feat: Add persistent storage support for Redis and APCu (v0.4.0)

- Add StorageFactory class for storage adapter selection with fallback
- Support Redis storage for shared metrics across instances
- Support APCu storage for high-performance single-server deployments
- Add Storage tab in admin settings with configuration UI
- Add connection testing for Redis and APCu adapters
- Support environment variables for Docker/containerized deployments
- Update Collector to use StorageFactory instead of hardcoded InMemory
- Add all translations (English and German)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,38 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.4.0] - 2026-02-02
+
+### Added
+
+- Persistent Storage Support:
+  - Redis storage adapter for shared metrics across multiple instances
+  - APCu storage adapter for single-server high-performance caching
+  - StorageFactory class for automatic adapter selection and fallback
+  - Connection testing with detailed error messages
+- New "Storage" tab in admin settings:
+  - Storage adapter selection (In-Memory, Redis, APCu)
+  - Redis configuration (host, port, password, database, key prefix)
+  - APCu configuration (key prefix)
+  - Connection test button
+  - Environment variables documentation
+- Environment variable configuration for Docker/containerized environments:
+  - `WP_PROMETHEUS_STORAGE_ADAPTER` - Select storage adapter
+  - `WP_PROMETHEUS_REDIS_HOST` - Redis server hostname
+  - `WP_PROMETHEUS_REDIS_PORT` - Redis server port
+  - `WP_PROMETHEUS_REDIS_PASSWORD` - Redis authentication
+  - `WP_PROMETHEUS_REDIS_DATABASE` - Redis database index (0-15)
+  - `WP_PROMETHEUS_REDIS_PREFIX` - Redis key prefix
+  - `WP_PROMETHEUS_APCU_PREFIX` - APCu key prefix
+- Automatic fallback to In-Memory storage if configured adapter fails
+- Docker Compose example in admin settings
+
+### Changed
+
+- Settings page now has 6 tabs: License, Metrics, Storage, Custom Metrics, Dashboards, Help
+- Updated translations with all new strings (English and German)
+- Collector now uses StorageFactory for storage adapter instantiation
+
 ## [0.3.0] - 2026-02-02
 
 ### Added

CLAUDE.md

@@ -234,7 +234,8 @@ wp-prometheus/
 │   ├── Metrics/
 │   │   ├── Collector.php        # Prometheus metrics collector
 │   │   ├── CustomMetricBuilder.php # Custom metric CRUD
-│   │   └── RuntimeCollector.php # Runtime metrics collector
+│   │   ├── RuntimeCollector.php # Runtime metrics collector
+│   │   └── StorageFactory.php   # Storage adapter factory
 │   ├── Installer.php         # Activation/Deactivation
 │   ├── Plugin.php            # Main plugin class
 │   └── index.php
@@ -290,6 +291,34 @@ add_action( 'wp_prometheus_collect_metrics', function( $collector ) {
 
 ## Session History
 
+### 2026-02-02 - Persistent Storage (v0.4.0)
+
+- Added persistent storage support for metrics:
+  - `StorageFactory.php` - Factory class for storage adapter instantiation
+  - Redis storage adapter for shared metrics across multiple instances
+  - APCu storage adapter for single-server high-performance caching
+  - Automatic fallback to In-Memory if configured adapter fails
+- Added new "Storage" tab in admin settings:
+  - Storage adapter selection (In-Memory, Redis, APCu)
+  - Redis configuration (host, port, password, database, key prefix)
+  - APCu configuration (key prefix)
+  - Connection test button with detailed error messages
+- Added environment variable support for Docker deployments:
+  - `WP_PROMETHEUS_STORAGE_ADAPTER` - Adapter selection
+  - `WP_PROMETHEUS_REDIS_HOST`, `_PORT`, `_PASSWORD`, `_DATABASE`, `_PREFIX`
+  - `WP_PROMETHEUS_APCU_PREFIX`
+  - Environment variables take precedence over admin settings
+- Updated `Collector.php` to use `StorageFactory::get_adapter()`
+- Updated Help tab with storage backends documentation
+- Updated translation files with all new strings
+- **Key Learning**: promphp/prometheus_client_php storage adapters
+  - Redis adapter requires options array with host, port, password, timeout
+  - APCu adapter just needs a prefix string
+  - Use `Redis::setPrefix()` before instantiation for custom key prefixes
+- **Key Learning**: Docker environment variable configuration
+  - Use `getenv()` with explicit false check (`false !== getenv()`)
+  - Environment variables should override WordPress options for containerized deployments
+
 ### 2026-02-02 - Custom Metrics & Dashboards (v0.3.0)
 
 - Added Custom Metric Builder with full admin UI:

PLAN.md

@@ -59,6 +59,7 @@ wp-prometheus/
 │   └── release.yml          # CI/CD pipeline
 ├── assets/
 │   ├── css/                  # Admin/Frontend styles
+│   ├── dashboards/           # Grafana dashboard templates
 │   └── js/
 │       └── admin.js          # Admin JavaScript
 ├── languages/                # Translation files
@@ -67,13 +68,17 @@ wp-prometheus/
 ├── releases/                 # Release packages
 ├── src/
 │   ├── Admin/
+│   │   ├── DashboardProvider.php
 │   │   └── Settings.php
 │   ├── Endpoint/
 │   │   └── MetricsEndpoint.php
 │   ├── License/
 │   │   └── Manager.php
 │   ├── Metrics/
-│   │   └── Collector.php
+│   │   ├── Collector.php
+│   │   ├── CustomMetricBuilder.php
+│   │   ├── RuntimeCollector.php
+│   │   └── StorageFactory.php
 │   ├── Installer.php
 │   ├── Plugin.php
 │   └── index.php
@@ -159,13 +164,57 @@ Alternatively, the token can be passed as a query parameter (for testing):
 https://example.com/metrics/?token=your-auth-token
 ```
 
+## Storage Configuration
+
+The plugin supports multiple storage backends for Prometheus metrics:
+
+### Available Adapters
+
+| Adapter   | Description                     | Use Case                              |
+| --------- | ------------------------------- | ------------------------------------- |
+| In-Memory | Default, no persistence         | Development, single request metrics   |
+| Redis     | Shared storage across instances | Production, load-balanced environments|
+| APCu      | High-performance local cache    | Production, single-server deployments |
+
+### Environment Variables
+
+For Docker or containerized environments, configure storage via environment variables:
+
+```bash
+# Storage adapter selection
+WP_PROMETHEUS_STORAGE_ADAPTER=redis
+
+# Redis configuration
+WP_PROMETHEUS_REDIS_HOST=redis
+WP_PROMETHEUS_REDIS_PORT=6379
+WP_PROMETHEUS_REDIS_PASSWORD=secret
+WP_PROMETHEUS_REDIS_DATABASE=0
+WP_PROMETHEUS_REDIS_PREFIX=WORDPRESS_PROMETHEUS_
+
+# APCu configuration
+WP_PROMETHEUS_APCU_PREFIX=wp_prom
+```
+
+### Docker Compose Example
+
+```yaml
+services:
+  wordpress:
+    image: wordpress:latest
+    environment:
+      WP_PROMETHEUS_STORAGE_ADAPTER: redis
+      WP_PROMETHEUS_REDIS_HOST: redis
+      WP_PROMETHEUS_REDIS_PORT: 6379
+    depends_on:
+      - redis
+
+  redis:
+    image: redis:alpine
+```
+
 ## Future Enhancements
 
-### Version 0.3.0
-
-- Custom metric builder in admin
-- Metric export/import
-- Grafana dashboard templates
+*No planned features at this time.*
 
 ## Dependencies
 

assets/js/admin.js

@@ -21,6 +21,9 @@
 
         // Runtime metrics reset handler.
         initResetRuntimeHandler();
+
+        // Storage tab handlers.
+        initStorageHandlers();
     });
 
     /**
@@ -613,4 +616,131 @@
         document.body.removeChild(a);
         URL.revokeObjectURL(url);
     }
+
+    /**
+     * Initialize storage tab handlers.
+     */
+    function initStorageHandlers() {
+        var $form = $('#wp-prometheus-storage-form');
+        var $adapterSelect = $('#storage-adapter');
+
+        // Show/hide adapter-specific config.
