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/Pipeline/SafeClusterExecutor.php

@@ -0,0 +1,72 @@
+<?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\Pipeline;
+
+use SplQueue;
+use Predis\CommunicationException;
+use Predis\Connection\ConnectionInterface;
+
+/**
+ * Implements a pipeline executor strategy for connection clusters that does
+ * not fail when an error is encountered, but adds the returned error in the
+ * replies array.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class SafeClusterExecutor implements PipelineExecutorInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function execute(ConnectionInterface $connection, SplQueue $commands)
+    {
+        $size = count($commands);
+        $values = array();
+        $connectionExceptions = array();
+
+        foreach ($commands as $command) {
+            $cmdConnection = $connection->getConnection($command);
+
+            if (isset($connectionExceptions[spl_object_hash($cmdConnection)])) {
+                continue;
+            }
+
+            try {
+                $cmdConnection->writeCommand($command);
+            } catch (CommunicationException $exception) {
+                $connectionExceptions[spl_object_hash($cmdConnection)] = $exception;
+            }
+        }
+
+        for ($i = 0; $i < $size; $i++) {
+            $command = $commands->dequeue();
+
+            $cmdConnection = $connection->getConnection($command);
+            $connectionObjectHash = spl_object_hash($cmdConnection);
+
+            if (isset($connectionExceptions[$connectionObjectHash])) {
+                $values[$i] = $connectionExceptions[$connectionObjectHash];
+                continue;
+            }
+
+            try {
+                $response = $cmdConnection->readResponse($command);
+                $values[$i] = $response instanceof \Iterator ? iterator_to_array($response) : $response;
+            } catch (CommunicationException $exception) {
+                $values[$i] = $exception;
+                $connectionExceptions[$connectionObjectHash] = $exception;
+            }
+        }
+
+        return $values;
+    }
+}