Database Replication

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  1. Database Replication in MediaWiki

Database replication is a critical component for ensuring high availability, scalability, and data security in any robust MediaWiki installation. This article aims to provide a comprehensive overview of database replication, geared towards beginners, explaining the concepts, benefits, common strategies, and practical considerations for MediaWiki administrators. We will focus primarily on the replication setup commonly used with MariaDB/MySQL, as this is the standard database backend for MediaWiki.

What is Database Replication?

At its core, database replication is the process of copying data from one database server (the *master* or *primary*) to one or more other database servers (the *slaves* or *replicas*). This creates multiple copies of the same data, distributed across different servers. Changes made to the master database are then propagated to the slaves, ensuring that all copies remain synchronized, or as close to synchronized as possible.

Think of it like making photocopies of an important document. The original is the master, and the copies are the slaves. Whenever the original document is updated, you make new copies to distribute. Database replication automates this process, making it efficient and reliable.

Why Use Database Replication with MediaWiki?

Implementing database replication for your MediaWiki installation offers numerous advantages:

  • High Availability: If the master database server fails, a slave can be quickly promoted to become the new master, minimizing downtime and ensuring continued access to your wiki. This is arguably the most important benefit. Special:MyLanguage/Server Administration details more about server maintenance.
  • Scalability: Read operations (like displaying wiki pages) can be distributed across multiple slaves, reducing the load on the master server. This is particularly useful for high-traffic wikis. Help:Contents provides information on optimizing wiki performance.
  • Data Security: Replication provides a built-in backup solution. Slaves can be geographically distributed, offering protection against regional disasters. It also allows for performing backups on the slaves without impacting the performance of the master. Refer to Manual:Backups for comprehensive backup strategies.
  • Reporting and Analytics: You can run reports and analytics queries on the slaves without impacting the performance of the master server, which needs to handle live wiki traffic. This is particularly useful for complex queries.
  • Reduced Latency: By placing slaves closer to users in different geographic locations, you can reduce latency and improve the user experience.

Replication Strategies

Several replication strategies exist, each with its own trade-offs. Here's a breakdown of the most common ones:

  • Master-Slave Replication: This is the most basic and widely used replication strategy. One server acts as the master, and all changes are written to it. These changes are then replicated to one or more slaves. It's relatively simple to set up and maintain but has a single point of failure (the master).
  • Master-Master Replication: In this setup, two servers act as masters, and each can accept writes. Changes are replicated between the two masters. This provides higher availability but introduces complexity in resolving conflicts when the same data is modified on both masters simultaneously. This is rarely used with MediaWiki due to the potential for data corruption.
  • Chain Replication: Slaves are chained together, where each slave replicates data from the previous slave. This can reduce the load on the master but introduces latency.
  • Circular Replication: Slaves replicate data to each other in a circular fashion. This provides redundancy but can also be complex to manage.
  • Semi-Synchronous Replication: The master waits for at least one slave to acknowledge receipt of the data before committing the transaction. This provides stronger consistency than asynchronous replication but can introduce some performance overhead.
  • Group Replication: (Available in newer versions of MariaDB/MySQL) This offers a more sophisticated approach, where a group of servers collectively act as a single system. It provides fault tolerance and automatic failover. Manual:Configuration details important configuration options.

For MediaWiki, **Master-Slave Replication** and **Semi-Synchronous Replication** are the most commonly recommended strategies. Group Replication is becoming increasingly popular as well, but requires a more modern database setup.

Understanding Replication Methods

Within the chosen strategy, the *method* of replication dictates *how* the changes are transmitted from the master to the slaves.

  • Statement-Based Replication (SBR): The master logs the SQL statements executed and sends those statements to the slaves. This is less reliable as it can lead to inconsistencies if the data on the master and slave differ. It’s generally discouraged.
  • Row-Based Replication (RBR): The master logs the actual row changes made by each statement and sends those changes to the slaves. This is more reliable than SBR as it doesn’t rely on the same SQL statements being executed identically on both servers. This is the recommended method for MediaWiki.
  • Mixed-Based Replication: This combines SBR and RBR, using SBR for simple statements and RBR for more complex ones. It offers a compromise between performance and reliability.
    • Row-Based Replication (RBR)** is *strongly recommended* for MediaWiki due to its reliability and ability to handle complex queries and data types.

Setting Up Master-Slave Replication (MariaDB/MySQL)

This section provides a high-level overview of setting up Master-Slave Replication. The specific steps may vary depending on your operating system and database version. Consult the official MariaDB/MySQL documentation for detailed instructions.