+        $adapterSelect.on('change', function() {
+            var adapter = $(this).val();
+            $('#redis-config').toggle(adapter === 'redis');
+            $('#apcu-config').toggle(adapter === 'apcu');
+        });
+
+        // Save storage settings.
+        $form.on('submit', function(e) {
+            e.preventDefault();
+            saveStorageSettings();
+        });
+
+        // Test storage connection.
+        $('#test-storage').on('click', function() {
+            testStorageConnection();
+        });
+    }
+
+    /**
+     * Save storage settings via AJAX.
+     */
+    function saveStorageSettings() {
+        var $spinner = $('#wp-prometheus-storage-spinner');
+        var $message = $('#wp-prometheus-storage-message');
+        var $form = $('#wp-prometheus-storage-form');
+
+        $spinner.addClass('is-active');
+        $message.hide();
+
+        var formData = $form.serialize();
+        formData += '&action=wp_prometheus_save_storage';
+        formData += '&nonce=' + wpPrometheus.storageNonce;
+
+        $.ajax({
+            url: wpPrometheus.ajaxUrl,
+            type: 'POST',
+            data: formData,
+            success: function(response) {
+                $spinner.removeClass('is-active');
+
+                if (response.success) {
+                    var noticeClass = response.data.warning ? 'notice-warning' : 'notice-success';
+                    $message
+                        .removeClass('notice-error notice-success notice-warning')
+                        .addClass('notice ' + noticeClass)
+                        .html('<p>' + response.data.message + '</p>')
+                        .show();
+
+                    if (!response.data.warning) {
+                        setTimeout(function() {
+                            location.reload();
+                        }, 1500);
+                    }
+                } else {
+                    $message
+                        .removeClass('notice-success notice-warning')
+                        .addClass('notice notice-error')
+                        .html('<p>' + (response.data.message || 'An error occurred.') + '</p>')
+                        .show();
+                }
+            },
+            error: function() {
+                $spinner.removeClass('is-active');
+                $message
+                    .removeClass('notice-success notice-warning')
+                    .addClass('notice notice-error')
+                    .html('<p>Connection error. Please try again.</p>')
+                    .show();
+            }
+        });
+    }
+
+    /**
+     * Test storage connection via AJAX.
+     */
+    function testStorageConnection() {
+        var $spinner = $('#wp-prometheus-storage-spinner');
+        var $message = $('#wp-prometheus-storage-message');
+        var $form = $('#wp-prometheus-storage-form');
+
+        $spinner.addClass('is-active');
+        $message.hide();
+
+        var formData = $form.serialize();
+        formData += '&action=wp_prometheus_test_storage';
+        formData += '&nonce=' + wpPrometheus.storageNonce;
+
+        $.ajax({
+            url: wpPrometheus.ajaxUrl,
+            type: 'POST',
+            data: formData,
+            success: function(response) {
+                $spinner.removeClass('is-active');
+
+                if (response.success) {
+                    $message
+                        .removeClass('notice-error notice-warning')
+                        .addClass('notice notice-success')
+                        .html('<p>' + response.data.message + '</p>')
+                        .show();
+                } else {
+                    $message
+                        .removeClass('notice-success notice-warning')
+                        .addClass('notice notice-error')
+                        .html('<p>' + (response.data.message || 'Connection test failed.') + '</p>')
+                        .show();
+                }
+            },
+            error: function() {
+                $spinner.removeClass('is-active');
+                $message
+                    .removeClass('notice-success notice-warning')
+                    .addClass('notice notice-error')
+                    .html('<p>Connection error. Please try again.</p>')
+                    .show();
+            }
+        });
+    }
 })(jQuery);

languages/wp-prometheus-de_CH.po

@@ -3,7 +3,7 @@
 # This file is distributed under the GPL v2 or later.
 msgid ""
 msgstr ""
-"Project-Id-Version: WP Prometheus 0.3.0\n"
+"Project-Id-Version: WP Prometheus 0.4.0\n"
 "Report-Msgid-Bugs-To: https://src.bundespruefstelle.ch/magdev/wp-prometheus/issues\n"
 "POT-Creation-Date: 2026-02-02T00:00:00+00:00\n"
 "PO-Revision-Date: 2026-02-02T00:00:00+00:00\n"
@@ -617,3 +617,265 @@ msgstr "WP Prometheus erfordert installierte Composer-Abhaengigkeiten. Bitte fue
 #: wp-prometheus.php
 msgid "WP Prometheus requires PHP version %s or higher."
 msgstr "WP Prometheus erfordert PHP-Version %s oder hoeher."
+
+#: src/Admin/Settings.php
+msgid "Storage"
+msgstr "Speicher"
+
+#: src/Admin/Settings.php
+msgid "Metrics Storage Configuration"
+msgstr "Metriken-Speicherkonfiguration"
+
+#: src/Admin/Settings.php
+msgid "Configure how Prometheus metrics are stored. Persistent storage (Redis, APCu) allows metrics to survive between requests and aggregate data over time."
+msgstr "Konfigurieren Sie, wie Prometheus-Metriken gespeichert werden. Persistenter Speicher (Redis, APCu) ermoeglicht es, Metriken zwischen Anfragen zu erhalten und Daten ueber Zeit zu aggregieren."
+
+#: src/Admin/Settings.php
+msgid "Environment Override Active"
+msgstr "Umgebungsvariablen-Ueberschreibung aktiv"
+
+#: src/Admin/Settings.php
+msgid "Storage adapter is configured via environment variable. Admin settings will be ignored."
+msgstr "Speicher-Adapter ist ueber Umgebungsvariable konfiguriert. Admin-Einstellungen werden ignoriert."
+
+#: src/Admin/Settings.php
+msgid "Storage Fallback Active"
+msgstr "Speicher-Fallback aktiv"
+
+#: src/Admin/Settings.php
+msgid "Falling back to In-Memory storage."
+msgstr "Faellt zurueck auf In-Memory-Speicher."
+
+#: src/Admin/Settings.php
+msgid "Current Status:"
+msgstr "Aktueller Status:"
+
+#. translators: %s: Active adapter name
+#: src/Admin/Settings.php
+msgid "Using %s storage."
+msgstr "Verwende %s-Speicher."
+
+#: src/Admin/Settings.php
+msgid "Storage Adapter"
+msgstr "Speicher-Adapter"
+
+#: src/Admin/Settings.php
+msgid "unavailable"
+msgstr "nicht verfuegbar"
+
+#: src/Admin/Settings.php
+msgid "Select the storage backend for metrics. Redis and APCu require their respective PHP extensions."
+msgstr "Waehlen Sie das Speicher-Backend fuer Metriken. Redis und APCu erfordern ihre jeweiligen PHP-Erweiterungen."
+
+#: src/Admin/Settings.php
+msgid "Redis Configuration"
+msgstr "Redis-Konfiguration"
+
+#: src/Admin/Settings.php
+msgid "Host"
+msgstr "Host"
+
+#. translators: %s: Environment variable name
+#: src/Admin/Settings.php
+msgid "Can be overridden with %s environment variable."
+msgstr "Kann mit Umgebungsvariable %s ueberschrieben werden."
+
+#: src/Admin/Settings.php
+msgid "Port"
+msgstr "Port"
+
+#: src/Admin/Settings.php
+msgid "Password"
+msgstr "Passwort"
+
+#: src/Admin/Settings.php
+msgid "Leave empty if not required"
+msgstr "Leer lassen, falls nicht erforderlich"
+
+#: src/Admin/Settings.php
+msgid "Database"
+msgstr "Datenbank"
+
+#. translators: %s: Environment variable name
+#: src/Admin/Settings.php
+msgid "Redis database index (0-15). Can be overridden with %s."
+msgstr "Redis-Datenbankindex (0-15). Kann mit %s ueberschrieben werden."
+
+#: src/Admin/Settings.php
+msgid "Key Prefix"
+msgstr "Schluessel-Praefix"
+
+#: src/Admin/Settings.php
+msgid "Prefix for Redis keys. Useful when sharing Redis with other applications."
+msgstr "Praefix fuer Redis-Schluessel. Nuetzlich bei gemeinsamer Redis-Nutzung mit anderen Anwendungen."
+
+#: src/Admin/Settings.php
+msgid "APCu Configuration"
+msgstr "APCu-Konfiguration"
+
+#. translators: %s: Environment variable name
+#: src/Admin/Settings.php
+msgid "Prefix for APCu keys. Can be overridden with %s."
