HyperDB not failing over as I expect

I’m trying to move some sites from a single-host setup to something more robust, but HyperDB isn’t handling database failover as I’d expect. I don’t know if I’m misconfiguring something, or if I have unrealistic expectations…

Here’s (most of) my db-config.php, in the site root, alongside wp-config.php and friends:

// 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;
}

My intent with the above is that read queries will be more-or-less split between the two servers, with writes only going to the masterdb. And that part seems to work alright.

But when I power down the masterdb server, the site just stops responding. What I’d expect to happen, is that the site should still be visible, but no new posts/pages/etc. can be created.

Do I have unrealistic expectations? If so, any suggestions on workarounds that will get me some portion of what I want?

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