diff --git a/composer.json b/composer.json
index 11fcf45..5417248 100644
--- a/composer.json
+++ b/composer.json
@@ -29,7 +29,7 @@
             "Swow\\": "src/"
         },
         "files": [
-            "src/functions.php",
+            "src/bootstrap.php",
             "src/Debug/functions.php"
         ]
     },
diff --git a/src/Library.php b/src/Library.php
new file mode 100644
index 0000000..4c67fb1
--- /dev/null
+++ b/src/Library.php
@@ -0,0 +1,26 @@
+<?php
+/**
+ * This file is part of Swow
+ *
+ * @link    https://github.com/swow/swow
+ * @contact twosee <twosee@php.net>
+ *
+ * For the full copyright and license information,
+ * please view the LICENSE file that was distributed with this source code
+ */
+
+declare(strict_types=1);
+
+namespace Swow;
+
+final class Library
+{
+    public const VERSION = '0.2.0-alpha';
+    public const VERSION_ID = 200;
+    public const MAJOR_VERSION = 0;
+    public const MINOR_VERSION = 2;
+    public const RELEASE_VERSION = 0;
+    public const EXTRA_VERSION = 'alpha';
+
+    public const REQUIRED_EXTENSION_VERSION = '^0.2.0';
+}
diff --git a/src/bootstrap.php b/src/bootstrap.php
new file mode 100644
index 0000000..50d129a
--- /dev/null
+++ b/src/bootstrap.php
@@ -0,0 +1,27 @@
+<?php
+/**
+ * This file is part of Swow
+ *
+ * @link    https://github.com/swow/swow
+ * @contact twosee <twosee@php.net>
+ *
+ * For the full copyright and license information,
+ * please view the LICENSE file that was distributed with this source code
+ */
+
+declare(strict_types=1);
+
+use Composer\Semver\Semver;
+use Swow\Extension;
+use Swow\Library;
+
+if (!class_exists(Extension::class)) {
+    return;
+}
+
+if (!Semver::satisfies(Extension::VERSION, Library::REQUIRED_EXTENSION_VERSION)) {
+    throw new Error(sprintf(
+        '%s extension version mismatch, required: %s, actual: %s',
+        Swow::class, Library::REQUIRED_EXTENSION_VERSION, Extension::VERSION
+    ));
+}
diff --git a/src/functions.php b/src/functions.php
deleted file mode 100644
index 8a4bf30..0000000
--- a/src/functions.php
+++ /dev/null
@@ -1,14 +0,0 @@
-<?php
-/**
- * This file is part of Swow
- *
- * @link    https://github.com/swow/swow
- * @contact twosee <twosee@php.net>
- *
- * For the full copyright and license information,
- * please view the LICENSE file that was distributed with this source code
- */
-
-declare(strict_types=1);
-
-define('SWOW_LIBRARY', true);
diff --git a/tests/SwowTest.php b/tests/SwowTest.php
index 424919a..63996dd 100644
--- a/tests/SwowTest.php
+++ b/tests/SwowTest.php
@@ -14,9 +14,10 @@
 namespace Swow\Tests;
 
 use PHPUnit\Framework\TestCase;
+use Swow;
 
-use function defined;
 use function extension_loaded;
+use function strlen;
 
 /**
  * @internal
@@ -26,12 +27,25 @@ final class SwowTest extends TestCase
 {
     public function testSwowExtension(): void
     {
-        $this->assertTrue(extension_loaded('swow'));
+        $this->assertTrue(extension_loaded(Swow::class));
+        $this->assertGreaterThan(0, Swow\Extension::VERSION_ID);
     }
 
     public function testSwowLibrary(): void
     {
-        $this->assertTrue(defined('SWOW_LIBRARY'));
-        $this->assertTrue(SWOW_LIBRARY);
+        $this->assertTrue(Swow::isLibraryLoaded());
+        $this->assertSame(Swow\Library::VERSION, sprintf(
+            '%d.%d.%d%s%s',
+            Swow\Library::MAJOR_VERSION,
+            Swow\Library::MINOR_VERSION,
+            Swow\Library::RELEASE_VERSION,
+            strlen(Swow\Library::EXTRA_VERSION) > 0 ? '-' : '', Swow\Library::EXTRA_VERSION
+        ));
+        $this->assertSame(Swow\Library::VERSION_ID, (int) sprintf(
+            '%02d%02d%02d',
+            Swow\Library::MAJOR_VERSION,
+            Swow\Library::MINOR_VERSION,
+            Swow\Library::RELEASE_VERSION
+        ));
     }
 }