+msgstr "Praefix fuer APCu-Schluessel. Kann mit %s ueberschrieben werden."
+
+#: src/Admin/Settings.php
+msgid "Save Storage Settings"
+msgstr "Speicher-Einstellungen speichern"
+
+#: src/Admin/Settings.php
+msgid "Test Connection"
+msgstr "Verbindung testen"
+
+#: src/Admin/Settings.php
+msgid "Environment Variables"
+msgstr "Umgebungsvariablen"
+
+#: src/Admin/Settings.php
+msgid "For Docker or containerized environments, you can configure storage using environment variables. These take precedence over admin settings."
+msgstr "Fuer Docker- oder Container-Umgebungen koennen Sie den Speicher ueber Umgebungsvariablen konfigurieren. Diese haben Vorrang vor Admin-Einstellungen."
+
+#: src/Admin/Settings.php
+msgid "Variable"
+msgstr "Variable"
+
+#: src/Admin/Settings.php
+msgid "Example"
+msgstr "Beispiel"
+
+#: src/Admin/Settings.php
+msgid "Storage adapter to use"
+msgstr "Zu verwendender Speicher-Adapter"
+
+#: src/Admin/Settings.php
+msgid "Redis server hostname"
+msgstr "Redis-Server-Hostname"
+
+#: src/Admin/Settings.php
+msgid "Redis server port"
+msgstr "Redis-Server-Port"
+
+#: src/Admin/Settings.php
+msgid "Redis authentication password"
+msgstr "Redis-Authentifizierungspasswort"
+
+#: src/Admin/Settings.php
+msgid "Redis database index"
+msgstr "Redis-Datenbankindex"
+
+#: src/Admin/Settings.php
+msgid "Redis key prefix"
+msgstr "Redis-Schluessel-Praefix"
+
+#: src/Admin/Settings.php
+msgid "APCu key prefix"
+msgstr "APCu-Schluessel-Praefix"
+
+#: src/Admin/Settings.php
+msgid "Docker Compose Example"
+msgstr "Docker Compose-Beispiel"
+
+#: src/Admin/Settings.php
+msgid "Permission denied."
+msgstr "Zugriff verweigert."
+
+#: src/Admin/Settings.php
+msgid "Storage adapter is configured via environment variable and cannot be changed."
+msgstr "Speicher-Adapter ist ueber Umgebungsvariable konfiguriert und kann nicht geaendert werden."
+
+#: src/Admin/Settings.php
+msgid "Invalid storage adapter."
+msgstr "Ungueltiger Speicher-Adapter."
+
+#: src/Admin/Settings.php
+msgid "Storage settings saved successfully."
+msgstr "Speicher-Einstellungen erfolgreich gespeichert."
+
+#: src/Admin/Settings.php
+msgid "Storage settings saved, but connection test failed:"
+msgstr "Speicher-Einstellungen gespeichert, aber Verbindungstest fehlgeschlagen:"
+
+#: src/Metrics/StorageFactory.php
+msgid "In-Memory (default, no persistence)"
+msgstr "In-Memory (Standard, keine Persistenz)"
+
+#: src/Metrics/StorageFactory.php
+msgid "Redis (requires PHP Redis extension)"
+msgstr "Redis (erfordert PHP-Redis-Erweiterung)"
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu (requires APCu extension)"
+msgstr "APCu (erfordert APCu-Erweiterung)"
+
+#: src/Metrics/StorageFactory.php
+msgid "PHP Redis extension is not installed."
+msgstr "PHP-Redis-Erweiterung ist nicht installiert."
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "Redis connection failed: %s"
+msgstr "Redis-Verbindung fehlgeschlagen: %s"
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "Redis error: %s"
+msgstr "Redis-Fehler: %s"
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "Storage error: %s"
+msgstr "Speicherfehler: %s"
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu extension is not installed."
+msgstr "APCu-Erweiterung ist nicht installiert."
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu is installed but not enabled."
+msgstr "APCu ist installiert, aber nicht aktiviert."
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "APCu error: %s"
+msgstr "APCu-Fehler: %s"
+
+#: src/Metrics/StorageFactory.php
+msgid "In-Memory storage is always available."
+msgstr "In-Memory-Speicher ist immer verfuegbar."
+
+#: src/Metrics/StorageFactory.php
+msgid "Unknown storage adapter."
+msgstr "Unbekannter Speicher-Adapter."
+
+#: src/Metrics/StorageFactory.php
+msgid "Could not connect to Redis server."
+msgstr "Verbindung zum Redis-Server konnte nicht hergestellt werden."
+
+#: src/Metrics/StorageFactory.php
+msgid "Redis authentication failed."
+msgstr "Redis-Authentifizierung fehlgeschlagen."
+
+#. translators: %s: Redis host:port
+#: src/Metrics/StorageFactory.php
+msgid "Successfully connected to Redis at %s."
+msgstr "Erfolgreich mit Redis verbunden unter %s."
+
+#: src/Metrics/StorageFactory.php
+msgid "Redis ping failed."
+msgstr "Redis-Ping fehlgeschlagen."
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu is installed but not enabled. Check your php.ini settings."
+msgstr "APCu ist installiert, aber nicht aktiviert. Pruefen Sie Ihre php.ini-Einstellungen."
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu store operation failed."
+msgstr "APCu-Speicheroperation fehlgeschlagen."
+
+#. translators: %s: Memory info
+#: src/Metrics/StorageFactory.php
+msgid "APCu is working. Memory: %s used."
+msgstr "APCu funktioniert. Speicher: %s belegt."
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu fetch operation returned unexpected value."
+msgstr "APCu-Abrufoperation hat unerwarteten Wert zurueckgegeben."

languages/wp-prometheus.pot

@@ -2,7 +2,7 @@
 # This file is distributed under the GPL v2 or later.
 msgid ""
 msgstr ""
-"Project-Id-Version: WP Prometheus 0.3.0\n"
+"Project-Id-Version: WP Prometheus 0.4.0\n"
 "Report-Msgid-Bugs-To: https://src.bundespruefstelle.ch/magdev/wp-prometheus/issues\n"
 "POT-Creation-Date: 2026-02-02T00:00:00+00:00\n"
 "MIME-Version: 1.0\n"
@@ -614,3 +614,265 @@ msgstr ""
 #: wp-prometheus.php
 msgid "WP Prometheus requires PHP version %s or higher."
 msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Metrics Storage Configuration"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Configure how Prometheus metrics are stored. Persistent storage (Redis, APCu) allows metrics to survive between requests and aggregate data over time."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Environment Override Active"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage adapter is configured via environment variable. Admin settings will be ignored."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage Fallback Active"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Falling back to In-Memory storage."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Current Status:"
+msgstr ""
+
+#. translators: %s: Active adapter name
+#: src/Admin/Settings.php
+msgid "Using %s storage."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage Adapter"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "unavailable"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Select the storage backend for metrics. Redis and APCu require their respective PHP extensions."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis Configuration"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Host"
+msgstr ""
+
+#. translators: %s: Environment variable name
+#: src/Admin/Settings.php
+msgid "Can be overridden with %s environment variable."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Port"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Password"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Leave empty if not required"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Database"
+msgstr ""
+
+#. translators: %s: Environment variable name
+#: src/Admin/Settings.php
+msgid "Redis database index (0-15). Can be overridden with %s."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Key Prefix"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Prefix for Redis keys. Useful when sharing Redis with other applications."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "APCu Configuration"
+msgstr ""
+
+#. translators: %s: Environment variable name
+#: src/Admin/Settings.php
+msgid "Prefix for APCu keys. Can be overridden with %s."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Save Storage Settings"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Test Connection"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Environment Variables"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "For Docker or containerized environments, you can configure storage using environment variables. These take precedence over admin settings."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Variable"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Example"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage adapter to use"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis server hostname"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis server port"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis authentication password"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis database index"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Redis key prefix"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "APCu key prefix"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Docker Compose Example"
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Permission denied."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage adapter is configured via environment variable and cannot be changed."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Invalid storage adapter."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage settings saved successfully."
+msgstr ""
+
+#: src/Admin/Settings.php
+msgid "Storage settings saved, but connection test failed:"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "In-Memory (default, no persistence)"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Redis (requires PHP Redis extension)"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu (requires APCu extension)"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "PHP Redis extension is not installed."
