Replication patch from
http://cvs.drupal.org/viewcvs/drupal/contributions/sandbox/crackerjm/replication/

--- /dev/null	2005-02-16 21:06:48.000000000 +0200
+++ ./README.replication	2005-08-15 16:08:52.000000000 +0300
@@ -0,0 +1,34 @@
+
+This README.txt and patch are for Mysql Replication. This assumes that you have
+a master replication server and slave server(s) up and running already. If 
+you don't, then there is no purpose for reading this.  If you need help for
+replication, please read this link.  
+      http://dev.mysql.com/doc/mysql/en/Replication.html
+
+NOTE: This is meant for drupal 4.6.0
+
+Now, in includes/conf.php you have a line that looks simalar to this
+    $db_url = "mysql://username:password@localhost/database";
+Comment at the line so that it looks like this
+    //$db_url = "mysql://username:password@localhost/database";
+
+Add the following lines below it to look like this:
+     $db_url = array();
+     $db_url['default'] = "mysql://username:password@localhost/database";
+     $db_url['slave1'] = "mysql://username:password@host/database";
+     //$db_url['slave2'] = "mysql://username:password@host/database";
+
+The 'default' is your MASTER server, the slaves are just that.  make 
+sure that you have added the appropriate users to the servers so that
+if you are using round robin DNS or something to that effect, that 
+way you don't end up with permission denied. 
+
+patch your code with replication.patch and there ya go
+
+***********
+NEW: This revision of code supports the dead slave servers!
+  caveat though, if ALL of your slaves are dead, then you will
+  end up in a infinitely recursive loop searching for slave servers.
+  Hopefully in the next revision of this, I will try to come up with
+  a way to stop this recursion without the use of flags.
+***********
diff -urN drupal-4.6.0/includes/database.inc drupal-test/includes/database.inc
--- drupal-4.6.0/includes/database.inc	2005-04-08 09:24:10.000000000 -0500
+++ drupal-test/includes/database.inc	2005-05-29 09:30:18.710563836 -0500
@@ -119,11 +119,98 @@
       die('Unsupported database type');
     }
 
-    $db_conns[$name] = db_connect($connect_url);
-
+    if($name == 'default') {
+      // die on failure
+      $db_conns[$name] = db_connect($connect_url, true);
+    }
+    else {
+      $db_conns[$name] = db_connect($connect_url);
+    }
+      
   }
   // Set the active connection.
-  $active_db = $db_conns[$name];
+  if($name != 'default' && $db_conns[$name] === false){
+    print "Bad slave, killing $name\n";
+    db_find_slave(true); // kill bad slave
+    db_set_active(db_find_slave());
+  }
+  else {
+    $active_db = $db_conns[$name];
+  }
+}
+
+function _is_commit($query) {
+  $commits = array('insert', 'alter', 'update', 'delete', 'flush', 'lock');
+  foreach($commits as $type) {
+    if(preg_match("/^$type/i", $query)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+function db_find_slave($killslave = false) {
+  global $db_url;
+  static $slave ;
+
+  if(!_replication_ready())
+    return 'default';
+  
+  if(!is_array($slave)){
+    $slave = array();
+    // initialize a local copy
+    foreach($db_url as $key=>$value) {
+      if(stristr($key, 'slave')) {
+        $slave[] = $key;
+      }
+    }  
+    /**
+     * Since this is the first iteration of the loop, we reset the array to 
+     * ensure we return the first element.
+     */
+    return reset($slave);
+  }
+
+  if(empty($slave)) {
+    return 'default';
+  }
+
+  // Failed to connect, remove from the list
+  if($killslave){
+    array_shift($slave);
+  }
+
+  $url = next($slave);
+
+  if($url === false){
+    // walked past of the end of the array, start over at first element
+    $url = reset($slave);
+  }
+
+  return $url;
+}
+
+function _replication_ready() {
+  global $db_url;
+  static $ready;
+
+  if(isset($ready)) {
+    return $ready;
+  }
+  
+  if(is_array($db_url)) {
+    foreach($db_url as $key=>$value) {
+      if(stristr($key, 'slave')) {
+        // Found at least ONE element with the word slave in it
+        // break the search to save on cycles
+        $ready = true;
+        return true;
+      }
+    }
+  }
+  
+  $ready = false;
+  return false;
 }
 
