Retwis ported to modern Redis data types.

The example (and related documentation at redis.io) was not updated for
5 years. Redis had no sorted sets and hashes when the original code was
written (!). An update was really needed.

We also use a modern PHP client now: Predis. A copy is shipped within
this repository to make life easier to newcomers trying Redis for the
first time via this example.

Predis/Connection/MasterSlaveReplication.php

@@ -0,0 +1,261 @@
+<?php
+
+/*
+ * This file is part of the Predis package.
+ *
+ * (c) Daniele Alessandri <suppakilla@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Predis\Connection;
+
+use Predis\Command\CommandInterface;
+use Predis\Replication\ReplicationStrategy;
+
+/**
+ * Aggregated connection class used by to handle replication with a
+ * group of servers in a master/slave configuration.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class MasterSlaveReplication implements ReplicationConnectionInterface
+{
+    protected $strategy;
+    protected $master;
+    protected $slaves;
+    protected $current;
+
+    /**
+     *
+     */
+    public function __construct(ReplicationStrategy $strategy = null)
+    {
+        $this->slaves = array();
+        $this->strategy = $strategy ?: new ReplicationStrategy();
+    }
+
+    /**
+     * Checks if one master and at least one slave have been defined.
+     */
+    protected function check()
+    {
+        if (!isset($this->master) || !$this->slaves) {
+            throw new \RuntimeException('Replication needs a master and at least one slave.');
+        }
+    }
+
+    /**
+     * Resets the connection state.
+     */
+    protected function reset()
+    {
+        $this->current = null;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function add(SingleConnectionInterface $connection)
+    {
+        $alias = $connection->getParameters()->alias;
+
+        if ($alias === 'master') {
+            $this->master = $connection;
+        } else {
+            $this->slaves[$alias ?: count($this->slaves)] = $connection;
+        }
+
+        $this->reset();
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function remove(SingleConnectionInterface $connection)
+    {
+        if ($connection->getParameters()->alias === 'master') {
+            $this->master = null;
+            $this->reset();
+
+            return true;
+        } else {
+            if (($id = array_search($connection, $this->slaves, true)) !== false) {
+                unset($this->slaves[$id]);
+                $this->reset();
+
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getConnection(CommandInterface $command)
+    {
+        if ($this->current === null) {
+            $this->check();
+            $this->current = $this->strategy->isReadOperation($command) ? $this->pickSlave() : $this->master;
+
+            return $this->current;
+        }
+
+        if ($this->current === $this->master) {
+            return $this->current;
+        }
+
+        if (!$this->strategy->isReadOperation($command)) {
+            $this->current = $this->master;
+        }
+
+        return $this->current;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getConnectionById($connectionId)
+    {
+        if ($connectionId === 'master') {
+            return $this->master;
+        }
+
+        if (isset($this->slaves[$connectionId])) {
+            return $this->slaves[$connectionId];
+        }
+
+        return null;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function switchTo($connection)
+    {
+        $this->check();
+
+        if (!$connection instanceof SingleConnectionInterface) {
+            $connection = $this->getConnectionById($connection);
+        }
+        if ($connection !== $this->master && !in_array($connection, $this->slaves, true)) {
+            throw new \InvalidArgumentException('The specified connection is not valid.');
+        }
+
+        $this->current = $connection;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getCurrent()
+    {
+        return $this->current;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getMaster()
+    {
+        return $this->master;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getSlaves()
+    {
+        return array_values($this->slaves);
+    }
+
+    /**
+     * Returns the underlying replication strategy.
+     *
+     * @return ReplicationStrategy
+     */
+    public function getReplicationStrategy()
+    {
+        return $this->strategy;
+    }
+
+    /**
+     * Returns a random slave.
+     *
+     * @return SingleConnectionInterface
+     */
+    protected function pickSlave()
+    {
+        return $this->slaves[array_rand($this->slaves)];
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function isConnected()
+    {
+        return $this->current ? $this->current->isConnected() : false;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function connect()
+    {
+        if ($this->current === null) {
+            $this->check();
+            $this->current = $this->pickSlave();
+        }
+
+        $this->current->connect();
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function disconnect()
+    {
+        if ($this->master) {
+            $this->master->disconnect();
+        }
+
+        foreach ($this->slaves as $connection) {
+            $connection->disconnect();
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function writeCommand(CommandInterface $command)
+    {
+        $this->getConnection($command)->writeCommand($command);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function readResponse(CommandInterface $command)
+    {
+        return $this->getConnection($command)->readResponse($command);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function executeCommand(CommandInterface $command)
+    {
+        return $this->getConnection($command)->executeCommand($command);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function __sleep()
+    {
+        return array('master', 'slaves', 'strategy');
+    }
+}