module init

Error/Maintenance.php

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+<?php
+/**
+ * Alpine_Maintenance
+ *
+ * @copyright   Copyright (c) 2020 Alpine Consulting, Inc (www.alpineinc.com)
+ * @author      Michal Zymela ([email protected])
+ */
+
+namespace Alpine\Maintenance\Error;
+
+use Magento\Framework\App\Bootstrap;
+use Magento\Framework\App\Filesystem\DirectoryList;
+use Magento\Framework\App\Response\Http;
+use Magento\Framework\Filesystem;
+use Magento\Framework\Filesystem\Io\File as FileReader;
+use Magento\Framework\ObjectManagerInterface;
+use Magento\Framework\Phrase;
+
+class Maintenance
+{
+    /** @var int  */
+    const RESPONSE_CODE = 503;
+
+    /**
+     * @var ObjectManagerInterface
+     */
+    private $objectManager;
+    /**
+     * @var Filesystem
+     */
+    private $filesystem;
+    /**
+     * @var FileReader
+     */
+    private $fileReader;
+
+    /**
+     * Maintenance constructor.
+     *
+     * @param Filesystem $filesystem
+     * @param FileReader $fileReader
+     */
+    public function __construct(
+        Filesystem $filesystem,
+        FileReader $fileReader
+    ) {
+        $this->filesystem = $filesystem;
+        $this->fileReader = $fileReader;
+        $objectManagerFactory = Bootstrap::createObjectManagerFactory(BP, $_SERVER);
+        $this->objectManager = $objectManagerFactory->create($_SERVER);
+    }
+
+    /**
+     * Render maintenance page
+     */
+    public function renderPage()
+    {
+        $response = $this->objectManager->create(Http::class);
+        $response->setHttpResponseCode(self::RESPONSE_CODE);
+        $response->setBody(
+            $this->setTemplate()
+        );
+
+        $response->sendResponse();
+    }
+
+    /**
+     * @return bool|Phrase|string
+     */
+    private function setTemplate()
+    {
+        $templatePath = $this->filesystem
+            ->getDirectoryRead(DirectoryList::PUB)
+            ->getAbsolutePath('maintenance/index.html');
+
+        $template = $this->fileReader->read($templatePath);
+
+        if (!$template) {
+            return __('Maintenance html file not found. Upload your index.html file under pub/maintenance directory.');
+        }
+
+        return $template;
+    }
+}

Plugin/HttpPlugin.php

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+<?php
+/**
+ * Alpine_Maintenance
+ *
+ * @copyright   Copyright (c) 2020 Alpine Consulting, Inc (www.alpineinc.com)
+ * @author      Michal Zymela ([email protected])
+ */
+
+namespace Alpine\Maintenance\Plugin;
+
+use Alpine\Maintenance\Error\Maintenance;
+use Exception;
+use Magento\Framework\App\Bootstrap;
+use Magento\Framework\App\Http;
+
+class HttpPlugin
+{
+
+    /**
+     * @var Maintenance
+     */
+    private $maintenance;
+
+    /**
+     * HttpPlugin constructor.
+     *
+     * @param Maintenance $maintenance
+     */
+    public function __construct(Maintenance $maintenance)
+    {
+        $this->maintenance = $maintenance;
+    }
+
+    /**
+     * @param Http $subject
+     * @param callable $proceed
+     * @param Bootstrap $bootstrap
+     * @param Exception $exception
+     *
+     * @return bool
+     */
+    public function aroundCatchException(
+        Http $subject,
+        callable $proceed,
+        Bootstrap $bootstrap,
+        Exception $exception
+    ) {
+        if (!$bootstrap->isDeveloperMode() && $bootstrap->getErrorCode() === Bootstrap::ERR_MAINTENANCE) {
+            $this->maintenance->renderPage();
+
+            return true;
+        }
+
+        return $proceed($bootstrap, $exception);
+    }
+}

composer.json

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+{
+    "name": "alpine/maintenance",
+    "description": "Custom maintenance page from html file placed in pub/maintenance directory",
+    "require": {
+        "php": "~7.1.3||~7.2.0||~7.3.0",
+        "magento/magento-composer-installer": "*",
+        "magento/framework": "102.0.*"
+    },
+    "type": "magento2-module",
+    "version": "1.0.0",
+    "license": "OSL-3.0",
+    "authors": [
+        {
+            "name": "Alpine Consulting, Inc",
+            "email": "[email protected]"
+        }
+    ],
+    "autoload": {
+        "files": [
+            "registration.php"
+        ],
+        "psr-4": {
+            "Alpine\\Maintenance\\\\": ""
+        }
+    }
+}

etc/di.xml

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+<?xml version="1.0"?>
+<!--
+/**
+ * Alpine_Maintenance
+ *
+ * @copyright   Copyright (c) 2020 Alpine Consulting, Inc (www.alpineinc.com)
+ * @author      Michal Zymela ([email protected])
+ */
+-->
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
+    <type name="Magento\Framework\App\Http">
+        <plugin name="alpine_maintenance_http_plugin" type="Alpine\Maintenance\Plugin\HttpPlugin" sortOrder="99" />
+    </type>
+</config>

etc/module.xml

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+<?xml version="1.0"?>
+<!--
+/**
+ * Alpine_Maintenance
+ *
+ * @copyright   Copyright (c) 2020 Alpine Consulting, Inc (www.alpineinc.com)
+ * @author      Michal Zymela ([email protected])
+ */
+-->
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
+    <module name="Alpine_Maintenance" setup_version="1.0.0"/>
+</config>

registration.php

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+<?php
+/**
+ * Alpine_Maintenance
+ *
+ * @copyright   Copyright (c) 2020 Alpine Consulting, Inc (www.alpineinc.com)
+ * @author      Michal Zymela ([email protected])
+ */
+
+\Magento\Framework\Component\ComponentRegistrar::register(
+    \Magento\Framework\Component\ComponentRegistrar::MODULE,
+    'Alpine_Maintenance',
+    __DIR__
+);