+msgstr ""
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "Redis connection failed: %s"
+msgstr ""
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "Redis error: %s"
+msgstr ""
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "Storage error: %s"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu extension is not installed."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu is installed but not enabled."
+msgstr ""
+
+#. translators: %s: Error message
+#: src/Metrics/StorageFactory.php
+msgid "APCu error: %s"
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "In-Memory storage is always available."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Unknown storage adapter."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Could not connect to Redis server."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Redis authentication failed."
+msgstr ""
+
+#. translators: %s: Redis host:port
+#: src/Metrics/StorageFactory.php
+msgid "Successfully connected to Redis at %s."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "Redis ping failed."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu is installed but not enabled. Check your php.ini settings."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu store operation failed."
+msgstr ""
+
+#. translators: %s: Memory info
+#: src/Metrics/StorageFactory.php
+msgid "APCu is working. Memory: %s used."
+msgstr ""
+
+#: src/Metrics/StorageFactory.php
+msgid "APCu fetch operation returned unexpected value."
+msgstr ""

src/Admin/Settings.php

@@ -10,6 +10,7 @@ namespace Magdev\WpPrometheus\Admin;
 use Magdev\WpPrometheus\License\Manager as LicenseManager;
 use Magdev\WpPrometheus\Metrics\CustomMetricBuilder;
 use Magdev\WpPrometheus\Metrics\RuntimeCollector;
+use Magdev\WpPrometheus\Metrics\StorageFactory;
 
 // Prevent direct file access.
 if ( ! defined( 'ABSPATH' ) ) {
@@ -51,6 +52,7 @@ class Settings {
 		$this->tabs = array(
 			'license'    => __( 'License', 'wp-prometheus' ),
 			'metrics'    => __( 'Metrics', 'wp-prometheus' ),
+			'storage'    => __( 'Storage', 'wp-prometheus' ),
 			'custom'     => __( 'Custom Metrics', 'wp-prometheus' ),
 			'dashboards' => __( 'Dashboards', 'wp-prometheus' ),
 			'help'       => __( 'Help', 'wp-prometheus' ),
@@ -70,6 +72,8 @@ class Settings {
 		add_action( 'wp_ajax_wp_prometheus_import_metrics', array( $this, 'ajax_import_metrics' ) );
 		add_action( 'wp_ajax_wp_prometheus_download_dashboard', array( $this, 'ajax_download_dashboard' ) );
 		add_action( 'wp_ajax_wp_prometheus_reset_runtime_metrics', array( $this, 'ajax_reset_runtime_metrics' ) );
+		add_action( 'wp_ajax_wp_prometheus_save_storage', array( $this, 'ajax_save_storage' ) );
+		add_action( 'wp_ajax_wp_prometheus_test_storage', array( $this, 'ajax_test_storage' ) );
 	}
 
 	/**
@@ -183,6 +187,7 @@ class Settings {
 			'importNonce'            => wp_create_nonce( 'wp_prometheus_import' ),
 			'dashboardNonce'         => wp_create_nonce( 'wp_prometheus_dashboard' ),
 			'resetRuntimeNonce'      => wp_create_nonce( 'wp_prometheus_reset_runtime' ),
+			'storageNonce'           => wp_create_nonce( 'wp_prometheus_storage' ),
 			'confirmDelete'          => __( 'Are you sure you want to delete this metric?', 'wp-prometheus' ),
 			'confirmReset'           => __( 'Are you sure you want to reset all runtime metrics? This cannot be undone.', 'wp-prometheus' ),
 			'confirmRegenerateToken' => __( 'Are you sure you want to regenerate the auth token? You will need to update your Prometheus configuration.', 'wp-prometheus' ),
@@ -226,6 +231,9 @@ class Settings {
 					case 'metrics':
 						$this->render_metrics_tab();
 						break;
+					case 'storage':
+						$this->render_storage_tab();
+						break;
 					case 'custom':
 						$this->render_custom_metrics_tab();
 						break;
@@ -415,6 +423,282 @@ class Settings {
 		<?php
 	}
 
+	/**
+	 * Render storage tab content.
+	 *
+	 * @return void
+	 */
+	private function render_storage_tab(): void {
+		$configured_adapter = StorageFactory::get_configured_adapter();
+		$active_adapter     = StorageFactory::get_active_adapter();
+		$last_error         = StorageFactory::get_last_error();
+		$redis_config       = StorageFactory::get_redis_config();
+		$apcu_prefix        = StorageFactory::get_apcu_prefix();
+		$adapters           = StorageFactory::get_available_adapters();
+
+		// Check environment variable overrides.
+		$env_override = false !== getenv( 'WP_PROMETHEUS_STORAGE_ADAPTER' );
+		?>
+		<div class="wp-prometheus-storage">
+			<h2><?php esc_html_e( 'Metrics Storage Configuration', 'wp-prometheus' ); ?></h2>
+			<p class="description">
+				<?php esc_html_e( 'Configure how Prometheus metrics are stored. Persistent storage (Redis, APCu) allows metrics to survive between requests and aggregate data over time.', 'wp-prometheus' ); ?>
+			</p>
+
+			<?php if ( $env_override ) : ?>
+				<div class="notice notice-info" style="padding: 12px; margin: 15px 0;">
+					<strong><?php esc_html_e( 'Environment Override Active', 'wp-prometheus' ); ?></strong>
+					<p><?php esc_html_e( 'Storage adapter is configured via environment variable. Admin settings will be ignored.', 'wp-prometheus' ); ?></p>
+				</div>
+			<?php endif; ?>
+
+			<?php if ( ! empty( $last_error ) ) : ?>
+				<div class="notice notice-warning" style="padding: 12px; margin: 15px 0;">
+					<strong><?php esc_html_e( 'Storage Fallback Active', 'wp-prometheus' ); ?></strong>
+					<p><?php echo esc_html( $last_error ); ?></p>
+					<p><?php esc_html_e( 'Falling back to In-Memory storage.', 'wp-prometheus' ); ?></p>
+				</div>
+			<?php endif; ?>
+
+			<div class="notice notice-<?php echo $active_adapter === $configured_adapter && empty( $last_error ) ? 'success' : 'warning'; ?>" style="padding: 12px; margin: 15px 0;">
+				<strong><?php esc_html_e( 'Current Status:', 'wp-prometheus' ); ?></strong>
+				<?php
+				printf(
+					/* translators: %s: Active adapter name */
+					esc_html__( 'Using %s storage.', 'wp-prometheus' ),
+					'<code>' . esc_html( ucfirst( $active_adapter ) ) . '</code>'
+				);
+				?>
+			</div>
+
+			<form id="wp-prometheus-storage-form">
+				<table class="form-table" role="presentation">
+					<tr>
+						<th scope="row">
+							<label for="storage-adapter"><?php esc_html_e( 'Storage Adapter', 'wp-prometheus' ); ?></label>
+						</th>
+						<td>
+							<select name="adapter" id="storage-adapter" <?php disabled( $env_override ); ?>>
+								<?php foreach ( $adapters as $key => $label ) : ?>
+									<option value="<?php echo esc_attr( $key ); ?>" <?php selected( $configured_adapter, $key ); ?>>
+										<?php echo esc_html( $label ); ?>
+										<?php if ( ! StorageFactory::is_adapter_available( $key ) && 'inmemory' !== $key ) : ?>
+											(<?php esc_html_e( 'unavailable', 'wp-prometheus' ); ?>)
+										<?php endif; ?>
+									</option>
+								<?php endforeach; ?>
+							</select>
+							<p class="description">
+								<?php esc_html_e( 'Select the storage backend for metrics. Redis and APCu require their respective PHP extensions.', 'wp-prometheus' ); ?>
+							</p>
+						</td>
+					</tr>
+				</table>
+
+				<div id="redis-config" style="<?php echo 'redis' === $configured_adapter ? '' : 'display: none;'; ?>">
+					<h3><?php esc_html_e( 'Redis Configuration', 'wp-prometheus' ); ?></h3>
+					<table class="form-table" role="presentation">
+						<tr>
+							<th scope="row">
+								<label for="redis-host"><?php esc_html_e( 'Host', 'wp-prometheus' ); ?></label>
