Write a Plugin for WordPress Multi-Site

WordPress multisite functionality is now integrated into WordPress 3.0. Currently, we must go through a non-trivial process to turn on multisite capability. However, there are already plugins that try to automate this process in one-click. As this process becomes easier and more streamlined, the multisite capability will likely see wider adoption.

Here, we consider how we can make our plugins compatible with WordPress multisite.

WordPress Multi-Site vs WordPress Single-Site

As its name suggests, WordPress multisite allows us to control multiple blogs from a single WordPress dashboard. WordPress.com for example, uses this multisite capability, so it is a great place to visit if you want to quickly test out the multisite interface and functions.

From a plugins perspective, the most important difference between WordPress multisite and WordPress single is when we write data – both writing to the database and writing to local file(s).

In the single WordPress configuration, we usually have an entire database and an entire WordPress installation area dedicated to a single blog. Plugins can be assured that there is only one posts table (wp_posts), one comments table (wp_comments), one links table (wp_links) etc.

Similarly, each plugin has their own file space that need only be dedicated to a single blog. Some plugins may cache files within this space.

In a multisite WordPress configuration, our database will contain tables for all of our blogs. Therefore, we may have multiple posts tables (wp_posts, wp2_posts, wp3_posts) multiple comments tables, multiple links tables, etc.

Furthermore, each blog no longer has its own installation file space. Instead, all blogs share a common plugin file-space.

Some plugins may need to create local pages/files that are specific to a blog (e.g., a caching plugin). In a multisite configuration, it is necessary for these plugins to create separate file areas for each blog and keep track of each of these areas. This results in more record keeping by the plugin application.

WordPress Multisite Plugin – Database Table Names

When writing a WordPress multisite plugin, it is absolutely crucial that you DO NOT make any database calls to hard-coded table names. Instead, always extract the names from the $wpdb global object.

For example, if you need to execute an SQL command on the posts table, then DO NOT use wp_posts as the table name –

$post_count = $wpdb->get_var("SELECT COUNT(*) FROM wp_posts");

Instead, use the posts table name contained within the $wpdb global object –

$post_count = $wpdb->get_var("SELECT COUNT(*) FROM $wpdb->posts");

The $wpdb global object always points to the correct database table names in both the single and multisite WordPress configurations. By tying our plugin to the $wpdb object, we can be assured that our plugin is accessing the right tables for the current blog.

The list of tables pointed to by the $wpdb global object include –

• comments
• commentmeta
• links
• options
• postmeta
• posts
• terms
• term_relationships
• term_taxonomy

Global multisite tables include –

• usermeta
• users
• blogs
• blog_versions
• registration_log
• signups
• site
• sitecategories

WordPress Multisite Plugin – Creating a Table

Sometimes, it is necessary to create our own tables. In this case, we must ensure that we obtain the proper table prefix ($wpdb->prefix). In the example below, we create a term metadata table for our plugin. This metadata table can then be used to store metadata for WordPress taxonomy objects such as tags, categories, or our own custom terms.

// Create a term metadata table where $type = metadata type
global $wpdb;
$table_name = $wpdb->prefix . $type . 'meta';
if ($wpdb->get_var( "SHOW TABLES LIKE '{$table_name}'") != $table_name) {
		  
	shiba_create_metadata_table($table_name, $type);
}

function shiba_create_metadata_table($table_name, $type) {
	global $wpdb;

	if (!empty ($wpdb->charset))
		$charset_collate = "DEFAULT CHARACTER SET {$wpdb->charset}";
	if (!empty ($wpdb->collate))
		$charset_collate .= " COLLATE {$wpdb->collate}";
			
		$sql = "CREATE TABLE IF NOT EXISTS {$table_name} (
		  	meta_id bigint(20) NOT NULL AUTO_INCREMENT,
		  	{$type}_id bigint(20) NOT NULL default 0,
	
			meta_key varchar(255) DEFAULT NULL,
			meta_value longtext DEFAULT NULL,
		   		
		  	UNIQUE KEY meta_id (meta_id)
		) {$charset_collate};";
	
	require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
	dbDelta($sql);
}

Line 2 – Set our database table name by pre-pending it with the right table prefix.
Line 3 – Check to see if table already exists. Only create a new table if one does not already exists.
Line 5 – Create our taxonomy metadata table.

