MDL-69121 core: Add ZSTD/Gzip compression options to Redis sessions

config-dist.php

@@ -324,6 +324,9 @@ $CFG->admin = 'admin';
 //      Use the igbinary serializer instead of the php default one. Note that phpredis must be compiled with
 //      igbinary support to make the setting to work. Also, if you change the serializer you have to flush the database!
 //      $CFG->session_redis_serializer_use_igbinary = false; // Optional, default is PHP builtin serializer.
+//      $CFG->session_redis_compressor = 'none'; // Optional, possible values are:
+//                                               // 'gzip' - PHP GZip compression
+//                                               // 'zstd' - PHP Zstandard compression
 //
 // Please be aware that when selecting Memcached for sessions that it is advised to use a dedicated
 // memcache server. The memcached extension does not provide isolated environments for individual uses.

lib/classes/session/redis.php

@@ -40,6 +40,19 @@ defined('MOODLE_INTERNAL') || die();
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 class redis extends handler {
+    /**
+     * Compressor: none.
+     */
+    const COMPRESSION_NONE      = 'none';
+    /**
+     * Compressor: PHP GZip.
+     */
+    const COMPRESSION_GZIP      = 'gzip';
+    /**
+     * Compressor: PHP Zstandard.
+     */
+    const COMPRESSION_ZSTD      = 'zstd';
+
     /** @var string $host save_path string  */
     protected $host = '';
     /** @var int $port The port to connect to */
@@ -56,6 +69,8 @@ class redis extends handler {
     protected $lockretry = 100;
     /** @var int $serializer The serializer to use */
     protected $serializer = \Redis::SERIALIZER_PHP;
+    /** @var int $compressor The compressor to use */
+    protected $compressor = self::COMPRESSION_NONE;
     /** @var string $lasthash hash of the session data content */
     protected $lasthash = null;
 
@@ -122,6 +137,10 @@ class redis extends handler {
         if (isset($CFG->session_redis_lock_expire)) {
             $this->lockexpire = (int)$CFG->session_redis_lock_expire;
         }
+
+        if (isset($CFG->session_redis_compressor)) {
+            $this->compressor = $CFG->session_redis_compressor;
+        }
     }
 
     /**
@@ -268,7 +287,8 @@ class redis extends handler {
             if ($this->requires_write_lock()) {
                 $this->lock_session($id);
             }
-            $sessiondata = $this->connection->get($id);
+            $sessiondata = $this->uncompress($this->connection->get($id));
+
             if ($sessiondata === false) {
                 if ($this->requires_write_lock()) {
                     $this->unlock_session($id);
@@ -285,6 +305,53 @@ class redis extends handler {
         return $sessiondata;
     }
 
+    /**
+     * Compresses session data.
+     *
+     * @param mixed $value
+     * @return string
+     */
+    private function compress($value) {
+        switch ($this->compressor) {
+            case self::COMPRESSION_NONE:
+                return $value;
+            case self::COMPRESSION_GZIP:
+                return gzencode($value);
+            case self::COMPRESSION_ZSTD:
+                return zstd_compress($value);
+            default:
+                debugging("Invalid compressor: {$this->compressor}");
+                return $value;
+        }
+    }
+
+    /**
+     * Uncompresses session data.
+     *
+     * @param string $value
+     * @return mixed
+     */
+    private function uncompress($value) {
+        if ($value === false) {
+            return false;
+        }
+
+        switch ($this->compressor) {
+            case self::COMPRESSION_NONE:
+                break;
+            case self::COMPRESSION_GZIP:
+                $value = gzdecode($value);
+                break;
+            case self::COMPRESSION_ZSTD:
+                $value = zstd_uncompress($value);
+                break;
+            default:
+                debugging("Invalid compressor: {$this->compressor}");
+        }
+
+        return $value;
+    }
+
     /**
      * Write the serialized session data to our session store.
      *
@@ -312,6 +379,8 @@ class redis extends handler {
         // There can be race conditions on new sessions racing each other but we can
         // address that in the future.
         try {
+            $data = $this->compress($data);
+
             $this->connection->setex($id, $this->timeout, $data);
         } catch (RedisException $e) {
             error_log('Failed talking to redis: '.$e->getMessage());

lib/tests/session_redis_test.php

@@ -116,6 +116,30 @@ class core_session_redis_testcase extends advanced_testcase {
         $this->assertSessionNoLocks();
     }
 
+    public function test_compression_read_and_write_works() {
+        global $CFG;
+
+        $CFG->session_redis_compressor = \core\session\redis::COMPRESSION_GZIP;
+
+        $sess = new \core\session\redis();
+        $sess->init();
+        $this->assertTrue($sess->handler_write('sess1', 'DATA'));
+        $this->assertSame('DATA', $sess->handler_read('sess1'));
+        $this->assertTrue($sess->handler_close());
+
+        if (extension_loaded('zstd')) {
+            $CFG->session_redis_compressor = \core\session\redis::COMPRESSION_ZSTD;
+
+            $sess = new \core\session\redis();
+            $sess->init();
+            $this->assertTrue($sess->handler_write('sess2', 'DATA'));
+            $this->assertSame('DATA', $sess->handler_read('sess2'));
+            $this->assertTrue($sess->handler_close());
+        }
+
+        $CFG->session_redis_compressor = \core\session\redis::COMPRESSION_NONE;
+    }
+
     public function test_session_blocks_with_existing_session() {
         $sess = new \core\session\redis();
         $sess->init();