Source

File: wp-includes/rest-api.php

function rest_get_server() {
	/* @var WP_REST_Server $wp_rest_server */
	global $wp_rest_server;

	if ( empty( $wp_rest_server ) ) {
		/**
		 * Filters the REST Server Class.
		 *
		 * This filter allows you to adjust the server class used by the API, using a
		 * different class to handle requests.
		 *
		 * @since 4.4.0
		 *
		 * @param string $class_name The name of the server class. Default 'WP_REST_Server'.
		 */
		$wp_rest_server_class = apply_filters( 'wp_rest_server_class', 'WP_REST_Server' );
		$wp_rest_server       = new $wp_rest_server_class;

		/**
		 * Fires when preparing to serve an API request.
		 *
		 * Endpoint objects should be created and register their hooks on this action rather
		 * than another action to ensure they're only loaded when needed.
		 *
		 * @since 4.4.0
		 *
		 * @param WP_REST_Server $wp_rest_server Server object.
		 */
		do_action( 'rest_api_init', $wp_rest_server );
	}

	return $wp_rest_server;
}