+							</th>
+							<td>
+								<input type="text" name="redis_host" id="redis-host" class="regular-text"
+									   value="<?php echo esc_attr( $redis_config['host'] ); ?>"
+									   placeholder="127.0.0.1">
+								<p class="description">
+									<?php
+									printf(
+										/* translators: %s: Environment variable name */
+										esc_html__( 'Can be overridden with %s environment variable.', 'wp-prometheus' ),
+										'<code>WP_PROMETHEUS_REDIS_HOST</code>'
+									);
+									?>
+								</p>
+							</td>
+						</tr>
+						<tr>
+							<th scope="row">
+								<label for="redis-port"><?php esc_html_e( 'Port', 'wp-prometheus' ); ?></label>
+							</th>
+							<td>
+								<input type="number" name="redis_port" id="redis-port" class="small-text"
+									   value="<?php echo esc_attr( $redis_config['port'] ); ?>"
+									   placeholder="6379" min="1" max="65535">
+								<p class="description">
+									<?php
+									printf(
+										/* translators: %s: Environment variable name */
+										esc_html__( 'Can be overridden with %s environment variable.', 'wp-prometheus' ),
+										'<code>WP_PROMETHEUS_REDIS_PORT</code>'
+									);
+									?>
+								</p>
+							</td>
+						</tr>
+						<tr>
+							<th scope="row">
+								<label for="redis-password"><?php esc_html_e( 'Password', 'wp-prometheus' ); ?></label>
+							</th>
+							<td>
+								<input type="password" name="redis_password" id="redis-password" class="regular-text"
+									   value="<?php echo esc_attr( $redis_config['password'] ); ?>"
+									   placeholder="<?php esc_attr_e( 'Leave empty if not required', 'wp-prometheus' ); ?>">
+								<p class="description">
+									<?php
+									printf(
+										/* translators: %s: Environment variable name */
+										esc_html__( 'Can be overridden with %s environment variable.', 'wp-prometheus' ),
+										'<code>WP_PROMETHEUS_REDIS_PASSWORD</code>'
+									);
+									?>
+								</p>
+							</td>
+						</tr>
+						<tr>
+							<th scope="row">
+								<label for="redis-database"><?php esc_html_e( 'Database', 'wp-prometheus' ); ?></label>
+							</th>
+							<td>
+								<input type="number" name="redis_database" id="redis-database" class="small-text"
+									   value="<?php echo esc_attr( $redis_config['database'] ); ?>"
+									   placeholder="0" min="0" max="15">
+								<p class="description">
+									<?php
+									printf(
+										/* translators: %s: Environment variable name */
+										esc_html__( 'Redis database index (0-15). Can be overridden with %s.', 'wp-prometheus' ),
+										'<code>WP_PROMETHEUS_REDIS_DATABASE</code>'
+									);
+									?>
+								</p>
+							</td>
+						</tr>
+						<tr>
+							<th scope="row">
+								<label for="redis-prefix"><?php esc_html_e( 'Key Prefix', 'wp-prometheus' ); ?></label>
+							</th>
+							<td>
+								<input type="text" name="redis_prefix" id="redis-prefix" class="regular-text"
+									   value="<?php echo esc_attr( $redis_config['prefix'] ); ?>"
+									   placeholder="WORDPRESS_PROMETHEUS_">
+								<p class="description">
+									<?php esc_html_e( 'Prefix for Redis keys. Useful when sharing Redis with other applications.', 'wp-prometheus' ); ?>
+								</p>
+							</td>
+						</tr>
+					</table>
+				</div>
+
+				<div id="apcu-config" style="<?php echo 'apcu' === $configured_adapter ? '' : 'display: none;'; ?>">
+					<h3><?php esc_html_e( 'APCu Configuration', 'wp-prometheus' ); ?></h3>
+					<table class="form-table" role="presentation">
+						<tr>
+							<th scope="row">
+								<label for="apcu-prefix"><?php esc_html_e( 'Key Prefix', 'wp-prometheus' ); ?></label>
+							</th>
+							<td>
+								<input type="text" name="apcu_prefix" id="apcu-prefix" class="regular-text"
+									   value="<?php echo esc_attr( $apcu_prefix ); ?>"
+									   placeholder="wp_prom">
+								<p class="description">
+									<?php
+									printf(
+										/* translators: %s: Environment variable name */
+										esc_html__( 'Prefix for APCu keys. Can be overridden with %s.', 'wp-prometheus' ),
+										'<code>WP_PROMETHEUS_APCU_PREFIX</code>'
+									);
+									?>
+								</p>
+							</td>
+						</tr>
+					</table>
+				</div>
+
+				<p class="submit">
+					<button type="submit" class="button button-primary" <?php disabled( $env_override ); ?>>
+						<?php esc_html_e( 'Save Storage Settings', 'wp-prometheus' ); ?>
+					</button>
+					<button type="button" id="test-storage" class="button button-secondary">
+						<?php esc_html_e( 'Test Connection', 'wp-prometheus' ); ?>
+					</button>
+					<span id="wp-prometheus-storage-spinner" class="spinner" style="float: none;"></span>
+				</p>
+			</form>
+
+			<div id="wp-prometheus-storage-message" style="display: none; margin-top: 10px;"></div>
+
+			<hr style="margin: 30px 0;">
+
+			<h3><?php esc_html_e( 'Environment Variables', 'wp-prometheus' ); ?></h3>
+			<p class="description">
+				<?php esc_html_e( 'For Docker or containerized environments, you can configure storage using environment variables. These take precedence over admin settings.', 'wp-prometheus' ); ?>
+			</p>
+
+			<table class="widefat striped" style="margin: 15px 0; max-width: 800px;">
+				<thead>
+					<tr>
+						<th><?php esc_html_e( 'Variable', 'wp-prometheus' ); ?></th>
+						<th><?php esc_html_e( 'Description', 'wp-prometheus' ); ?></th>
+						<th><?php esc_html_e( 'Example', 'wp-prometheus' ); ?></th>
+					</tr>
+				</thead>
+				<tbody>
+					<tr>
+						<td><code>WP_PROMETHEUS_STORAGE_ADAPTER</code></td>
+						<td><?php esc_html_e( 'Storage adapter to use', 'wp-prometheus' ); ?></td>
+						<td><code>redis</code></td>
+					</tr>
+					<tr>
+						<td><code>WP_PROMETHEUS_REDIS_HOST</code></td>
+						<td><?php esc_html_e( 'Redis server hostname', 'wp-prometheus' ); ?></td>
+						<td><code>redis</code></td>
+					</tr>
+					<tr>
+						<td><code>WP_PROMETHEUS_REDIS_PORT</code></td>
+						<td><?php esc_html_e( 'Redis server port', 'wp-prometheus' ); ?></td>
+						<td><code>6379</code></td>
+					</tr>
+					<tr>
+						<td><code>WP_PROMETHEUS_REDIS_PASSWORD</code></td>
+						<td><?php esc_html_e( 'Redis authentication password', 'wp-prometheus' ); ?></td>
+						<td><code>secret123</code></td>
+					</tr>
+					<tr>
+						<td><code>WP_PROMETHEUS_REDIS_DATABASE</code></td>
+						<td><?php esc_html_e( 'Redis database index', 'wp-prometheus' ); ?></td>
+						<td><code>0</code></td>
+					</tr>
+					<tr>
+						<td><code>WP_PROMETHEUS_REDIS_PREFIX</code></td>
+						<td><?php esc_html_e( 'Redis key prefix', 'wp-prometheus' ); ?></td>
+						<td><code>MYSITE_PROM_</code></td>
+					</tr>
+					<tr>
+						<td><code>WP_PROMETHEUS_APCU_PREFIX</code></td>
+						<td><?php esc_html_e( 'APCu key prefix', 'wp-prometheus' ); ?></td>
+						<td><code>wp_prom</code></td>
+					</tr>
+				</tbody>
+			</table>
+
+			<h4><?php esc_html_e( 'Docker Compose Example', 'wp-prometheus' ); ?></h4>
+			<pre style="background: #f1f1f1; padding: 15px; overflow-x: auto; margin: 15px 0;">services:
+  wordpress:
+    image: wordpress:latest
+    environment:
+      WP_PROMETHEUS_STORAGE_ADAPTER: redis
+      WP_PROMETHEUS_REDIS_HOST: redis
+      WP_PROMETHEUS_REDIS_PORT: 6379
+    depends_on:
+      - redis
+
+  redis:
+    image: redis:alpine</pre>
+		</div>
+		<?php
+	}
+
 	/**
 	 * Render custom metrics tab content.
