command to prevent any further changes to databases.

flush tables with read lock;

This master node will become desynced from the rest of the cluster, which can continue running as normal. It will sync with other nodes again when we unlock the tables later. Also note that if this master node is serving a website, the above command can cause the website go offline.

Then obtain the binary log GTID (global transaction ID) position with the following command.

show variables like 'gtid_binlog_pos';

Do NOT exit MariaDB monitor yet, because exiting it now will release the lock. Now open another terminal window and SSH into your master server. Use mysqldump utility to dump the database to a .sql file.

sudo mysqldump -u root database_name > database_name.sql

Your can obtain the database name by running the following command at the MariaDB monitor.

show databases;

You can also dump all databases with the following command:

sudo mysqldump -u root --all-databases > all-databases.sql

After the database is dumped to disk, you can unlock tables on master server by running the following command at the MariaDB monitor.

unlock tables;

Use the scp command or whatever method you prefer to copy this SQL file to your slave server.

To import the single database, log into the slave MariaDB monitor.

sudo mysql -u root

Create a blank database with the same name.

create database database_name;

Exit MariaDB monitor.

exit;

Import the database into the slave MariaDB server with the following command.

sudo mysql -u root database_name < database_name.sql

If you are going to enable replication on all databases, then you can import all databases to slave MariaDB server with the following command:

sudo mysql -u root < all-databases.sql

To keep data consistent with the master, it is advisable to enable read-only mode on the slave. Replication will work as usual in read-only mode. Open the 50-server.cnf file on the slave.

sudo nano /etc/mysql/mariadb.conf.d/50-server.cnf

Add the following line in [mysqld] unit to enable read-only mode.

read-only = 1

Users with SUPER privilege (like root) can still write to the database, so you should be careful when granting privileges to users. If you don’t want anyone to be able to change/delete the database, you can add the following line in [mysqld] unit.

innodb-read-only = 1

Save and close the file. Then restart MariaDB for the change to take effect.

sudo systemctl restart mariadb

Step 4: Enabling TLS Encryption on Gelera Master

Note: If the slave and master are in a private network, your don’t have to do this step. Skip to step 5.

If the slave is going to connect to the master over the public Internet, it is necessary to enable TLS encryption to prevent traffic snooping. Your server may have a web server with TLS enabled, so you can use that TLS certificate for MariaDB as well. For example, I have enabled TLS in my Nginx web server with Let’s Encrypt. To enable TLS in MariaDB, open the 50-server.cnf file.

sudo nano /etc/mysql/mariadb.conf.d/50-server.cnf

Find the Security Features section in [mysqld] unit. Add the following lines:

ssl-ca = /etc/letsencrypt/live/www.linuxbabe.com/chain.pem
ssl-cert = /etc/letsencrypt/live/www.linuxbabe.com/cert.pem
ssl-key = /etc/letsencrypt/live/www.linuxbabe.com/privkey.pem

Save and close the file. The mysql user needs permission to access the above SSL files, so you need to grant read permission with the following commands.

sudo setfacl -R -m "u:mysql:rx" /etc/letsencrypt/archive/

sudo setfacl -R -m "u:mysql:rx" /etc/letsencrypt/live/

Then restart the master node for the changes to take effect.

sudo systemctl restart mariadb

Your may be wondering why you don’t need to grant the www-data user read permission of the SSL files. That’s because Apache or Nginx has a master process running as root user. However, all MariaDB processes are running as mysql user.

After MariaDB restarts, log into MariaDB monitor and run the following command to check if SSL is successfully enabled.

show global variables like "have_ssl";

If the value is “Yes”, then SSL is enabled.

If the value is “DISABLED”, that means there are something wrong in your SSL configuration. Check the MariaDB error log (/var/log/mysql/error.log) to find the reason.

The MariaDB server binary from Debian/Ubuntu repository is statically linked with MariaDB’s bundled YaSSL library. The MariaDB binary from the MariaDB.org repository is dynamically linked with the system’s TLS library, usually OpenSSL. You can log into MariaDB monitor and run the following command to check which SSL library your MariaDB server is using.

show variables like "version_ssl_library";

If you would like to use TLS 1.3 in MariaDB, then you need to install MariaDB from MariaDB.org repository and install OpenSSL 1.1.1 on your Ubuntu system.

You can test TLS connection by logging in from the slave with the following command. --ssl option enforces secure connection.

mysql -h Master_IP_Address -u replicant -p --ssl

Once you are logged in, run

status;

In the output, you can see the SSL cipher in use.

Step 5: Connect the Slave to the Master

We are going to use GTID-based replication. Log into the slave MariaDB monitor and run the following command to set the gtid_slave_pos variable. Its value should be the same as the gtid_binlog_pos variable on the master.

MariaDB [(none)]> set global gtid_slave_pos = "0-1-13";

Then run the following command on the MariaDB monitor to create a connection profile.

change master 'galera01' to
    master_host='master_IP_address',
    master_user='replicant',
    master_password='replicant_password',
    master_port=3306,
    master_connect_retry=10,
    master_use_gtid=slave_pos,
    master_ssl=1;

In the first line, the connection name is set to galera01. master_use_gtid=slave_pos enables GTID (Global Transaction ID) in MariaDB replication, so you don’t need to use the old-style master_log_file and master_log_pos any more.

