Additional in-process cache plugin InProcessCache

Some of our caching systems, like the disk cache or memcached, have
significant overhead (network connections or disk I/O).

This plugin adds an additional layer of in-process cache, so we don't
need to reconnect to external cache systems when we've already
received a data item from the cache. There are some concurrency issues
here, but typically they won't be important at the level of a single
web hit.

plugins/InProcessCache/InProcessCachePlugin.php

@@ -0,0 +1,197 @@
+<?php
+/**
+ * StatusNet - the distributed open-source microblogging tool
+ * Copyright (C) 2010, StatusNet, Inc.
+ *
+ * Extra level of caching, in memory
+ *
+ * PHP version 5
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * @category  Cache
+ * @package   StatusNet
+ * @author    Evan Prodromou <evan@status.net>
+ * @copyright 2010 StatusNet, Inc.
+ * @license   http://www.fsf.org/licensing/licenses/agpl-3.0.html AGPL 3.0
+ * @link      http://status.net/
+ */
+
+if (!defined('STATUSNET')) {
+    // This check helps protect against security problems;
+    // your code file can't be executed directly from the web.
+    exit(1);
+}
+
+/**
+ * Extra level of caching
+ *
+ * This plugin adds an extra level of in-process caching to any regular
+ * cache system like APC, XCache, or Memcache.
+ * 
+ * Note that since most caching plugins return false for StartCache*
+ * methods, you should add this plugin before them, i.e.
+ *
+ *     addPlugin('InProcessCache');
+ *     addPlugin('XCache');
+ *
+ * @category  Cache
+ * @package   StatusNet
+ * @author    Evan Prodromou <evan@status.net>
+ * @copyright 2010 StatusNet, Inc.
+ * @license   http://www.fsf.org/licensing/licenses/agpl-3.0.html AGPL 3.0
+ * @link      http://status.net/
+ */
+
+class InProcessCachePlugin extends Plugin
+{
+    private $_items = array();
+    private $_hits  = array();
+
+    /**
+     * Get an item from the cache
+     * 
+     * Called before other cache systems are called (iif this
+     * plugin was loaded correctly, see class comment). If we
+     * have the data, return it, and don't hit the other cache
+     * systems.
+     *
+     * @param string &$key   Key to fetch
+     * @param mixed  &$value Resulting value or false for miss
+     *
+     * @return boolean false if found, else true
+     */
+
+    function onStartCacheGet(&$key, &$value)
+    {
+        if (array_key_exists($key, $this->_items)) {
+            $value = $this->_items[$key];
+            if (array_key_exists($key, $this->_hits)) {
+                $this->_hits[$key]++;
+            } else {
+                $this->_hits[$key] = 1;
+            }
+            Event::handle('EndCacheGet', array($key, &$value));
+            return false;
+        }
+        return true;
+    }
+
+    /**
+     * Called at the end of a cache get
+     *
+     * If we don't already have the data, we cache it. This
+     * keeps us from having to call the external cache if the
+     * key is requested again.
+     *
+     * @param string $key    Key to fetch
+     * @param mixed  &$value Resulting value or false for miss
+     *
+     * @return boolean hook value, true
+     */
+
+    function onEndCacheGet($key, &$value)
+    {
+        if (!array_key_exists($key, $this->_items) || 
+            $this->_items[$key] != $value) {
+            $this->_items[$key] = $value;
+        }
+        return true;
+    }
+
+    /**
+     * Called at the end of setting a cache element
+     * 
+     * Always set the cache element; may overwrite existing
+     * data.
+     *
+     * @param string  $key    Key to fetch
+     * @param mixed   $value  Resulting value or false for miss
+     * @param integer $flag   ignored
+     * @param integer $expiry ignored
+     *
+     * @return boolean true
+     */
+
+    function onEndCacheSet($key, $value, $flag, $expiry)
+    {
+        $this->_items[$key] = $value;
+        return true;
+    }
+
+    /**
+     * Called at the end of deleting a cache element
+     *
+     * If stuff's deleted from the other cache, we
+     * delete it too.
+     *
+     * @param string  &$key     Key to delete
+     * @param boolean &$success Success flag; ignored
+     *
+     * @return boolean true
+     */
+     
+    function onStartCacheDelete(&$key, &$success)
+    {
+        if (array_key_exists($key, $this->_items)) {
+            unset($this->_items[$key]);
+        }
+        return true;
+    }
+
+    /**
+     * Version info
+     *
+     * @param array &$versions Array of version blocks
+     *
+     * @return boolean true
+     */
+
+    function onPluginVersion(&$versions)
+    {
+        $url = 'http://status.net/wiki/Plugin:InProcessCache';
+
+        $versions[] = array('name' => 'InProcessCache',
+                            'version' => STATUSNET_VERSION,
+                            'author' => 'Evan Prodromou',
+                            'homepage' => $url,
+                            'description' =>
+                            _m('Additional in-process cache for plugins.'));
+        return true;
+    }
+
+    /**
+     * Cleanup function; called at end of process
+     *
+     * If the inprocess/stats config value is true, we dump
+     * stats to the log file
+     *
+     * @return boolean true
+     */
+
+    function cleanup()
+    {
+        if (common_config('inprocess', 'stats')) {
+            $this->log(LOG_INFO, "cache size: " . 
+                       count($this->_items));
+            $sum = 0;
+            foreach ($this->_hits as $hitcount) {
+                $sum += $hitcount;
+            }
+            $this->log(LOG_INFO, $sum . " hits on " . 
+                       count($this->_hits) . " keys");
+        }
+        return true;
+    }
+}