 	 *
@@ -872,6 +1156,38 @@ class Settings {
     );
     $gauge->set( 42, array( 'value1', 'value2' ) );
 } );</pre>
+
+		<h3><?php esc_html_e( 'Storage Backends', 'wp-prometheus' ); ?></h3>
+		<p><?php esc_html_e( 'The plugin supports multiple storage backends for metrics persistence:', 'wp-prometheus' ); ?></p>
+		<table class="widefat striped" style="margin: 15px 0;">
+			<thead>
+				<tr>
+					<th><?php esc_html_e( 'Adapter', 'wp-prometheus' ); ?></th>
+					<th><?php esc_html_e( 'Description', 'wp-prometheus' ); ?></th>
+					<th><?php esc_html_e( 'Use Case', 'wp-prometheus' ); ?></th>
+				</tr>
+			</thead>
+			<tbody>
+				<tr>
+					<td><strong>In-Memory</strong></td>
+					<td><?php esc_html_e( 'Default storage, no persistence between requests', 'wp-prometheus' ); ?></td>
+					<td><?php esc_html_e( 'Development, testing', 'wp-prometheus' ); ?></td>
+				</tr>
+				<tr>
+					<td><strong>Redis</strong></td>
+					<td><?php esc_html_e( 'Shared storage, survives restarts', 'wp-prometheus' ); ?></td>
+					<td><?php esc_html_e( 'Production, load-balanced environments', 'wp-prometheus' ); ?></td>
+				</tr>
+				<tr>
+					<td><strong>APCu</strong></td>
+					<td><?php esc_html_e( 'Fast local cache, process-specific', 'wp-prometheus' ); ?></td>
+					<td><?php esc_html_e( 'Production, single-server deployments', 'wp-prometheus' ); ?></td>
+				</tr>
+			</tbody>
+		</table>
+		<p class="description">
+			<?php esc_html_e( 'Configure storage in the Storage tab. For Docker environments, use environment variables like WP_PROMETHEUS_STORAGE_ADAPTER.', 'wp-prometheus' ); ?>
+		</p>
 		<?php
 	}
 
@@ -1210,4 +1526,102 @@ class Settings {
 
 		wp_send_json_success( array( 'message' => __( 'Runtime metrics have been reset.', 'wp-prometheus' ) ) );
 	}
+
+	/**
+	 * AJAX handler for saving storage settings.
+	 *
+	 * @return void
+	 */
+	public function ajax_save_storage(): void {
+		check_ajax_referer( 'wp_prometheus_storage', 'nonce' );
+
+		if ( ! current_user_can( 'manage_options' ) ) {
+			wp_send_json_error( array( 'message' => __( 'Permission denied.', 'wp-prometheus' ) ) );
+		}
+
+		// Check if environment variable override is active.
+		if ( false !== getenv( 'WP_PROMETHEUS_STORAGE_ADAPTER' ) ) {
+			wp_send_json_error( array( 'message' => __( 'Storage adapter is configured via environment variable and cannot be changed.', 'wp-prometheus' ) ) );
+		}
+
+		$adapter = isset( $_POST['adapter'] ) ? sanitize_key( $_POST['adapter'] ) : 'inmemory';
+
+		// Validate adapter.
+		$valid_adapters = array_keys( StorageFactory::get_available_adapters() );
+		if ( ! in_array( $adapter, $valid_adapters, true ) ) {
+			wp_send_json_error( array( 'message' => __( 'Invalid storage adapter.', 'wp-prometheus' ) ) );
+		}
+
+		// Build config array.
+		$config = array(
+			'adapter' => $adapter,
+		);
+
+		// Redis config.
+		if ( 'redis' === $adapter ) {
+			$config['redis'] = array(
+				'host'     => isset( $_POST['redis_host'] ) ? sanitize_text_field( wp_unslash( $_POST['redis_host'] ) ) : '127.0.0.1',
+				'port'     => isset( $_POST['redis_port'] ) ? absint( $_POST['redis_port'] ) : 6379,
+				'password' => isset( $_POST['redis_password'] ) ? sanitize_text_field( wp_unslash( $_POST['redis_password'] ) ) : '',
+				'database' => isset( $_POST['redis_database'] ) ? absint( $_POST['redis_database'] ) : 0,
+				'prefix'   => isset( $_POST['redis_prefix'] ) ? sanitize_key( $_POST['redis_prefix'] ) : 'WORDPRESS_PROMETHEUS_',
+			);
+		}
+
+		// APCu config.
+		if ( 'apcu' === $adapter ) {
+			$config['apcu_prefix'] = isset( $_POST['apcu_prefix'] ) ? sanitize_key( $_POST['apcu_prefix'] ) : 'wp_prom';
+		}
+
+		// Save configuration.
+		StorageFactory::save_config( $config );
+
+		// Test if the new configuration works.
+		$test_result = StorageFactory::test_connection( $adapter, $config['redis'] ?? array() );
+
+		if ( $test_result['success'] ) {
+			wp_send_json_success( array(
+				'message' => __( 'Storage settings saved successfully.', 'wp-prometheus' ) . ' ' . $test_result['message'],
+			) );
+		} else {
+			wp_send_json_success( array(
+				'message' => __( 'Storage settings saved, but connection test failed:', 'wp-prometheus' ) . ' ' . $test_result['message'],
+				'warning' => true,
+			) );
+		}
+	}
+
+	/**
+	 * AJAX handler for testing storage connection.
+	 *
+	 * @return void
+	 */
+	public function ajax_test_storage(): void {
+		check_ajax_referer( 'wp_prometheus_storage', 'nonce' );
+
+		if ( ! current_user_can( 'manage_options' ) ) {
+			wp_send_json_error( array( 'message' => __( 'Permission denied.', 'wp-prometheus' ) ) );
+		}
+
+		$adapter = isset( $_POST['adapter'] ) ? sanitize_key( $_POST['adapter'] ) : 'inmemory';
+
+		// Build test config from form data.
+		$config = array();
+		if ( 'redis' === $adapter ) {
+			$config = array(
+				'host'     => isset( $_POST['redis_host'] ) ? sanitize_text_field( wp_unslash( $_POST['redis_host'] ) ) : '127.0.0.1',
+				'port'     => isset( $_POST['redis_port'] ) ? absint( $_POST['redis_port'] ) : 6379,
+				'password' => isset( $_POST['redis_password'] ) ? sanitize_text_field( wp_unslash( $_POST['redis_password'] ) ) : '',
+				'database' => isset( $_POST['redis_database'] ) ? absint( $_POST['redis_database'] ) : 0,
+			);
+		}
+
+		$result = StorageFactory::test_connection( $adapter, $config );
+
+		if ( $result['success'] ) {
+			wp_send_json_success( array( 'message' => $result['message'] ) );
+		} else {
+			wp_send_json_error( array( 'message' => $result['message'] ) );
+		}
+	}
 }

src/Metrics/Collector.php

@@ -8,9 +8,9 @@
 namespace Magdev\WpPrometheus\Metrics;
 
 use Prometheus\CollectorRegistry;
-use Prometheus\Storage\InMemory;
 use Prometheus\RenderTextFormat;
 use Magdev\WpPrometheus\Metrics\CustomMetricBuilder;
+use Magdev\WpPrometheus\Metrics\StorageFactory;
 
 // Prevent direct file access.
 if ( ! defined( 'ABSPATH' ) ) {
@@ -42,7 +42,7 @@ class Collector {
      * Constructor.
      */
     public function __construct() {
-        $this->registry = new CollectorRegistry( new InMemory() );
+        $this->registry = new CollectorRegistry( StorageFactory::get_adapter() );
     }
 
     /**

src/Metrics/StorageFactory.php

@@ -0,0 +1,502 @@
+<?php
+/**
+ * Storage factory for Prometheus metrics.
+ *
+ * @package WP_Prometheus
+ */
+
+namespace Magdev\WpPrometheus\Metrics;
+
+use Prometheus\Storage\Adapter;
+use Prometheus\Storage\InMemory;
+use Prometheus\Storage\Redis;
+use Prometheus\Storage\APC;
+use Prometheus\Exception\StorageException;
+
+// Prevent direct file access.
