Database replication

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  1. Database Replication in MediaWiki

Database replication is a crucial aspect of maintaining a highly available, scalable, and reliable MediaWiki installation. It’s the process of copying data from one database server (the *master*) to one or more other database servers (the *slaves*). This article will provide a comprehensive overview of database replication in the context of MediaWiki, targeted at beginners, covering the benefits, different types, setup considerations, and common troubleshooting steps. We will focus primarily on MySQL replication, as it is the most commonly used database system with MediaWiki.

Why Use Database Replication?

Several compelling reasons drive the implementation of database replication for MediaWiki deployments:

  • High Availability:* If the master database server fails, one of the slave servers can be quickly promoted to become the new master, minimizing downtime and ensuring continuous operation of the wiki. This is crucial for wikis with a large user base or critical information. Consider the impact of downtime on Special:Statistics page views.
  • Scalability:* Read operations (such as displaying pages) can be distributed across multiple slave servers, reducing the load on the master server and improving overall performance. This is particularly important for wikis experiencing high traffic. Analyzing Help:Contents usage would highlight read-heavy operations.
  • Data Redundancy:* Replication provides a backup of the database, protecting against data loss due to hardware failures, software errors, or human mistakes. While not a replacement for regular backups, it offers an additional layer of protection. Regularly reviewing Manual:Backups is still essential.
  • Reporting and Analytics:* Data can be replicated to a separate server dedicated to reporting and analytics, without impacting the performance of the live wiki. This allows for complex queries and data analysis without slowing down the primary wiki site. Understanding Special:Search query patterns can inform reporting needs.
  • Geographic Distribution:* Slaves can be located in different geographic regions, providing faster access to the wiki for users in those regions and improving overall responsiveness.

Types of Database Replication

There are several different types of database replication, each with its own advantages and disadvantages:

  • Master-Slave Replication:* This is the most common type of replication. One server is designated as the master, and all writes are performed on the master. Changes are then replicated to one or more slave servers. Slaves can only read from the master. This is readily implemented using MySQL's built-in replication features.
  • Master-Master Replication:* In this configuration, two or more servers act as masters, and writes can be performed on any of them. This is more complex to set up and manage, as it requires conflict resolution mechanisms to handle situations where the same data is modified on multiple masters simultaneously. Generally not recommended for MediaWiki.
  • Semi-Synchronous Replication:* A hybrid approach where the master waits for at least one slave to acknowledge receipt of the transaction before committing it. This provides a higher level of data consistency than asynchronous replication but can introduce some performance overhead.
  • Asynchronous Replication:* The master does not wait for any acknowledgement from the slaves before committing transactions. This offers the best performance but has the risk of data loss if the master fails before the changes are replicated to the slaves. This is the default for standard MySQL replication.
  • Group Replication:* A more advanced form of replication where a group of servers work together to provide a highly available and consistent database. This is more complex to set up and manage but offers the highest level of reliability.

For most MediaWiki installations, **Master-Slave replication with asynchronous replication** is the most practical and cost-effective solution. It provides a good balance between performance, reliability, and ease of management.

Setting up Master-Slave Replication (MySQL)

The following steps outline the general process for setting up master-slave replication in a MySQL environment used by MediaWiki. *Always back up your database before making any changes!* Refer to Manual:Configuration for recommended backup procedures.

1. Configure the Master Server:

  * Enable binary logging:  Edit the `my.cnf` file and add or modify the following lines:
    ```
    log_bin = mysql-bin
    server-id = 1  # Unique ID for the master server
    binlog_format = ROW # Recommended for MediaWiki
    ```
  * Restart the MySQL server.
  * Create a replication user:
    ```sql
    CREATE USER 'repl'@'%' IDENTIFIED BY 'your_password';
    GRANT REPLICATION SLAVE ON *.* TO 'repl'@'%';
    FLUSH PRIVILEGES;
    ```

2. Configure the Slave Server:

  * Edit the `my.cnf` file and add or modify the following lines:
    ```
    server-id = 2  # Unique ID for the slave server (different from the master)
    relay-log = mysql-relay-bin
    log_slave_updates = 1 # Optional: logs updates received from master
    ```
  * Restart the MySQL server.

3. Take a Database Dump from the Master:

  * Use `mysqldump` to create a consistent snapshot of the database:
    ```bash
    mysqldump -u root -p --all-databases --single-transaction --flush-logs --master-data=2 > backup.sql
    ```
    The `--master-data=2` option includes the binary log file name and position, which are needed for the slave to start replicating from the correct point.

