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Understanding Inbound and Outbound Processing in Drupal: Principles and Examples

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Introduction

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In a Drupal site, when a user interacts with the system (for example, by accessing a URL, submitting a form, or consuming an API), Drupal processes the request through an entry and exit cycle — this is what we call Inbound and Outbound processing.

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These two mechanisms allow Drupal to:

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Let us look at all of this in detail with concrete examples.

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1. Inbound Processing: Handling What Comes In

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Inbound Processing applies to everything the user sends to the system:

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Goal

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Example: Modifying an Incoming URL

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Imagine we want to redirect paths /products to /product while keeping Drupal's internal logic intact.

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use Symfony\\Component\\HttpKernel\\Event\\RequestEvent;\nuse Symfony\\Component\\EventDispatcher\\EventSubscriberInterface;\n\nclass InboundPathSubscriber implements EventSubscriberInterface {\n\n  public static function getSubscribedEvents() {\n    return ['kernel.request' => ['onKernelRequest', 30]];\n  }\n\n  public function onKernelRequest(RequestEvent $event) {\n    $request = $event->getRequest();\n    $path = $request->getPathInfo();\n\n    if ($path === '/products') {\n      $request->server->set('REQUEST_URI', '/product');\n    }\n  }\n}
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Here, we use the kernel.request event to intercept the URL before Drupal resolves the router.

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2. Outbound Processing: Manipulating What Goes Out

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Outbound Processing occurs when Drupal generates a response (HTML, JSON, etc.). It is often used to:

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Example: Dynamic Link Rewriting

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Suppose you want to transform all /user/ID links into /profile/ID during HTML rendering:

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use Drupal\\Core\\PathProcessor\\OutboundPathProcessorInterface;\nuse Symfony\\Component\\HttpFoundation\\Request;\nuse Drupal\\Core\\Render\\BubbleableMetadata;\n\nclass OutboundPathProcessor implements OutboundPathProcessorInterface {\n  public function processOutbound($path, &$options = [], Request $request = NULL, BubbleableMetadata $bubbleable_metadata = NULL) {\n    if (strpos($path, '/user/') === 0) {\n      return str_replace('/user/', '/profile/', $path);\n    }\n    return $path;\n  }\n}
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Declaration in services.yml:

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services:\n  my_module.path_processor.outbound:\n    class: Drupal\\my_module\\OutboundPathProcessor\n    tags:\n      - { name: path_processor_outbound, priority: 100 }
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When to Use Inbound vs Outbound?

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SituationInboundOutbound
Modify the path before it is resolvedYesNo
Modify a link before displaying itNoYes
Rewrite URL parametersYesYes
Add HTTP headersNoYes
Handle conditional redirectionYesNo
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Combined Use: A Typical Multilingual Site Scenario

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PathProcessorManager orchestrates these operations in Drupal Core.

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Debugging & Best Practices

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Conclusion

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The Inbound and Outbound systems in Drupal are powerful tools for controlling the flow of HTTP data. Whether you are building a multilingual site, a headless site, or simply want to customize your site's paths, these hooks give you fine-grained control over data entry and exit — without modifying Drupal core.

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