Database library to handle multiple masters and multiple slaves

In a large scale mysql deployment there could be multiple masters and multiple slaves. Masters are generally in circular replication. And are used for running all inserts, updates and deletes. Slaves are used to run selects.

When you are dealing with multiple mysql instances running in a large scale environment, it is important to take care of lags between masters and slaves. To handle such scenarios, the code should be capable of firing query on a server dynamically. Which means that for each query, I as a developer should have the flexibility to decide which server the query should go.

A list of existing scenarios :

1. All registrations / username generation process should happen on a single master. If you generate usernames at both masters, there may be scenarios where, due to lag between mysql masters, the user is not reflected. And in such a case, the user may register again and land on another master. Creating the same username again and breaking the circular replication. So all registrations and check for “username exists” should happen on a single master.

2. For all other Insert, Update and Delete operations, the user should be stuck to a single master. Why ? Assume there is a lag of around 30 minutes between the masters and slaves. The user inserts a record and immediately wants to see what record has been inserted. If we fetch the record from another master or slave, the record will not be available, because it has not yet been replicated. To take care of this scenario, whenever a record is inserted the immediate select has to be from the same server.

3. For all other selects, the query can be fired on any of the slaves. For example, the user logs into the site and sees his own profile. We show him his profile using one of the slave servers. This can be cached as well. The point here is that for data which has not been updated recently – the query can be fired on any of the slaves.

The following piece of code/library handles most of the scenarios. Please feel free to suggest modifications or improvements.


/**
* Created by : Jayant Kumar
* Description : php database library to handle multiple masters & multiple slaves
**/
class DatabaseList // jk : Base class
{
public $db = array();
public function setDb($db)
{
$this->db = $db;
}
public function getDb()
{
return $this->db;
}
}
class SDatabaseList extends DatabaseList // jk : Slave mysql servers
{
function __construct()
{
$this->db[0] = array(‘ip’=>’10.20.1.11’, ‘u’=>’user11’, ‘p’=>’pass11’, ‘db’=>’database1’);
$this->db[1] = array(‘ip’=>’10.20.1.12’, ‘u’=>’user12’, ‘p’=>’pass12’, ‘db’=>’database1’);
$this->db[2] = array(‘ip’=>’10.20.1.13’, ‘u’=>’user13’, ‘p’=>’pass13’, ‘db’=>’database1’);
//print_r($db);
}
}
class MDatabaseList extends DatabaseList // jk : Master mysql servers
{
function __construct()
{
$this->db[0] = array(‘ip’=>’10.20.1.1’, ‘u’=>’user1’, ‘p’=>’pass1’, ‘db’=>’database1’);
$this->db[1] = array(‘ip’=>’10.20.1.2’, ‘u’=>’user2’, ‘p’=>’pass2’, ‘db’=>’database2’);
//print_r($db);
}
}
class MemcacheList extends DatabaseList // jk : memcache servers
{
function __construct()
{
$this->db[0] = array(‘ip’=>’localhost’, ‘port’=>11211);
}
}
Interface DatabaseSelectionStrategy  // jk : Database interface
{
public function getCurrentDb();
}
class StickyDbSelectionStrategy implements DatabaseSelectionStrategy // jk : sticky db . For update / delete / insert
{
private $dblist;
private $uid;
private $sessionDb;
private $sessionTimeout = 3600;
function __construct(DatabaseList $dblist)
{
$this->dblist = $dblist;
}
public function setUserId($uid)
{
$this->uid = $uid;
}
public function setSessionDb($sessionDb)
{
$this->sessionDb = $sessionDb->db;
}
private function getDbForUser() // jk : get db for this user. If not found – assign him random master db.
{
$memc = new Memcache;
foreach ($this->sessionDb as $key => $value) {
$memc->addServer($value[‘ip’], $value[‘port’]);
}
$dbIp = $memc->get($this->uid);
if($dbIp == null)
{
$masterlist = new MDatabaseList();
$randomdb = new RandomDbSelectionStrategy($masterlist);
$mdb = $randomdb->getCurrentDb();
$dbIp = $mdb[‘ip’];
$memc->set($this->uid, $dbIp, false, $this->sessionTimeout);
}
return $dbIp;
}
public function getCurrentDb()
{
$dbIp = $this->getDbForUser();
foreach ($this->dblist->db as $key => $value) 
{
if($value[‘ip’] == $dbIp)
return $value;
}
}
}
class RandomDbSelectionStrategy implements DatabaseSelectionStrategy // jk : select random db from list
{
private $dblist;
function __construct(DatabaseList $dblist)
{
//print_r($dblist);
$this->dblist = $dblist;
}
public function getCurrentDb()
{
//print_r($this->dblist);
$cnt = sizeof($this->dblist->db);
$rnd = rand(0,$cnt-1);
$current = $this->dblist->db[$rnd];
return $current;
}
}
class SingleDbSelectionStrategy implements DatabaseSelectionStrategy // jk : select one master db – to generate unique keys
{
private $dblist;
function __construct(DatabaseList $dblist)
{
$this->dblist = $dblist;
}
public function getCurrentDb()
{
//print_r($this->dblist);
return $this->dblist->db[0];
}
}
Interface Database
{
public function getIp();
public function getDbConnection();
}
class DatabaseFactory implements Database // cmt : database factory
{
private $db;
public function getIp()
{
return $this->db[‘ip’];
}
public function getDbConnection($type = ‘slave’, $uid = 0)
{
$dbStrategy;
switch($type)
{
case ‘slave’:
$dblist = new SDatabaseList();
//print_r($dblist);
$dbStrategy = new RandomDbSelectionStrategy($dblist);
break;
case ‘master’:
$dblist = new MDatabaseList();
//print_r($dblist);
$dbStrategy = new StickyDbSelectionStrategy($dblist);
$dbStrategy->setSessionDb(new MemcacheList());
$dbStrategy->setUserId($uid);
break;
case ‘unique’:
$dblist = new MDatabaseList();
//print_r($dblist);
$dbStrategy = new SingleDbSelectionStrategy($dblist);
break;
}
$this->db = $dbStrategy->getCurrentDb();
print_r($this->db);
// return mysql_connect($this->db[‘ip’], $this->db[‘u’], $this->db[‘p’], $this->db[‘db’]);
}
}
// tst :  test this out…
$factory = new DatabaseFactory();
echo ‘Slave : ‘; $factory->getDbConnection(‘slave’);
echo ‘Slave2 : ‘; $factory->getDbConnection(‘slave’);
echo ‘Unique : ‘; $factory->getDbConnection(‘unique’);
echo ‘New Master 100: ‘; $factory->getDbConnection(‘master’,100);
echo ‘New Master 101: ‘; $factory->getDbConnection(‘master’,101);
echo ‘New Master 102: ‘; $factory->getDbConnection(‘master’,102);
echo ‘old Master 100: ‘; $factory->getDbConnection(‘master’,100);
echo ‘old Master 102: ‘; $factory->getDbConnection(‘master’,102);
?>

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