4. Restore the Database Dump on the Slave:

  * Copy the `backup.sql` file to the slave server.
  * Import the dump using:
    ```bash
    mysql -u root -p < backup.sql
    ```

5. Configure the Slave to Connect to the Master:

  * Connect to the MySQL server on the slave.
  * Execute the following command, replacing the placeholders with the correct values:
    ```sql
    CHANGE MASTER TO
      MASTER_HOST='master_host_ip',
      MASTER_USER='repl',
      MASTER_PASSWORD='your_password',
      MASTER_LOG_FILE='binary_log_file_name',  # From the mysqldump output
      MASTER_LOG_POS=binary_log_position;       # From the mysqldump output
    ```

6. Start Replication on the Slave:

  * Execute the following command:
    ```sql
    START SLAVE;
    ```

7. Verify Replication:

  * On the slave, execute:
    ```sql
    SHOW SLAVE STATUS\G
    ```
  * Check the output for the following:
    * `Slave_IO_Running: Yes`
    * `Slave_SQL_Running: Yes`
    * `Seconds_Behind_Master: 0` (or a small number, indicating that the slave is keeping up with the master)

Important Considerations for MediaWiki Replication

  • `$wgReplicationMaster` in LocalSettings.php:* This crucial setting in `LocalSettings.php` tells MediaWiki which server is the current master. It *must* be updated whenever you promote a slave to become the master. Incorrect configuration here will result in broken wiki functionality. See Configuration:Settings for more details.
  • `$wgSharedDB` in LocalSettings.php:* If you are using shared databases (common in some setups), ensure this setting is configured correctly to avoid conflicts.
  • Caching:* Replication doesn't automatically handle cache invalidation. You may need to implement a caching strategy that takes replication delay into account. Consider using a shared cache like Memcached or Redis. Help:Caching provides a detailed overview.
  • Load Balancing:* To distribute read traffic across multiple slaves, you'll need to set up a load balancer (e.g., HAProxy, Nginx). The load balancer directs requests to the available slaves.
  • Replication Filters:* You can configure replication filters to exclude specific databases or tables from replication. This can be useful for excluding databases that are not critical or for reducing the replication load.
  • Monitoring:* Regularly monitor the replication status to ensure that the slaves are keeping up with the master and that there are no errors. Tools like Nagios, Zabbix, or Prometheus can be used for monitoring.
  • Failover Testing:* Periodically test the failover process to ensure that you can quickly and reliably promote a slave to become the master in case of a failure. This is critical for maintaining high availability.

Troubleshooting Common Replication Issues

  • Slave is not starting:* Check the MySQL error logs on the slave server for clues. Common causes include incorrect master credentials, network connectivity issues, or an invalid binary log file name or position.
  • Slave is falling behind:* This can be caused by a slow network connection, a heavily loaded master server, or a slow slave server. Investigate the performance of both servers and the network connection between them.
  • Errors during replication:* Check the MySQL error logs on both the master and slave servers for details about the errors. Common causes include data inconsistencies, unique key violations, or incorrect character set settings.
  • `SHOW SLAVE STATUS\G` shows errors:* Carefully examine the output of this command for error messages. The error messages provide valuable information about the cause of the problem. Look for `Last_Error` and `Last_IO_Error`.
  • Data inconsistencies:* If you suspect data inconsistencies, you can use tools like `pt-table-checksum` and `pt-table-sync` from the Percona Toolkit to verify and synchronize the data. These tools are invaluable for maintaining data integrity.
  • Binary Log Rotation:* Ensure that your binary log rotation strategy is appropriate for your needs. If the binary logs are rotated too frequently, the slave may not be able to keep up.

Advanced Replication Strategies

  • Galera Cluster:* A synchronous multi-master cluster solution. Offers strong consistency and automatic failover but can be more complex to set up.
  • Percona XtraDB Cluster:* Another synchronous multi-master cluster solution based on Galera.
  • Virtualization and Containerization:* Using virtualization (e.g., VMware, KVM) or containerization (e.g., Docker) can simplify the setup and management of replication.
  • Cloud-Based Replication:* Cloud providers (e.g., AWS, Google Cloud, Azure) offer managed database services with built-in replication features.

Security Considerations

  • Secure Network Connectivity:* Ensure that the network connection between the master and slave servers is secure. Use firewalls and encryption to protect the data in transit.
  • Restrict Access to Replication User:* The replication user should only have the necessary privileges to replicate data. Restrict access to this user to the master and slave servers.
  • Regularly Audit Replication Configuration:* Periodically review the replication configuration to ensure that it is still secure and that there are no vulnerabilities.

Understanding and implementing database replication is a significant step towards building a robust and scalable MediaWiki deployment. By carefully planning and configuring replication, you can ensure that your wiki remains available, reliable, and responsive, even under heavy load. Don't underestimate the importance of thorough testing and monitoring. Regularly consult the MediaWiki FAQ and the official MySQL documentation for the latest information and best practices.

Percona's MySQL Replication Best Practices MySQL Replication Documentation DigitalOcean MySQL Replication Tutorial LinuxBabe MySQL Replication Tutorial SitePoint MySQL Replication Guide Guru99 MySQL Replication Tutorial Scalable DBA MySQL Replication Best Practices Database Management MySQL Replication Best Practices Percona Toolkit HAProxy Load Balancer Nginx Load Balancer Memcached Caching System Redis Caching System Prometheus Monitoring System Zabbix Monitoring System Nagios Monitoring System Datadog Monitoring New Relic Monitoring SolarWinds Monitoring Grafana Data Visualization InfluxDB Time Series Database Elastic Stack (ELK) Splunk Data Analytics Atlassian Tools PagerDuty Incident Management Statuspage Incident Communication Opsgenie Incident Management Dynatrace Application Performance Monitoring AppDynamics Application Performance Monitoring IBM Cloud AWS Google Cloud Azure

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