Add daemon application section

docs/building-extensions/daemon/index.md

@@ -0,0 +1,95 @@
+---
+title: Daemon Process
+---
+
+Creating a Daemon Process
+=========================
+
+:::danger[Developer Notice]
+
+Creating a script which directly includes loads and boots the CMS framework is not recommended and only needed in
+rare cases. Depending on the usecase you should create a [console plugin](../plugins/plugin-examples/basic-console-plugin-helloworld.md)
+and extend the Joomla! console application.
+
+:::
+
+## Introduction
+
+Creating a daemon application means to create a long-running process which might be running in the background.
+This application is started in the console or by a supervisor like systemd.
+
+```bash
+./cli/mydaemon.php
+```
+
+## Location
+
+You can put your php script file in the cli subfolder of the joomla root directory. 
+Example: `/var/lib/www/public_html/cli/mydaemon.php`
+
+We use this path in the following code as our base path
+
+## Sample code
+
+A working example you can copy-paste, the first line is the [Shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) and 
+definies the program loader, in our case the php cli interpreter.
+
+```php
+#!/usr/bin/php
+<?php
+// We need to classify this script as Joomla entry point
+define('_JEXEC', 1);
+
+// Set the Joomla basepath one folder up relative to our current directory /cli
+define('JPATH_BASE', dirname(__DIR__));
+
+// Load the definitions and the framework
+require_once JPATH_BASE . '/includes/defines.php';
+require_once JPATH_BASE . '/includes/framework.php';
+
+// Add a simple 'echo' logger
+\Joomla\CMS\Log\Log::addLogger(['logger' => 'Echo']);
+
+/**
+ * The application which handles our business logic 
+ */
+class MyDaemon extends \Joomla\CMS\Application\DaemonApplication
+{
+    // Set the name of the application
+    public $name = 'MyDaemon';
+
+    public function __construct() {
+        // Load the configuration
+        $config = new \Joomla\Registry\Registry(new \JConfig());
+
+        // We set the `pid file` manually but should be done in the configuration.php 
+        $config->set('application_pid_file', '/run/mydaemon.pid');
+
+        parent::__construct(null, $config);
+    }
+
+    // This function needs to be implmented since it's required by the CMSApplicationInterface  
+    public function getName() {
+      return $this->name;
+   }
+
+    // This function holds our business logic
+    public function doExecute()
+    {
+        $this->out("Hello Joomla!");
+    }
+}
+
+// Run the application
+\Joomla\CMS\Application\DaemonApplication::getInstance('MyDaemon')->execute();
+```
+
+## Parameters
+
+Joomla autodetects the `-f` parameter and executes the application in foreground and doesn't daemonize the process.
+
+```bash
+./cli/mydaemon.php -f
+```
+
+Using the `-f` flag makes it easier to debug and also the usage in the supervisor daemons allows a direct monitoring.