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * StorageFactory class.
+ *
+ * Creates and configures storage adapters for Prometheus metrics.
+ * Supports configuration via WordPress options or environment variables.
+ */
+class StorageFactory {
+
+	/**
+	 * Available storage adapters.
+	 */
+	public const ADAPTER_INMEMORY = 'inmemory';
+	public const ADAPTER_REDIS    = 'redis';
+	public const ADAPTER_APCU     = 'apcu';
+
+	/**
+	 * Environment variable names.
+	 */
+	private const ENV_STORAGE_ADAPTER  = 'WP_PROMETHEUS_STORAGE_ADAPTER';
+	private const ENV_REDIS_HOST       = 'WP_PROMETHEUS_REDIS_HOST';
+	private const ENV_REDIS_PORT       = 'WP_PROMETHEUS_REDIS_PORT';
+	private const ENV_REDIS_PASSWORD   = 'WP_PROMETHEUS_REDIS_PASSWORD';
+	private const ENV_REDIS_DATABASE   = 'WP_PROMETHEUS_REDIS_DATABASE';
+	private const ENV_REDIS_PREFIX     = 'WP_PROMETHEUS_REDIS_PREFIX';
+	private const ENV_APCU_PREFIX      = 'WP_PROMETHEUS_APCU_PREFIX';
+
+	/**
+	 * Default Redis prefix.
+	 */
+	private const DEFAULT_REDIS_PREFIX = 'WORDPRESS_PROMETHEUS_';
+
+	/**
+	 * Default APCu prefix.
+	 */
+	private const DEFAULT_APCU_PREFIX = 'wp_prom';
+
+	/**
+	 * Singleton instance of the storage adapter.
+	 *
+	 * @var Adapter|null
+	 */
+	private static ?Adapter $instance = null;
+
+	/**
+	 * Last error message.
+	 *
+	 * @var string
+	 */
+	private static string $last_error = '';
+
+	/**
+	 * Get the storage adapter instance.
+	 *
+	 * @return Adapter
+	 */
+	public static function get_adapter(): Adapter {
+		if ( null === self::$instance ) {
+			self::$instance = self::create_adapter();
+		}
+		return self::$instance;
+	}
+
+	/**
+	 * Reset the singleton instance (useful for testing or config changes).
+	 *
+	 * @return void
+	 */
+	public static function reset(): void {
+		self::$instance = null;
+		self::$last_error = '';
+	}
+
+	/**
+	 * Get the last error message.
+	 *
+	 * @return string
+	 */
+	public static function get_last_error(): string {
+		return self::$last_error;
+	}
+
+	/**
+	 * Get available storage adapters.
+	 *
+	 * @return array<string, string>
+	 */
+	public static function get_available_adapters(): array {
+		return array(
+			self::ADAPTER_INMEMORY => __( 'In-Memory (default, no persistence)', 'wp-prometheus' ),
+			self::ADAPTER_REDIS    => __( 'Redis (requires PHP Redis extension)', 'wp-prometheus' ),
+			self::ADAPTER_APCU     => __( 'APCu (requires APCu extension)', 'wp-prometheus' ),
+		);
+	}
+
+	/**
+	 * Check if a storage adapter is available on this system.
+	 *
+	 * @param string $adapter Adapter name.
+	 * @return bool
+	 */
+	public static function is_adapter_available( string $adapter ): bool {
+		switch ( $adapter ) {
+			case self::ADAPTER_INMEMORY:
+				return true;
+
+			case self::ADAPTER_REDIS:
+				return extension_loaded( 'redis' );
+
+			case self::ADAPTER_APCU:
+				return extension_loaded( 'apcu' ) && apcu_enabled();
+
+			default:
+				return false;
+		}
+	}
+
+	/**
+	 * Get the configured storage adapter name.
+	 *
+	 * @return string
+	 */
+	public static function get_configured_adapter(): string {
+		// Check environment variable first.
+		$env_adapter = getenv( self::ENV_STORAGE_ADAPTER );
+		if ( false !== $env_adapter && ! empty( $env_adapter ) ) {
+			return strtolower( $env_adapter );
+		}
+
+		// Fall back to WordPress option.
+		return get_option( 'wp_prometheus_storage_adapter', self::ADAPTER_INMEMORY );
+	}
+
+	/**
+	 * Get the active storage adapter name (may differ from configured if fallback occurred).
+	 *
+	 * @return string
+	 */
+	public static function get_active_adapter(): string {
+		// Ensure adapter is created.
+		self::get_adapter();
+
+		$configured = self::get_configured_adapter();
+		if ( self::is_adapter_available( $configured ) && empty( self::$last_error ) ) {
+			return $configured;
+		}
+
+		return self::ADAPTER_INMEMORY;
+	}
+
+	/**
+	 * Create the storage adapter based on configuration.
+	 *
+	 * @return Adapter
+	 */
+	private static function create_adapter(): Adapter {
+		$adapter = self::get_configured_adapter();
+		self::$last_error = '';
+
+		switch ( $adapter ) {
+			case self::ADAPTER_REDIS:
+				return self::create_redis_adapter();
+
+			case self::ADAPTER_APCU:
+				return self::create_apcu_adapter();
+
+			case self::ADAPTER_INMEMORY:
+			default:
+				return new InMemory();
+		}
+	}
+
+	/**
+	 * Create Redis storage adapter.
+	 *
+	 * @return Adapter
+	 */
+	private static function create_redis_adapter(): Adapter {
+		if ( ! extension_loaded( 'redis' ) ) {
+			self::$last_error = __( 'PHP Redis extension is not installed.', 'wp-prometheus' );
+			return new InMemory();
+		}
+
+		$config = self::get_redis_config();
+
+		try {
+			Redis::setPrefix( $config['prefix'] );
+
+			$redis = new Redis( array(
+				'host'                   => $config['host'],
+				'port'                   => $config['port'],
+				'password'               => $config['password'] ?: null,
+				'timeout'                => 0.5,
+				'read_timeout'           => 10,
+				'persistent_connections' => true,
+			) );
+
+			// Test connection by triggering initialization.
+			// The Redis adapter connects lazily, so we need to check it works.
+			return $redis;
+		} catch ( StorageException $e ) {
+			self::$last_error = sprintf(
+				/* translators: %s: Error message */
+				__( 'Redis connection failed: %s', 'wp-prometheus' ),
+				$e->getMessage()
+			);
+			return new InMemory();
+		} catch ( \RedisException $e ) {
+			self::$last_error = sprintf(
+				/* translators: %s: Error message */
+				__( 'Redis error: %s', 'wp-prometheus' ),
+				$e->getMessage()
+			);
+			return new InMemory();
+		} catch ( \Exception $e ) {
+			self::$last_error = sprintf(
+				/* translators: %s: Error message */
+				__( 'Storage error: %s', 'wp-prometheus' ),
+				$e->getMessage()
+			);
+			return new InMemory();
+		}
+	}
+
+	/**
+	 * Create APCu storage adapter.
+	 *
+	 * @return Adapter
+	 */
+	private static function create_apcu_adapter(): Adapter {
+		if ( ! extension_loaded( 'apcu' ) ) {
+			self::$last_error = __( 'APCu extension is not installed.', 'wp-prometheus' );
+			return new InMemory();
+		}
+
+		if ( ! apcu_enabled() ) {
+			self::$last_error = __( 'APCu is installed but not enabled.', 'wp-prometheus' );
+			return new InMemory();
+		}
+
+		$prefix = self::get_apcu_prefix();
+
+		try {
+			return new APC( $prefix );
+		} catch ( StorageException $e ) {
+			self::$last_error = sprintf(
+				/* translators: %s: Error message */
+				__( 'APCu error: %s', 'wp-prometheus' ),
+				$e->getMessage()
+			);
+			return new InMemory();
+		}
+	}
+
+	/**
+	 * Get Redis configuration.
+	 *
+	 * @return array{host: string, port: int, password: string, database: int, prefix: string}
+	 */
+	public static function get_redis_config(): array {
+		// Check environment variables first.
