Index: bootstrap.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/bootstrap.inc,v
retrieving revision 1.167
diff -u -r1.167 bootstrap.inc
--- bootstrap.inc	25 May 2007 21:01:29 -0000	1.167
+++ bootstrap.inc	27 May 2007 22:45:36 -0000
@@ -263,7 +263,7 @@
   global $base_url, $base_path, $base_root;
 
   // Export the following settings.php variables to the global namespace
-  global $db_url, $db_prefix, $cookie_domain, $conf, $installed_profile;
+  global $db_url, $db_slave_url, $db_prefix, $cookie_domain, $conf, $installed_profile;
   $conf = array();
 
   if (file_exists('./'. conf_path() .'/settings.php')) {
Index: database.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/database.inc,v
retrieving revision 1.69
diff -u -r1.69 database.inc
--- database.inc	25 May 2007 12:46:43 -0000	1.69
+++ database.inc	27 May 2007 22:45:36 -0000
@@ -115,8 +115,8 @@
  * @return the name of the previously active database or FALSE if non was found.
  */
 function db_set_active($name = 'default') {
-  global $db_url, $db_type, $active_db;
-  static $db_conns;
+  global $db_url, $db_slave_url, $db_type, $active_db, $active_slave_db;
+  static $db_conns, $db_slave_conns;
 
   if (empty($db_url)) {
     include_once 'includes/install.inc';
@@ -124,12 +124,31 @@
   }
 
   if (!isset($db_conns[$name])) {
-    // Initiate a new connection, using the named DB URL specified.
+    // If the $name doesn't have a corresponding URL, use the default.
+    if (!isset($db_url[$name])) {
+      $name = 'default';
+    }
+    
+    // Initiate a new connection, using the specifed DB URL.
     if (is_array($db_url)) {
-      $connect_url = array_key_exists($name, $db_url) ? $db_url[$name] : $db_url['default'];
+      $connect_url = $db_url[$name];
+      if (is_array($db_slave_url[$name])) {
+        $slave_index = mt_rand(0, count($db_slave_url[$name]));
+        $slave_connect_url = $db_slave_url[$name][$slave_index];
+      }
+      else {
+        $slave_connect_url = $db_slave_url[$name];        
+      }
     }
     else {
       $connect_url = $db_url;
+      if (is_array($db_slave_url)) {
+        $slave_index = mt_rand(0, count($db_slave_url) - 1);
+        $slave_connect_url = $db_slave_url[$slave_index];
+      }
+      else {
+        $slave_connect_url = $db_slave_url;        
+      }
     }
 
     $db_type = substr($connect_url, 0, strpos($connect_url, '://'));
@@ -147,11 +166,20 @@
     }
 
     $db_conns[$name] = db_connect($connect_url);
+    if (!empty($slave_connect_url)) {
+      $db_slave_conns[$name] = db_connect($slave_connect_url); 
+    }
   }
 
   $previous_db = $active_db;
-  // Set the active connection.
+  // Set the active connections.
   $active_db = $db_conns[$name];
+  if (isset($db_slave_conns[$name])) {
+    $active_slave_db = $db_slave_conns[$name];
+  }
+  else {
+    unset($active_slave_db);
+  }
 
   return array_search($previous_db, $db_conns);
 }
Index: database.mysqli.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/database.mysqli.inc,v
retrieving revision 1.37
diff -u -r1.37 database.mysqli.inc
--- database.mysqli.inc	25 May 2007 21:01:30 -0000	1.37
+++ database.mysqli.inc	27 May 2007 22:45:36 -0000
@@ -125,14 +125,19 @@
  * Helper function for db_query().
  */
 function _db_query($query, $debug = 0) {
-  global $active_db, $queries;
+  global $active_db, $active_slave_db, $queries;
 
   if (variable_get('dev_query', 0)) {
     list($usec, $sec) = explode(' ', microtime());
     $timer = (float)$usec + (float)$sec;
   }
 