WordPress Multisite Plugin – Activation

When a plugin gets activated, we may run certain initialization functions by tying them to the register_activation_hook function. For example, we may create our plugin table during the activation process.

In a single blog configuration, we run our activation function, create a single table, and we are all done. However, in a multisite configuration, we will need to create tables for each blog within our network. Therefore, we need to iterate over each blog and run our plugin activation function for each and every one of them.

register_activation_hook( __FILE__, 'shiba_activate' );
function shiba_activate($networkwide) {
	global $wpdb;
				
	if (function_exists('is_multisite') && is_multisite()) {
		// check if it is a network activation - if so, run the activation function for each blog id
		if ($networkwide) {
	                $old_blog = $wpdb->blogid;
			// Get all blog ids
			$blogids = $wpdb->get_col("SELECT blog_id FROM $wpdb->blogs");
			foreach ($blogids as $blog_id) {
				switch_to_blog($blog_id);
				_shiba_activate();
			}
			switch_to_blog($old_blog);
			return;
		}	
	} 
	_shiba_activate();		
}

function _shiba_activate() {
        // Add initial plugin options here
	$current_theme = get_current_theme();
	add_option('shiba_bg_theme', $current_theme);

	// Create term metadata table if necessary
	global $wpdb;
        $type = 'shiba_term';
        $table_name = $wpdb->prefix . $type . 'meta';
	if ($wpdb->get_var( "SHOW TABLES LIKE '{$table_name}'") != $table_name) {
		shiba_create_metadata_table($table_name, $type);
        }
}

Line 5 – Check if multisite capability is on.
Line 7 – Check if plugin has been activated for the entire blog network.
Lines 9-13 – Iterate over each blog within our network, and run our activation function for each blog.
Line 17 – If multisite capability is not on, or if plugin is only activated for a single blog, then just run our activation function once.

WordPress Multisite Plugin – Adding a New Blog

We may also want to run our plugin activation function whenever a new blog is created.

add_action( 'wpmu_new_blog', 'new_blog', 10, 6); 		

function new_blog($blog_id, $user_id, $domain, $path, $site_id, $meta ) {
	global $wpdb;

	if (is_plugin_active_for_network('shiba-custom-background/shiba-custom-background.php')) {
		$old_blog = $wpdb->blogid;
		switch_to_blog($blog_id);
		_shiba_activate();
		switch_to_blog($old_blog);
	}
}

WordPress Multisite Plugin – Deactivation, and Uninstall

In the same way that we want to initialize each blog when our plugin is network activated, we will also want to clean up every blog when our plugin is network deactivated. We can combine both activate and deactivate functions as follows.

register_activation_hook( __FILE__, 'shiba_activate' );
register_deactivation_hook( __FILE__, 'shiba_deactivate' );
	
function my_network_propagate($pfunction, $networkwide) {
	global $wpdb;

	if (function_exists('is_multisite') && is_multisite()) {
		// check if it is a network activation - if so, run the activation function 
		// for each blog id
		if ($networkwide) {
			$old_blog = $wpdb->blogid;
			// Get all blog ids
			$blogids = $wpdb->get_col("SELECT blog_id FROM {$wpdb->blogs}");
			foreach ($blogids as $blog_id) {
				switch_to_blog($blog_id);
				call_user_func($pfunction, $networkwide);
			}
			switch_to_blog($old_blog);
			return;
		}	
	} 
	call_user_func($pfunction, $networkwide);
}

function shiba_activate($networkwide) {
	my_network_propagate('_my_activate', $networkwide);
}

function shiba_deactivate($networkwide) {
	my_network_propagate('_my_deactivate', $networkwide);
}

Finally, we just need to repeat our deactivation function when a blog is deleted, and make sure that our uninstall.php file executes clean up operations for all the blogs in our WordPress multisite network. This process is very similar to what we have done above so we will not repeat it again here.

Relevant WordPress Multi-Site Files

• Multi-site administration functions.
• Multi-site WordPress API.
• WordPress database object.

A Better Way

I am just starting to play around with WordPress multisite so if you know of a better way to initialize and clean-up plugins for a multisite configuration please let me know.

Many thanks to Ian Dunn for letting us know about the network wide activation updates in WP 3.4.