+		$env_host = getenv( self::ENV_REDIS_HOST );
+		$env_port = getenv( self::ENV_REDIS_PORT );
+		$env_password = getenv( self::ENV_REDIS_PASSWORD );
+		$env_database = getenv( self::ENV_REDIS_DATABASE );
+		$env_prefix = getenv( self::ENV_REDIS_PREFIX );
+
+		// Get WordPress options as fallback.
+		$options = get_option( 'wp_prometheus_redis_config', array() );
+
+		return array(
+			'host'     => ( false !== $env_host && ! empty( $env_host ) ) ? $env_host : ( $options['host'] ?? '127.0.0.1' ),
+			'port'     => ( false !== $env_port && ! empty( $env_port ) ) ? (int) $env_port : ( (int) ( $options['port'] ?? 6379 ) ),
+			'password' => ( false !== $env_password ) ? $env_password : ( $options['password'] ?? '' ),
+			'database' => ( false !== $env_database && ! empty( $env_database ) ) ? (int) $env_database : ( (int) ( $options['database'] ?? 0 ) ),
+			'prefix'   => ( false !== $env_prefix && ! empty( $env_prefix ) ) ? $env_prefix : ( $options['prefix'] ?? self::DEFAULT_REDIS_PREFIX ),
+		);
+	}
+
+	/**
+	 * Get APCu prefix.
+	 *
+	 * @return string
+	 */
+	public static function get_apcu_prefix(): string {
+		$env_prefix = getenv( self::ENV_APCU_PREFIX );
+		if ( false !== $env_prefix && ! empty( $env_prefix ) ) {
+			return $env_prefix;
+		}
+
+		return get_option( 'wp_prometheus_apcu_prefix', self::DEFAULT_APCU_PREFIX );
+	}
+
+	/**
+	 * Save storage configuration.
+	 *
+	 * @param array $config Configuration array.
+	 * @return void
+	 */
+	public static function save_config( array $config ): void {
+		if ( isset( $config['adapter'] ) ) {
+			update_option( 'wp_prometheus_storage_adapter', sanitize_key( $config['adapter'] ) );
+		}
+
+		if ( isset( $config['redis'] ) && is_array( $config['redis'] ) ) {
+			$redis_config = array(
+				'host'     => sanitize_text_field( $config['redis']['host'] ?? '127.0.0.1' ),
+				'port'     => absint( $config['redis']['port'] ?? 6379 ),
+				'password' => sanitize_text_field( $config['redis']['password'] ?? '' ),
+				'database' => absint( $config['redis']['database'] ?? 0 ),
+				'prefix'   => sanitize_key( $config['redis']['prefix'] ?? self::DEFAULT_REDIS_PREFIX ),
+			);
+			update_option( 'wp_prometheus_redis_config', $redis_config );
+		}
+
+		if ( isset( $config['apcu_prefix'] ) ) {
+			update_option( 'wp_prometheus_apcu_prefix', sanitize_key( $config['apcu_prefix'] ) );
+		}
+
+		// Reset the singleton to apply new configuration.
+		self::reset();
+	}
+
+	/**
+	 * Test storage adapter connection.
+	 *
+	 * @param string $adapter Adapter name.
+	 * @param array  $config  Optional configuration to test.
+	 * @return array{success: bool, message: string}
+	 */
+	public static function test_connection( string $adapter, array $config = array() ): array {
+		switch ( $adapter ) {
+			case self::ADAPTER_REDIS:
+				return self::test_redis_connection( $config );
+
+			case self::ADAPTER_APCU:
+				return self::test_apcu_connection( $config );
+
+			case self::ADAPTER_INMEMORY:
+				return array(
+					'success' => true,
+					'message' => __( 'In-Memory storage is always available.', 'wp-prometheus' ),
+				);
+
+			default:
+				return array(
+					'success' => false,
+					'message' => __( 'Unknown storage adapter.', 'wp-prometheus' ),
+				);
+		}
+	}
+
+	/**
+	 * Test Redis connection.
+	 *
+	 * @param array $config Redis configuration.
+	 * @return array{success: bool, message: string}
+	 */
+	private static function test_redis_connection( array $config ): array {
+		if ( ! extension_loaded( 'redis' ) ) {
+			return array(
+				'success' => false,
+				'message' => __( 'PHP Redis extension is not installed.', 'wp-prometheus' ),
+			);
+		}
+
+		$redis_config = ! empty( $config ) ? $config : self::get_redis_config();
+
+		try {
+			$redis = new \Redis();
+
+			$connected = $redis->connect(
+				$redis_config['host'],
+				$redis_config['port'],
+				0.5 // timeout
+			);
+
+			if ( ! $connected ) {
+				return array(
+					'success' => false,
+					'message' => __( 'Could not connect to Redis server.', 'wp-prometheus' ),
+				);
+			}
+
+			if ( ! empty( $redis_config['password'] ) ) {
+				$authenticated = $redis->auth( $redis_config['password'] );
+				if ( ! $authenticated ) {
+					return array(
+						'success' => false,
+						'message' => __( 'Redis authentication failed.', 'wp-prometheus' ),
+					);
+				}
+			}
+
+			if ( $redis_config['database'] > 0 ) {
+				$redis->select( $redis_config['database'] );
+			}
+
+			// Test with a ping.
+			$pong = $redis->ping();
+			$redis->close();
+
+			if ( $pong ) {
+				return array(
+					'success' => true,
+					'message' => sprintf(
+						/* translators: %s: Redis host:port */
+						__( 'Successfully connected to Redis at %s.', 'wp-prometheus' ),
+						$redis_config['host'] . ':' . $redis_config['port']
+					),
+				);
+			}
+
+			return array(
+				'success' => false,
+				'message' => __( 'Redis ping failed.', 'wp-prometheus' ),
+			);
+		} catch ( \RedisException $e ) {
+			return array(
+				'success' => false,
+				'message' => sprintf(
+					/* translators: %s: Error message */
+					__( 'Redis error: %s', 'wp-prometheus' ),
+					$e->getMessage()
+				),
+			);
+		}
+	}
+
+	/**
+	 * Test APCu connection.
+	 *
+	 * @param array $config APCu configuration.
+	 * @return array{success: bool, message: string}
+	 */
+	private static function test_apcu_connection( array $config ): array {
+		if ( ! extension_loaded( 'apcu' ) ) {
+			return array(
+				'success' => false,
+				'message' => __( 'APCu extension is not installed.', 'wp-prometheus' ),
+			);
+		}
+
+		if ( ! apcu_enabled() ) {
+			return array(
+				'success' => false,
+				'message' => __( 'APCu is installed but not enabled. Check your php.ini settings.', 'wp-prometheus' ),
+			);
+		}
+
+		// Test with a simple store/fetch.
+		$test_key = 'wp_prometheus_test_' . time();
+		$test_value = 'test_' . wp_rand();
+
+		$stored = apcu_store( $test_key, $test_value, 5 );
+		if ( ! $stored ) {
+			return array(
+				'success' => false,
+				'message' => __( 'APCu store operation failed.', 'wp-prometheus' ),
+			);
+		}
+
+		$fetched = apcu_fetch( $test_key );
+		apcu_delete( $test_key );
+
+		if ( $fetched === $test_value ) {
+			$info = apcu_cache_info( true );
+			return array(
+				'success' => true,
+				'message' => sprintf(
+					/* translators: %s: Memory info */
+					__( 'APCu is working. Memory: %s used.', 'wp-prometheus' ),
+					size_format( $info['mem_size'] ?? 0 )
+				),
+			);
+		}
+
+		return array(
+			'success' => false,
+			'message' => __( 'APCu fetch operation returned unexpected value.', 'wp-prometheus' ),
+		);
+	}
+}

wp-prometheus.php

@@ -3,7 +3,7 @@
  * Plugin Name: WP Prometheus
  * Plugin URI: https://src.bundespruefstelle.ch/magdev/wp-prometheus
  * Description: Prometheus metrics endpoint for WordPress with extensible hooks for custom metrics.
- * Version: 0.3.0
+ * Version: 0.4.0
  * Requires at least: 6.4
  * Requires PHP: 8.3
  * Author: Marco Graetsch
@@ -26,7 +26,7 @@ if ( ! defined( 'ABSPATH' ) ) {
  *
  * @var string
  */
-define( 'WP_PROMETHEUS_VERSION', '0.3.0' );
+define( 'WP_PROMETHEUS_VERSION', '0.4.0' );
 
 /**
  * Plugin file path.