The last line enforces TLS encryption. If the master and slave are in a secure private network, then you don’t have to enforce TLS encryption, so you can remove the last line. Notice that the last line ends with a semicolon.

Then start this connection.

MariaDB [(none)]> start slave 'galera01';

Check the status.

MariaDB [(none)]> show slave 'galera01' status\G;

If you see no errors in the output, that means replication is running smoothly. You should see the following two “Yes” indicating everything is going well. If one of them is not “Yes”, then something is wrong.

 Slave_IO_Running: Yes
 Slave_SQL_Running: Yes

Now if you make a change in your master database server, it will be replicated to the slave database server. The slave I/O thread connects to the master and requests binary log. If there are new transactions in the binary log, the slave I/O thread writes them to the relay log on the slave server. Then the slave SQL thread reads the relay log and executes new transactions in the database.

To stop replication, run:

MariaDB [(none)]> stop slave 'galera01';

If you want the replication to restart from a clean state, you can reset the replication.

MariaDB [(none)]> reset slave 'galera01';

By default, when MariaDB server restarts, it resumes all stopped replication tasks.

TroubleShooting

Slave_IO_Running: Connecting

If you have the Slave_IO_Running: Connecting status,  you can try logging into the Master server from the slave host as the replicant user.

mysql -h master_server_ip -u replicant -p

If you can’t log in, then you have a problem. Maybe it’s because you are blocked by the firewall on the Master server.

Last_SQL_Error: Error ‘Duplicate entry

The first time you check the slave status, you might see the following error.

Last_SQL_Error: Error 'Duplicate entry

This usually happens when the slave connects to the master the first time. If an error occurs, the replication will stop. We can skip this duplicate entry error by adding the following two line in [mysqld] unit in the MariaDB server configuration file (50-server.conf).

slave-skip-errors=1062
skip-slave-start

Restart MariaDB. Then start the slave replication again.

MariaDB [(none)]> start slave 'galera01';

Check the status.

MariaDB [(none)]> show slave 'galera01' status\G;

After a few moments, the Seconds_Behind_Master in the status output will get to zero. After that, you can remove the two lines in [mysqld] and restart MariaDB.

If there are other errors, you can add the error code like below.

slave-skip-errors = 1062, 1032

It’s recommended that you only use the slave-skip-errors option when you first start the replication. Use this option later could cause data inconsistency between master and slave.

Check if Replication is Working

First, change some data on the master, then run the following command on the master MariaDB monitor.

MariaDB [(none)]> show variables like 'gtid_binlog_pos';

Next, on the slave MariaDB monitor, run the following command.

MariaDB [(none)]> show variables like 'gtid_slave_pos';

If the two values are the same, then data changes on the master is replicated to the slave. You should change some data on each Galera node (e.g. create an empty database or table) to see if they are all replicated to the slave.

Changing the Master Node

If the Galera master node goes offline, you can quickly configure the slave to replicate from another Galera node. First, stop the current slave.

MariaDB [(none)]> stop slave 'galera01';

Then you need to change the IP address to another Galera node in the connection profile.

MariaDB [(none)]> change master 'galera01' to
    -> master_host='IP_address_of_another_Galera_node',
    -> master_user='replicant',
    -> master_password='replicant_password',
    -> master_port=3306,
    -> master_connect_retry=10,
    -> master_use_gtid=slave_pos,
    -> master_ssl=1;

Then start the slave.

MariaDB [(none)]> start slave 'galera01';

Because we are using global transaction ID and all Galera nodes share the same gtid_binlog_pos at a given time (Recall that we set wsrep_gtid_domain_id and server-id  to the same value on all nodes in the cluster), the slave can easily determine where to resume replication on the new master.

Set Up Email Alerts

It’s important that you keep a tab on the replication health status. If the slave hasn’t been able to replicate from the master for a long time, then you might not be able to restart the replication as the old binary log files on the master server are automatically deleted. In this case, you need to dump the database from the Galera master and copy it to the slave again, and start it over.

It’s a good idea to set up email alerts when replication fails so you can fix the problem in a timely manner.

Create a shell script under the root user directory.

sudo nano /root/replication-alert.sh

Add the following lines to this file. Replace you@example.com with your real email address.

#!/bin/bash

# Get the replication status
/usr/bin/mysql -u root -Bse "show slave 'galera01' status\G;" > /root/show-slave-status.txt

#Install Mutt mail client
apt-get install mutt -y > /dev/null

# If replication failed, send notification email. 
if ! grep -q "Slave_IO_Running: Yes" /root/show-slave-status.txt; then
        cat /root/show-slave-status.txt | mutt -s "MariaDB Replication - Slave IO Stoped" -- you@example.com
fi

if ! grep -q "Slave_SQL_Running: Yes" /root/show-slave-status.txt; then
        cat /root/show-slave-status.txt | mutt -s "MariaDB Replication - Slave SQL Stoped" -- you@example.com
fi

exit

Save and close the file. Then make it executable.

sudo chmod +x /root/replication-alert.sh

Next, create a Cron job to run this script periodically.

sudo crontab -e

Add the following line to execute this script every hour.

@hourly bash /root/replication-alert.sh

Save and close the crontab file.

Source : https://www.linuxbabe.com/mariadb/master-slave-replication-galera-cluster

Posted on: June 28, 2022, by : Julian's | 169 views