New REST API Notice in 5.1

Edit: On January 14, 2019, the Good and Bad Practices section was added to show both correct and incorrect code examples.

Starting in WordPress 5.1, if register_rest_route() is not called on the rest_api_init action hook, a “doing it wrong” notice will be triggered. This notice is being added in an effort to encourage best practices when registering REST API endpoints.

First, let’s look at what happens when WordPress loads to set up the REST API and explore a few reasons why this pattern is beneficial.

REST API Bootstrap Process

WordPress does its best to ensure that the REST API is only loaded when a REST request is being performed. To do this, the rest_api_loaded() function is run on the parse_request action and checks for a value in the rest_route query argument. This argument is populated with a value when the Rewrite API matches a WordPress REST API URL. When a value is present, the rest_get_server() function is called to instantiate the WP_REST_Server class, store it for use across WordPress, and to run the rest_api_init action hook.

Performance

When register_rest_route() is called, it invokes rest_get_server() to retrieve the global WP_REST_Server instance created in the bootstrap process. But, if the instance has not been set up yet, it is instantiated then and the rest_api_init action hook is run. This means that every function added to the rest_api_init hook will fire at that time. This could result in a large performance hit.

For example, say register_rest_route() is called on the init action, or, just called in a theme’s functions.php file. The REST API server would be set up for every WordPress request, even those that are not actually aimed at the REST API.

Missing Endpoints

If register_rest_route() is called too early, it’s also possible that endpoints will go missing and never be registered. This happens when other plugins are not given the chance to register their rest_api_init hooks.

For example, say register_rest_route() is called directly in an mu-plugin file. This will cause the REST API to be set up before regular plugins are run, so their rest_api_init hooks will not be registered.

Good and Bad Practices

Let’s look at a few code examples and detail why they are good or bad.

Bad Practice

register_rest_route(
	'hello-world/v1',
	'/phrase',
	array(
		'method'   => 'GET',
		'callback' => 'myplugin_get_endpoint_phrase',
	)
);

In this example, register_rest_route() is called directly in a file without being attached to an action hook. This means the function will be called as soon as the file is loaded by WordPress. All of the potential issues detailed above are possible, and a _doing_it_wrong() notice will be triggered.

Good Practice

function myplugin_register_endpoints() {
	register_rest_route(
		'hello-world/v1',
		'/phrase',
		array(
			'method'   => 'GET',
			'callback' => 'myplugin_get_endpoint_phrase',
		)
	);
}
add_action( 'rest_api_ini', `myplugin_register_endpoints` );

In this example, register_rest_route() is correctly placed inside of a function that is added to the rest_api_init action hook. It will only execute when the rest_api_init action hook is executed. The potential issues detailed above are avoided, and no _doing_it_wrong() notice is triggered.

Changes Required

Plugins and Themes

All plugins and themes should double check that their REST API endpoints are being registered correctly using the rest_api_init action hook. This best practice is also mentioned in the Routes and Endpoints section of the REST API Handbook. If this was a resource used when developing, chances are you won’t have to change anything!

Unit Tests

Because some unit tests require custom endpoints to exist, it is not uncommon for a test method to call register_rest_route() directly. If a test method calls the function before rest_api_init, a previously passing test method may now fail. This can be fixed in two ways.

The first way is to use rest_get_server() to create the WP_Rest_Server instance for your tests. Since rest_api_init is run within that function, this will prevent the notice. This approach can be seen in the Tests_REST_Server class. The wp_rest_server_class filter still allows you to replace the default WP_Rest_Server class with your own for testing purposes with this method.

The second way is to call do_action( 'rest_api_init' ); directly in your test method or setUp() method. This method is for scenarios where complete control is needed over the REST server setup process. This can approach can be seen in the Tests_REST_API class.

You can read more information about this change in the ticket on Trac.

#5-1, #dev-notes, #rest-api