Is HyperDB supposed to be able to handle read-only (only) database instances?

Question

I'm trying to make my WordPress database a bit more resilient, with MySQL master/slave replication. To go along with that, I've installed HyperDB, with the master as a read/write database, and the slave as read-only.

As long as both master and slave database servers are running, everything is alright. The Web server sends queries to both master and slave databases. But as soon as I shut down the master database server, my sites become non-responsive. I expected the sites to still function, just not to be able to post new articles and such.

Do I have unrealistic expectations of the HyperDB plugin, or is there something I might have misconfigured?

Below is the relevant section of db-config.php. It's not just straight $wpdb->add_database() calls because a little bit of logic is needed to work out the "correct" database server. For testing, I tried replacing all of it with simple add_database() calls and had the same issue.

// Returns wpdb-style array for use with add_database() copying settings from wp-config
function wshdb_get_template_db($host, $readpref = 1, $writepref = 1) {
    $db_tmpl = array(
            'host' => $host,
            'server' => '',  // only here to suppress a PHP error
            'dataset' => 'global',
            'lag_threshold' => NULL,
            'user' => DB_USER,
            'password' => DB_PASSWORD,
            'name' => DB_NAME,
            'timeout' => 0.2,
            'read' => $readpref,
            'write' => $writepref
    );
    return $db_tmpl;
}

// Populate the wpdb array based on where we're running
$tmp_dbhost = explode(".", strtolower(gethostname()));
$host = $tmp_dbhost[0];
switch($host) {
    // WP4 stage
    case 'webserver1':
    case 'webserver2':
            $wpdb->add_database(wshdb_get_template_db('masterdb'));
            $wpdb->add_database(wshdb_get_template_db('slavedb', 1, 0));
            break;

    // a bunch of other cases omitted

    default:
            $wpdb->add_database(wshdb_get_template_db('localhost'));
            break;
}

Answer 1

I just came across a similar issue. The problem was that the theme was updating options on the 'init' action hook.

function theme_setup() {
    update_option('thumbnail_size_w', 170);
    update_option('medium_size_w', 470);
    update_option('large_size_w', 970);
}
add_action('init', 'theme_setup');

That is called on every page load. This is better done using the 'after_switch_theme' action hook which only takes place when the theme is activated.

function theme_setup() {
    update_option('thumbnail_size_w', 170);
    update_option('medium_size_w', 470);
    update_option('large_size_w', 970);
}
add_action('after_switch_theme', 'theme_setup');

This change saved 3 database updates on every page load.