Read Replica just for API Requests

[michaelrimbach]

Is it possible to load the read replica settings just for WP JSON / WooCommerce API requests? My plan is that all read requests from API calls go to the read replica, everything else should go to the master db.

I thought it should work like this:

if( ! empty( DATABASE_HOST_RR ) && ( ( defined( 'REST_REQUEST' ) && REST_REQUEST ) || ( defined( 'WC_API_REQUEST' ) && WC_API_REQUEST ) || isset( $_GET['wc-api'] ) ) ) {

$wpdb->add_database( array(
	'host'     => DATABASE_HOST_RR,
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => DB_NAME,
	'write'    => 0,
	'read'     => 1,
	'dataset'  => 'global',
	'timeout'  => 0.2,
) );	

}

But I found out that the constant REST_REQUEST is not set when the ludicrousdb config is loaded.

Maybe the solution is to load the read replica settings via a custom plugin later (not in db-config.php file)?

[JJJ]

Hey @michaelrimbach. Good question, and one that I'm sure others will have eventually, once they've reached a scale in their store where this is a benefit to them.

Totally not testing this myself, but I'd do something like:

• Always define your API-only read-only database server, maybe with a specific dataset value like wcapi
• Use $wpdb->add_callback( 'wc_api_callback' ); to make yourself a custom callback
• Include a custom callback like the following:

/**
 * Woo Commerce dataset override
 */
function wc_api_callback( $query, $wpdb ) {
	if ( ( defined( 'REST_REQUEST' ) && REST_REQUEST ) || ( defined( 'WC_API_REQUEST' ) && WC_API_REQUEST ) || isset( $_GET['wc-api'] ) ) {
		return 'wcapi';
	}
}

This way, anytime those conditions are met, the correct wcapi dataset is used for any query that might be related to a WooCommerce REST API request, regardless of where it might be coming from (internal, external, whatever.)