support split master/slave backends for the primary database

db-config.php

@@ -5,19 +5,26 @@ $wpdb->persistent = true;
 $wpdb->max_connections = 30;
 
 define("NOBLOGS_BACKEND_CONFIG", "/etc/noblogs/backends");
+define("NOBLOGS_MASTER_CONFIG", "/etc/noblogs/master");
 
 include_once('r2db/db-hash.php');
 include_once('r2db/db-backends.php');
 
 // Add the global database (configured in wp-config.php), stores the global
 // blogs and users tables.
+
+// This is the read-only slave.
 $wpdb->add_database(array(
     "host" => DB_HOST,
     "user" => DB_USER,
     "password" => DB_PASSWORD,
     "name" => DB_NAME,
     "dataset" => "global",
-    "write" => 1, "read" => 1, "timeout" => 2
+    "write" => 0, "read" => 1, "timeout" => 2
     ));
 
+// This is the write-only master.
+$wpdb->add_database(noblogs_load_master(NOBLOGS_MASTER_CONFIG));
+
+// Add all the sharded blog databases.
 $wpdb_reverse_backend_map = noblogs_load_backends(NOBLOGS_BACKEND_CONFIG, $wpdb->hash_map);

r2db/db-backends.php

@@ -25,7 +25,7 @@ function noblogs_load_backends($db_config_file, $hashptr) {
         "password" => $backend_url_data["pass"],
         "name" => substr($backend_url_data["path"], 1),
         "dataset" => $dataset,
-        "read" => 1, "write" => 1, "timeout" => 2
+        "read" => 1, "write" => 1, "timeout" => 10
         );
     $wpdb->add_database($backend);
     $hashptr->addTarget($dataset);
@@ -34,3 +34,17 @@ function noblogs_load_backends($db_config_file, $hashptr) {
   fclose($fp);
   return $backend_map;
 }
+
+function noblogs_load_master($master_file) {
+  $master_url = trim(file_get_contents($master_file));
+  $mdata = parse_url($master_url);
+  return array(
+      "host" => $mdata["host"] . ":" . $mdata["port"],
+      "user" => $mdata["user"],
+      "password" => $mdata["pass"],
+      "name" => substr($mdata["path"], 1),
+      "dataset" => "global",
+      "read" => 0, "write" => 1, "timeout" => 10
+      );
+}
+