YOURLS plugin : Master and Slave Databases (requires YOURLS 1.10.3+)

plugin.php

<?php

/*
Plugin Name: MySQL Master & Slave for YOURLS 1.10.3+
Plugin URI: https://gist.github.com/ozh/553c2519878984271584939355e91b8e
Description: Configure a MySQL Master & Slave setup for YOURLS
Version: 0.1
Author: Ozh
Author URI: https://ozh.org/
*/

// Init when plugins load
yourls_add_action('plugins_loaded', 'mysql_master_slave_init');
function mysql_master_slave_init() {

    // the WRITE connection is the one used for all write operations (insert, update, delete)
    // It should probably be the same as the default YOURLS database settings
    $write = [
        'dns' => 'mysql:host=localhost;dbname=yourls_empty;charset=utf8mb4',
        'user' => 'root',
        'pass' => '',
    ];

    // the READ connection is used for read-only operations (select)
    // Use settings of your SLAVE database
    $read = [
        'dns' => 'mysql:host=localhost;dbname=yourls_empty;charset=utf8mb4',
        'user' => 'root',
        'pass' => '',
    ];

    MySQL_Master_Slave_Manager::init($read, $write);
}

// Handle database connections
class MySQL_Master_Slave_Manager {
    private static $connections = null;

    public static function init($read, $write) {
        self::$connections = new Aura\Sql\ConnectionLocator();

        self::$connections->setDefault(function () use ($write) {
            $ydb = new \YOURLS\Database\YDB( $write['dns'], $write['user'], $write['pass'], [], [] );
            $ydb->init();
            yourls_debug_log('Init default connection');
            return $ydb;
        });

        self::$connections->setWrite('master', function () use ($write) {
            $ydb = new \YOURLS\Database\YDB( $write['dns'], $write['user'], $write['pass'], [], [] );
            $ydb->init();
            yourls_debug_log('Init master connection');
            return $ydb;
        });

        self::$connections->setRead('slave', function () use ($read) {
            $ydb = new \YOURLS\Database\YDB( $read['dns'], $read['user'], $read['pass'], [], [] );
            $ydb->init();
            yourls_debug_log('Init slave connection');
            return $ydb;
        });
    }

    public static function getConnections() {
        return self::$connections;
    }

    public static function isInitialized() {
        return self::$connections !== null;
    }
}

// Intercept all yourls_get_db() calls and use the appropriate connection based on the context
yourls_add_filter('get_db', 'mysql_master_slave_get_db');
function mysql_master_slave_get_db($db, $context) {
    if (!MySQL_Master_Slave_Manager::isInitialized()) {
        return $db;
    }

    // if $context starts with 'other-', use the default connection
    if (strpos($context, 'other-') === 0) {
        // log context, but not if the context is 'other-debug_log' which is in function yourls_debug_log(), otherwise infinite loop
        if ($context != 'other-debug_log') {
            yourls_debug_log('DB context : '.$context);
        }
        return $db;
    }

    $connections = MySQL_Master_Slave_Manager::getConnections();

    // if $context starts with 'read-', use the read connection
    if (strpos($context, 'read-') === 0) {
        yourls_debug_log('DB context : '. $context . ' - Using read connection');
        return $connections->getRead();
    }

    // if $context starts with 'write-', use the write connection
    if (strpos($context, 'write-') === 0) {
        yourls_debug_log('DB context : '. $context . ' - Using write connection');
        return $connections->getWrite();
    }

    // otherwise, use the default connection
    return $db;
}