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/Option/ClientCluster.php

@@ -0,0 +1,96 @@
+<?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\Option;
+
+use Predis\Connection\ClusterConnectionInterface;
+use Predis\Connection\PredisCluster;
+use Predis\Connection\RedisCluster;
+
+/**
+ * Option class that returns a connection cluster to be used by a client.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class ClientCluster extends AbstractOption
+{
+    /**
+     * Checks if the specified value is a valid instance of ClusterConnectionInterface.
+     *
+     * @param  ClusterConnectionInterface $cluster Instance of a connection cluster.
+     * @return ClusterConnectionInterface
+     */
+    protected function checkInstance($cluster)
+    {
+        if (!$cluster instanceof ClusterConnectionInterface) {
+            throw new \InvalidArgumentException('Instance of Predis\Connection\ClusterConnectionInterface expected');
+        }
+
+        return $cluster;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function filter(ClientOptionsInterface $options, $value)
+    {
+        if (is_callable($value)) {
+            return $this->checkInstance(call_user_func($value, $options, $this));
+        }
+
+        $initializer = $this->getInitializer($options, $value);
+
+        return $this->checkInstance($initializer());
+    }
+
+    /**
+     * Returns an initializer for the specified FQN or type.
+     *
+     * @param  string                 $fqnOrType Type of cluster or FQN of a class implementing ClusterConnectionInterface.
+     * @param  ClientOptionsInterface $options   Instance of the client options.
+     * @return \Closure
+     */
+    protected function getInitializer(ClientOptionsInterface $options, $fqnOrType)
+    {
+        switch ($fqnOrType) {
+            case 'predis':
+                return function () {
+                    return new PredisCluster();
+                };
+
+            case 'redis':
+                return function () use ($options) {
+                    $connectionFactory = $options->connections;
+                    $cluster = new RedisCluster($connectionFactory);
+
+                    return $cluster;
+                };
+
+            default:
+                // TODO: we should not even allow non-string values here.
+                if (is_string($fqnOrType) && !class_exists($fqnOrType)) {
+                    throw new \InvalidArgumentException("Class $fqnOrType does not exist");
+                }
+
+                return function () use ($fqnOrType) {
+                    return new $fqnOrType();
+                };
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getDefault(ClientOptionsInterface $options)
+    {
+        return new PredisCluster();
+    }
+}