WordPress drop-in (mu-plugin or include) to help manage features in certain server load conditions.

server-load.php

<?php
/**
 * Plugin name: Server Load
 * Plugin URI: https://gist.github.com/crstauf/67de8505ae628d9589c14e62e6ef69eb
 * Author: Caleb Stauffer
 * Author URI: https://develop.calebstauffer.com
 * Version: 1.0
 */

class ServerLoad {

	/**
	 * @var string Version number.
	 */
	const VERSION = '1.0';

	/**
	 * @var string $load
	 */
	protected $load = 'normal';

	/**
	 * @var string[] $loads
	 */
	protected $loads = array(
		'normal',
		'high',
		'extreme',
	);

	/**
	 * @var array $constants
	 */
	protected $constants = array(
		'EXTREME_SERVER_LOAD' => 'extreme',
		   'HIGH_SERVER_LOAD' => 'high',
	);

	/**
	 * Get instance.
	 *
	 * @return self
	 */
	static function instance() {
		static $_instance = null;

		if ( is_null( $_instance ) )
			$_instance = new self;

		return $_instance;
	}

	/**
	 * Construct.
	 *
	 * @uses $this::check_constants()
	 */
	protected function __construct() {
		$this->check_constants();

		add_action( 'plugins_loaded', array( $this, 'action__plugins_loaded' ) );
	}

	/**
	 * Getter.
	 *
	 * @param string $key
	 */
	function __get( $key ) {
		if ( 'load' !== $key )
			return null;

		return $this->load;
	}

	/**
	 * Setter.
	 *
	 * @param string $key
	 * @param mixed $value
	 * @uses $this::set()
	 */
	function __set( $key, $value ) {
		if ( 'load' !== $key )
			return null;

		$this->set( $value );
	}

	static function load() {
		return static::instance()->load;
	}

	/**
	 * Check constants.
	 */
	protected function check_constants() {
		foreach ( $this->constants as $constant => $load ) {

			# If constant not defined, define false.
			if ( !defined( $constant ) )
				define( $constant, false );

			# If constant is defined, set load.
			else if ( constant( $constant ) )
				$this->load = $load;

		}
	}

	/**
	 * Set load.
	 *
	 * @param string $load
	 * @uses $this::is_valid_load()
	 */
	protected function set( string $load ) {
		if ( !$this->is_valid_load( $load ) ) {
			trigger_error( sprintf( '<code>%s</code> is not a valid load level.', $load ) );
			return;
		}

		$this->load = $load;
	}

	/**
	 * Check if load is valid.
	 *
	 * @param string $load
	 * @return bool
	 */
	protected function is_valid_load( string $load ) {
		return in_array( $load, $this->loads );
	}

	/**
	 * Check if is specified load.
	 *
	 * @param string|array $loads
	 * @return bool
	 */
	static function is( $loads ) {
		$loads = ( array ) $loads;

		if ( in_array( 'high', $loads ) )
			$loads[] = 'extreme';

		return in_array( static::instance()->load, $loads );
	}

	/**
	 * Action: plugins_loaded
	 *
	 * - add warning to Query Monitor if server load isn't normal
	 */
	function action__plugins_loaded() {
		if ( static::is( 'normal' ) )
			return;

		do_action( 'qm/warning', sprintf( 'Server load: %s', $this->load ) );
	}

}

if ( !function_exists( 'add_action_if_load' ) ) {

	/**
	 * Add action if specified load.
	 *
	 * @see add_action()
	 * @param string|array $loads
	 * @uses ServerLoad::is()
	 * @uses add_action()
	 */
	function add_action_if_load( $loads, string $hook, callable $callback, int $priority = 10, int $accepted_args = 1 ) {
		if ( !ServerLoad::is( $loads ) )
			return;

		add_action( $hook, $callback, $priority, $accepted_args );
	}

}

if ( !function_exists( 'add_filter_if_load' ) ) {

	/**
	 * Add filter if specified load.
	 *
	 * @see add_filter()
	 * @param string|array $loads
	 * @uses ServerLoad::is()
	 * @uses add_filter()
	 */
	function add_filter_if_load( $loads, string $hook, callable $callback, int $priority = 10, int $accepted_args = 1 ) {
		if ( !ServerLoad::is( $loads ) )
			return;

		add_filter( $hook, $callback, $priority, $accepted_args );
	}

}

if ( !function_exists( 'remove_action_if_load' ) ) {

	/**
	 * Remove action if specified load.
	 *
	 * @see add_action()
	 * @param string|array $loads
	 * @uses ServerLoad::is()
	 * @uses remove_action()
	 */
	function remove_action_if_load( $loads, string $hook, callable $callback, int $priority = 10, int $accepted_args = 1 ) {
		if ( !ServerLoad::is( $loads ) )
			return;

		if ( class_exists( 'QM_Util' ) )
			do_action( 'qm/info', sprintf( '<code>%s</code> on <code>%s</code> action prevented due to %s server load.', QM_Util::populate_callback( array( 'function' => $callback ) )['function'], $hook, ServerLoad::load() ) );

		remove_action( $hook, $callback, $priority, $accepted_args );
	}

}

if ( !function_exists( 'remove_filter_if_load' ) ) {

	/**
	 * Remove filter if specified load.
	 *
	 * @see add_filter()
	 * @param string|array $loads
	 * @uses ServerLoad::is()
	 * @uses add_filter()
	 */
	function remove_filter_if_load( $loads, string $hook, callable $callback, int $priority = 10, int $accepted_args = 1 ) {
		if ( !ServerLoad::is( $loads ) )
			return;

		if ( class_exists( 'QM_Util' ) )
			do_action( 'qm/info', sprintf( '<code>%s</code> on <code>%s</code> filter prevented due to %s server load.', QM_Util::populate_callback( array( 'function' => $callback ) )['function'], $hook, ServerLoad::load() ) );

		remove_filter( $hook, $callback, $priority, $accepted_args );
	}

}

ServerLoad::instance();

?>