 /**
diff -urN drupal-4.6.0/includes/database.mysql.inc drupal-test/includes/database.mysql.inc
--- drupal-4.6.0/includes/database.mysql.inc	2005-04-14 13:50:23.000000000 -0500
+++ drupal-test/includes/database.mysql.inc	2005-05-29 09:28:22.490546558 -0500
@@ -20,7 +20,7 @@
  * performance, however, when the overhead to connect to your database is high
  * (e.g. your database and web server live on different machines).
  */
-function db_connect($url) {
+function db_connect($url, $critical = false) {
   $url = parse_url($url);
 
   // Allow for non-standard MySQL port.
@@ -28,10 +28,16 @@
      $url['host'] = $url['host'] .':'. $url['port'];
   }
 
-  $connection = mysql_connect($url['host'], $url['user'], $url['pass'], TRUE) or die(mysql_error());
-  mysql_select_db(substr($url['path'], 1)) or die('unable to select database');
+  // For replication setups, we will assume that dieing is bad 
+  $connection = mysql_pconnect($url['host'], $url['user'], $url['pass'], TRUE);
+  if(!is_resource($connection) && $critical) {
+    die(mysql_error());
+  }
+  // We die on bad slaves.  These should be taken care of
+  // maybe hook this somehow or something
+  mysql_select_db(substr($url['path'], 1)) or die(mysql_error());
 
-  return $connection;
+  return ($connection)?$connection:false;
 }
 
 /**
@@ -40,14 +46,38 @@
 function _db_query($query, $debug = 0) {
   global $active_db;
   global $queries;
+  global $db_url;
 
   if (variable_get('dev_query', 0)) {
     list($usec, $sec) = explode(' ', microtime());
     $timer = (float)$usec + (float)$sec;
   }
 
+  /**
+   * If the db_url isn't an array, then we can assume that this server is not
+   * ready for a replication setup.  This outer if is purely for a speed as
+   * calling PHP's builtin functions are way faster than user functions.
+   * So, if its an array and has more than one element, we shall atleast check
+   * to see if it's possibly ready for a replication enviorment.  However, 
+   * if no [slave#] elements are found, then we assume normal operations.
+   */
+  if(is_array($db_url) && sizeof($db_url) > 1 && _replication_ready() )
+  {
+    if (_is_commit($query) === true){
+      db_set_active('default');
+    }
+    else
+    {
+      // no checks as _replication_ready() verifies 
+      // that there is atleast one slave
+      db_set_active(db_find_slave());
+    }
+  }
+
+
   $result = mysql_query($query, $active_db);
 
+
   if (variable_get('dev_query', 0)) {
     list($usec, $sec) = explode(' ', microtime());
     $stop = (float)$usec + (float)$sec;
@@ -67,6 +97,7 @@
   }
 }
 
+
 /**
  * Fetch one result row from the previous query as an object.
  *
@@ -239,4 +270,4 @@
  * @} End of "ingroup database".
  */
 
-?>
\ No newline at end of file
+?>
--- ./sites/default/settings.php~	2005-08-15 16:41:07.000000000 +0300
+++ ./sites/default/settings.php	2005-08-15 16:42:47.000000000 +0300
@@ -77,8 +77,15 @@
  * Database URL format:
  * $db_url = 'mysql://username:password@localhost/database';
  * $db_url = 'pgsql://username:password@localhost/database';
+ *
+ * Database URL format with MySQL replication:
+ * $db_url = array();
+ * $db_url['default'] = "mysql://username:password@localhost/database";
+ * $db_url['slave1'] = "mysql://username:password@host/database";
  */
-$db_url = 'mysql://username:password@localhost/database';
+$db_url = array();
+$db_url['default'] = 'mysql://mysql:@localhost/drupal';
+#$db_url['slave1'] = "mysql://mysql:@mysql-slave/drupal";
 $db_prefix = '';
 
 /**