-  $result = mysqli_query($active_db, $query);
+  if (isset($active_slave_db) && strtoupper(substr(ltrim($query), 0, 6)) == 'SELECT') {
+    $result = mysqli_query($active_slave_db, $query);
+  }
+  else {
+    $result = mysqli_query($active_db, $query);
+  }
 
   if (variable_get('dev_query', 0)) {
     $bt = debug_backtrace();
Index: database.mysql.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/database.mysql.inc,v
retrieving revision 1.73
diff -u -r1.73 database.mysql.inc
--- database.mysql.inc	25 May 2007 21:01:30 -0000	1.73
+++ database.mysql.inc	27 May 2007 22:45:36 -0000
@@ -134,14 +134,19 @@
  * Helper function for db_query().
  */
 function _db_query($query, $debug = 0) {
-  global $active_db, $queries;
+  global $active_db, $active_slave_db, $queries;
 
   if (variable_get('dev_query', 0)) {
     list($usec, $sec) = explode(' ', microtime());
     $timer = (float)$usec + (float)$sec;
   }
 
-  $result = mysql_query($query, $active_db);
+  if (isset($active_slave_db) && strtoupper(substr(ltrim($query), 0, 6)) == 'SELECT') {
+    $result = mysql_query($active_slave_db, $query);    
+  }
+  else {
+    $result = mysql_query($active_db, $query);
+  }
 
   if (variable_get('dev_query', 0)) {
     $bt = debug_backtrace();
Index: database.pgsql.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/database.pgsql.inc,v
retrieving revision 1.46
diff -u -r1.46 database.pgsql.inc
--- database.pgsql.inc	25 May 2007 12:46:43 -0000	1.46
+++ database.pgsql.inc	27 May 2007 22:45:36 -0000
@@ -115,14 +115,19 @@
  * Helper function for db_query().
  */
 function _db_query($query, $debug = 0) {
-  global $active_db, $last_result, $queries;
+  global $active_db, $active_slave_db, $last_result, $queries;
 
   if (variable_get('dev_query', 0)) {
     list($usec, $sec) = explode(' ', microtime());
     $timer = (float)$usec + (float)$sec;
   }
 
-  $last_result = pg_query($active_db, $query);
+  if (isset($active_slave_db) && strtoupper(substr(ltrim($query), 0, 6)) == 'SELECT') {
+    $result = pg_query($active_slave_db, $query);    
+  }
+  else {
+    $result = pg_query($active_db, $query);
+  }
 
   if (variable_get('dev_query', 0)) {
     $bt = debug_backtrace();
Index: session.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/session.inc,v
retrieving revision 1.40
diff -u -r1.40 session.inc
--- session.inc	25 May 2007 15:04:41 -0000	1.40
+++ session.inc	27 May 2007 22:45:36 -0000
@@ -29,7 +29,7 @@
   }
 
   // Otherwise, if the session is still active, we have a record of the client's session in the database.
-  $user = db_fetch_object(db_query("SELECT u.*, s.* FROM {users} u INNER JOIN {sessions} s ON u.uid = s.uid WHERE s.sid = '%s'", $key));
+  $user = db_fetch_object(db_query("/* Master */ SELECT u.*, s.* FROM {users} u INNER JOIN {sessions} s ON u.uid = s.uid WHERE s.sid = '%s'", $key));
 
   // We found the client's session record and they are an authenticated user
   if ($user && $user->uid > 0) {
@@ -61,9 +61,9 @@
     return TRUE;
   }
 
-  $result = db_query("SELECT sid FROM {sessions} WHERE sid = '%s'", $key);
+  db_query("UPDATE {sessions} SET uid = %d, cache = %d, hostname = '%s', session = '%s', timestamp = %d WHERE sid = '%s'", $user->uid, isset($user->cache) ? $user->cache : '', ip_address(), $value, time(), $key);
 
-  if (!db_num_rows($result)) {
+  if (!db_affected_rows()) {
     // Only save session data when when the browser sends a cookie. This keeps
     // crawlers out of session table. This reduces memory and server load,
     // and gives more useful statistics. We can't eliminate anonymous session
@@ -73,8 +73,6 @@
     }
   }
   else {
-    db_query("UPDATE {sessions} SET uid = %d, cache = %d, hostname = '%s', session = '%s', timestamp = %d WHERE sid = '%s'", $user->uid, isset($user->cache) ? $user->cache : '', ip_address(), $value, time(), $key);
-
     // TODO: this can be an expensive query. Perhaps only execute it every x minutes. Requires investigation into cache expiration.
     if ($user->uid) {
       db_query("UPDATE {users} SET access = %d WHERE uid = %d", time(), $user->uid);