    • 1. Configure the Master Server:**
  • Enable binary logging: Edit the `my.cnf` (or `my.ini`) file and add the following lines under the `[mysqld]` section:
   ```
   log_bin = /var/log/mysql/mysql-bin.log
   server-id = 1  # Unique ID for the master server
   binlog_format = ROW # Use Row-Based Replication
   ```
  • Restart the MySQL/MariaDB service.
  • Create a replication user:
   ```sql
   CREATE USER 'repl'@'%' IDENTIFIED BY 'your_replication_password';
   GRANT REPLICATION SLAVE ON *.* TO 'repl'@'%';
   FLUSH PRIVILEGES;
   ```
  • Lock the tables for backup:
   ```sql
   FLUSH TABLES WITH READ LOCK;
   SHOW MASTER STATUS;
   ```

   Record the `File` and `Position` values from the `SHOW MASTER STATUS` output.  These are crucial for configuring the slave.
  • Take a database backup. This backup will be restored on the slave server.
  • Unlock the tables:
   ```sql
   UNLOCK TABLES;
   ```
    • 2. Configure the Slave Server:**
  • Edit the `my.cnf` (or `my.ini`) file and add the following lines under the `[mysqld]` section:
   ```
   server-id = 2  # Unique ID for the slave server (different from the master)
   relay-log = /var/log/mysql/mysql-relay-bin.log
   log_bin = /var/log/mysql/mysql-bin.log # optional - enables binary logging on the slave, useful for cascading replication
   binlog_format = ROW # Must match the master's binlog_format
   ```
  • Restart the MySQL/MariaDB service.
  • Restore the database backup taken from the master server.
  • Configure the replication connection:
   ```sql
   CHANGE MASTER TO
     MASTER_HOST='your_master_ip_address',
     MASTER_USER='repl',
     MASTER_PASSWORD='your_replication_password',
     MASTER_LOG_FILE='the_file_value_from_show_master_status',
     MASTER_LOG_POS=the_position_value_from_show_master_status;
   ```
  • Start the replication:
   ```sql
   START SLAVE;
   ```
  • Check the replication status:
   ```sql
   SHOW SLAVE STATUS\G
   ```

   Look for `Slave_IO_Running: Yes` and `Slave_SQL_Running: Yes`.  Also, check `Seconds_Behind_Master` to see how far behind the slave is.
    • 3. Monitoring Replication:**

Regularly monitor the replication status using `SHOW SLAVE STATUS\G`. Pay attention to:

  • `Seconds_Behind_Master`: Indicates the delay in replication. A high value suggests a performance issue.
  • `Last_IO_Error` and `Last_SQL_Error`: Indicate errors during replication. Investigate and resolve these errors immediately.
  • `Slave_IO_Running` and `Slave_SQL_Running`: Must be `Yes` for replication to be functioning correctly.

Semi-Synchronous Replication

To enable semi-synchronous replication, you'll need to modify the master configuration:

  • In `my.cnf`, add:
   ```
   plugin-load-add = semi_sync_master.so
   semi_sync_master_min_threads = 1 # Minimum number of slaves required to acknowledge
   ```
  • Restart the master.
  • On the slave, add:
   ```
   plugin-load-add = semi_sync_slave.so
   ```
  • Restart the slave.
  • Enable semi-synchronous replication on the master:
   ```sql
   UPDATE GLOBAL VARIABLES SET semi_sync_master_min_threads = 1;
   ```

Important Considerations for MediaWiki

  • **`$wgReplicationMasterHost`:** In your `LocalSettings.php` file, define the master database host using this variable. This tells MediaWiki which server to write to.
  • **`$wgReplicationLagThreshold`:** Adjust this value to control how MediaWiki handles replication lag. If the slave is behind the master by more than this threshold, MediaWiki may display a warning to users.
  • **Database Collation:** Ensure that the master and slaves use the *same* collation to avoid data inconsistencies. UTF-8 is highly recommended.
  • **Firewall Configuration:** Ensure that the slave servers can connect to the master server on the MySQL/MariaDB port (typically 3306).
  • **Regular Testing:** Regularly test the failover process to ensure that you can quickly and smoothly switch to a slave server in case of a master failure.
  • **Monitoring Tools:** Implement monitoring tools to track replication status and performance. Special:Statistics offers basic statistics, but dedicated monitoring solutions are recommended.

Advanced Topics

  • **Cascading Replication:** Slaves replicate from other slaves, reducing the load on the master.
  • **Galera Cluster:** (Part of MariaDB) Provides synchronous multi-master replication with automatic failover.
  • **Percona XtraDB Cluster:** Another popular synchronous multi-master replication solution.
  • **Database Sharding:** Dividing the database into multiple shards, each hosted on a separate server. This is a more complex solution for very large wikis.

Troubleshooting Common Issues

  • **Replication Errors:** Check the error logs on both the master and slave servers.
  • **Replication Lag:** Investigate network latency, disk I/O, and database server load.
  • **Slave Server Not Connecting:** Verify firewall settings and network connectivity.
  • **Data Inconsistencies:** Ensure that the master and slaves use the same collation and that replication is configured correctly.

External Resources

Special:MyLanguage/Manual:Configuration Help:Contents Manual:Backups Manual:Database Manual:Upgrading Manual:Performance Extension:ReplicationLag Special:Statistics Manual:FAQ Special:Search

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