PHP redis session handler - changing number of servers

Question

I am using phpredis as a session handler (https://github.com/phpredis/phpredis). My current connection string looks like this:

session.save_path = "tcp://10.0.1.11:7005?weight=1&timeout=0.2&persistent=1&read_timeout=0.5,
tcp://10.0.1.12:7005?weight=1&timeout=0.2&persistent=1&read_timeout=0.5"

but i need to add more redis servers and move existing sessions among them.

My new connection string will look like this:

session.save_path = "tcp://10.0.1.11:7005?weight=1&timeout=0.2&persistent=1&read_timeout=0.5,
tcp://10.0.1.12:7005?weight=1&timeout=0.2&persistent=1&read_timeout=0.5,
tcp://10.0.1.13:7005?weight=1&timeout=0.2&persistent=1&read_timeout=0.5,
tcp://10.0.1.14:7005?weight=1&timeout=0.2&persistent=1&read_timeout=0.5,
tcp://10.0.1.15:7005?weight=1&timeout=0.2&persistent=1&read_timeout=0.5"

so there will be 3 more servers and sessions distribution by key will change with number of servers.

How can i move existing sessions from old servers to new servers to have each one on right server then? Are there any existing tools for this? Has anyone had a similar problem and have a ready solution for it?

Answer 1

I have ready php script that can migrate all sessions to new servers while preserving the same layout as phpredis session handler is using. It support batching and redis->migrate() or manual redis->dump() and redis->restore(). Maybe it will be useful to someone with the same problem.

<?php
class RedisPoolMember {
    public Redis $redis_sock;
    public string $hostname;
    public int $port;
    public int $weight;
    public $next;

    public function __construct(string $hostname, int $port, int $weight) {
        $this->redis_sock = new Redis(); 
        $this->redis_sock->connect($hostname, $port);
        $this->weight = $weight;
        $this->hostname = $hostname;
        $this->port = $port;
        $this->next = null;
    }
}

class RedisPool {
    public $totalWeight = 0;
    public $count = 0;
    public $head = null;
    public $lock_status;

    public function add(string $hostname, int $port, int $weight) {
        
        $rpm = new RedisPoolMember($hostname, $port, $weight);
        $rpm->next = $this->head;
        $this->head = $rpm;

        $this->totalWeight += $weight;
        $this->count++;
    }
    
    public function get(string $key): ?RedisPoolMember {
        $pos = unpack("L", substr($key, 0, 4))[1]; // Assuming a little-endian order
        $pos %= $this->totalWeight;

        $rpm = $this->head;

        for ($i = 0; $i < $this->totalWeight;) {
            if ($pos >= $i && $pos < $i + $rpm->weight) {
                return $rpm;
            }
            $i += $rpm->weight;
            $rpm = $rpm->next;
        }

        return null;
    }
}

function saveKeyPool(RedisPool &$pool, string $sessionPrefix, string $key, $value) {
    $sid = substr($key, strlen($sessionPrefix));
    $rpm = $pool->get($sid);
    $rpm->redis_sock->restore($key, $value);
}

function readKeyPool(RedisPool &$pool, string $sessionPrefix, string $key) {
    $sid = substr($key, strlen($sessionPrefix));
    $rpm = $pool->get($sid);
    return $rpm->redis_sock->dump($key);
}

function migrateKey(RedisPool &$oldPool, RedisPool &$newPool, string $sessionPrefix, string $key) {
    echo('.');
    $sid = substr($key, strlen($sessionPrefix));
    $rpmOld = $oldPool->get($sid);
    $rpmNew = $newPool->get($sid);
    //echo("Migrate key $key to {$rpmNew->hostname}:{$rpmNew->port}\n");
    $rpmOld->redis_sock->migrate($rpmNew->hostname, $rpmNew->port, $key, 0, 0, true, true);
}

function getKeyDestinationRpm(RedisPool &$newPool, string $sessionPrefix, string $key) {
    $sid = substr($key, strlen($sessionPrefix));
    $rpmNew = $newPool->get($sid);
    return $rpmNew->hostname.':'.$rpmNew->port;
}

function processBatch(Redis &$oldRedis, array &$batch) {
    foreach($batch as $server=>$keys) {
        $server = explode(':', $server);
        $oldRedis->migrate($server[0], $server[1], $keys, 0, 0, true, true);
    }
}

$sessionPrefix = 'PHPREDIS_SESSION:';
$oldRedisServers = [['hostname'=>'10.0.1.11', 'port'=>7005], ['hostname'=>'10.0.1.12', 'port'=>7005]];

$redisPoolOld = new RedisPool();
$redisPoolOld->add('10.0.1.11', 7005, 1);
$redisPoolOld->add('10.0.1.12', 7005, 1);


$redisPoolNew = new RedisPool();
$redisPoolNew->add('10.0.1.11', 7010, 1);
$redisPoolNew->add('10.0.1.11', 7011, 1);
$redisPoolNew->add('10.0.1.11', 7012, 1);
$redisPoolNew->add('10.0.1.11', 7013, 1);
$redisPoolNew->add('10.0.1.11', 7014, 1);
$redisPoolNew->add('10.0.1.11', 7015, 1);
$redisPoolNew->add('10.0.1.12', 7010, 1);
$redisPoolNew->add('10.0.1.12', 7011, 1);
$redisPoolNew->add('10.0.1.12', 7012, 1);
$redisPoolNew->add('10.0.1.12', 7013, 1);
$redisPoolNew->add('10.0.1.12', 7014, 1);
$redisPoolNew->add('10.0.1.12', 7015, 1);

foreach($oldRedisServers as $redisServerData) {
    $oldRedis = new Redis();
    $oldRedis->connect($redisServerData['hostname'], $redisServerData['port']);
    $count = 0;
    // Get all keys
    $it = NULL;
    do {
        // Use scan to get the next batch of keys and update the iterator
        $arr_keys = $oldRedis->scan($it, '*', 1000);
        $batch = [];
        if ($arr_keys !== FALSE) {
            foreach ($arr_keys as $str_key) {
                //migrateKey($redisPoolOld, $redisPoolNew, $sessionPrefix, $str_key);
                $batch[getKeyDestinationRpm($redisPoolNew, $sessionPrefix, $str_key)] []= $str_key;
            }
        }
        processBatch($oldRedis, $batch);
        $count += 1000;
        echo("\n{$redisServerData['hostname']}:{$redisServerData['port']} - $count migrated\n"); //1000 records mark
    } while ($it > 0);
}

//test functions
//$key = 'PHPREDIS_SESSION:d09d7358fa84cf68455fee3564e126a5';
//$value = readKeyPool($redisPoolOld, $sessionPrefix, $key);
//saveKeyPool($redisPoolNew, $sessionPrefix, $key, $value);
//migrateKey($redisPoolOld, $redisPoolNew, $sessionPrefix, $key);
//var_dump(readKeyPool($redisPoolOld, $sessionPrefix, 'PHPREDIS_SESSION:d09d7358fa84cf68455fee3564e126a5'));
?>