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Understanding Adobe Experience Manager (AEM) Dispatcher - Satish Kumar Choudhary - Tech Consultant Based out of New Jersey, USA

Understanding Adobe Experience Manager (AEM) Dispatcher

Understanding Adobe Experience Manager (AEM) Dispatcher

A Sample AEM Architecture Diagram

Basic AEM Architecture
Basic AEM Architecture

What is a Dispatcher?

Dispatcher is Adobe Experience Manager’s caching and/or load balancing tool. Dispatcher also helps to protect your AEM server from attack.

The Dispatcher helps realize an environment i.e both fast and dynamic. It works as a part of a static HTML server, with the aim of :

  • Storing as much of the site content as is possible, in the form of static website.
  • Accessing the layout as little as possible.

For caching, the Dispatcher module uses the Web server’s ability to serve static content. The Dispatcher places the cached documents in the document root of the Web server.

How Dispatcher Performs Caching ?

  • The Cache Directory
    • For caching, the Dispatcher module uses the web server’s ability to serve static content. The Dispatcher places the cached documents in the document root of the web server.
  • Methods for Caching
    • Content Updates
    • Auto-Invalidation

Methods for Caching - Content Updates

  • It deletes the modified file(s) from the cache.
  • It deletes all files that start with the same handle from the cache.
  • It touches the so-called statfile ; this updates the timestamp of the statfile to indicate the date of the last change.

Notes:

  • Content Updates are typically used in conjunction with an authoring system which “knows” what must be replaced.
  • Files that are affected by a content update are removed, but Not Replaced immediately.
  • You may have several statfiles, If a page is updated, AEM looks for the next parent folder containing a statfile, and touches that file.

Methods for Caching - Auto-Invalidation

Invalidates parts of the cache – without physically deleting any files.

  • if the cached document is newer, the Dispatcher returns it.
  • if it is older, the Dispatcher retrieves the current version from the AEM instance.

Notes:

  • Auto invalidation is typically used when the inter-relations are complex.
  • Auto-invalidation does not involve any action by the dispatcher at update time, except for touching the statfile.

Dispatcher - Workflow

AEM Dispatcher Workflow
AEM Dispatcher Workflow. Reference: https://docs.adobe.com/content/help/en/experience-manager-dispatcher/using/dispatcher.html

Determining whether a document is subject to caching

The Dispatcher checks the request against the list of cacheable documents. If the document is not in this list, the Dispatcher requests the document from the AEM instance.

You can define which documents the Dispatcher caches in the configuration file .

  • By default the Dispatcher configuration is stored in the dispatcher.any text file.
Determining whether a document is subject to caching
Determining whether a document is subject to caching
Determining whether a document is subject to caching

What Files Are Not Cacheable?

  • If the document is not in the dispatcher configuration file
  • If the request URI contains a query parameter- Question mark “?”
  • The file extension is missing. The web server needs the extension to determine the document type (the MIME-type).
  • The authentication header is set
  • If the HTTP method is not GET. e.g POST

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