The Ultimate Guide to Acing AEM Dispatcher Interview Questions

As an Adobe Experience Manager (AEM) professional, you know that the Dispatcher plays a crucial role in ensuring a fast, secure, and efficient website experience. However, during job interviews, you may encounter various questions related to the Dispatcher, and being well-prepared can make all the difference. In this comprehensive guide, we’ll explore some of the most commonly asked AEM Dispatcher interview questions and provide detailed explanations to help you ace your next interview.

Understanding the Fundamentals

Before diving into the interview questions, let’s start with a brief overview of the AEM Dispatcher.

The AEM Dispatcher is a caching and load balancing tool designed to enhance the performance and security of your AEM website. It acts as a static HTML server, storing as much content as possible in the form of a static website, reducing the load on the AEM instance. The Dispatcher also helps protect your AEM server from potential attacks by implementing security measures.

AEM Dispatcher Interview Questions

1. What is the AEM Dispatcher?

The AEM Dispatcher is a caching and load balancing tool that enhances the performance and security of your AEM website. It stores website content as a static HTML website to reduce the load on the AEM instance and access the layout as infrequently as possible. Additionally, the Dispatcher helps protect the AEM server from attacks.

1. How does the Dispatcher perform caching?

The Dispatcher utilizes the web server’s ability to serve static content. It stores cached documents in the web server’s document root. The Dispatcher has two primary methods for updating the cache content when changes are made to the website:

• Content Updates: This method removes the pages that have changed, as well as files directly associated with them.
• Auto-Invalidation: This method automatically invalidates portions of the cache that may be outdated after an update, effectively flagging relevant pages as being out of date without deleting anything.

1. How does the Dispatcher perform load balancing? What are the benefits of load balancing?

The Dispatcher performs load balancing by distributing user requests (load) across different clustered AEM instances. The main benefits of load balancing include:

• Increased Processing Power: By sharing document requests between multiple AEM instances, each instance has fewer documents to process, resulting in faster response times.
• Increased Fail-safe Coverage: If the Dispatcher does not receive responses from an instance, it automatically relays requests to one of the other instances. This ensures that if an instance becomes unavailable, the only effect is a proportional slowdown of the site.

1. What is a statfile?

The /statfile property identifies the file used by the Dispatcher to register the time of the most recent content update. The statfile can be any file on the web server, and when content is updated, the Dispatcher updates the timestamp. The default statfile is named .stat and is stored in the docroot.

1. What is the dispatcher.any file?

The dispatcher.any file is the default configuration file for the Dispatcher. It contains a series of single-valued or multi-valued properties that control the behavior of the Dispatcher. Property names are prefixed with a forward slash (/), and multi-valued properties enclose child items using braces ({ }).

1. What types of requests are not cached by the Dispatcher?

By default, the Dispatcher does not cache the following types of requests:

• Requests that do not return an HTTP code 200 (success)
• Requests with suffixes (e.g., .html, .pdf)
• Requests with request parameters (e.g., ?query=value)
• Requests programmatically set with the response.setHeader("Dispatcher", "no-cache") header

1. How can you restrict external access to resources in the Dispatcher?

The Dispatcher filter can be used to allow or deny external access to specific areas of AEM. To protect your instance, you should configure the Dispatcher to restrict external access as much as possible. First, deny access to all files, and then allow or deny access to specific areas. For example:

/filter {    /0001 { /glob "*" /type "deny" }    /0002 { /type "allow" /url "/libs/cq/workflow/content/console*" }    /0003 { /type "deny" /url "/libs/cq/workflow/content/console/archive*" }}

1. How does the Dispatcher return documents?

The Dispatcher checks the request against the list of cacheable documents defined in the dispatcher.any configuration file. If the document is not in this list, the Dispatcher requests the document from the AEM instance. The Dispatcher always requests the document directly from the AEM instance in the following cases:

• If the HTTP method is not GET (e.g., POST for form data, HEAD for HTTP headers)
• If the request URI contains a question mark (?), indicating a dynamic page
• If the file extension is missing
• If the authentication header is set (this can be configured)

1. How do you implement permission-sensitive caching?

To implement permission-sensitive caching, follow the instructions in the official Adobe documentation: Permissions-Sensitive Caching in AEM Dispatcher.

1. Can you cache only files with specific extensions?

Yes, you can cache only files with specific extensions by specifying them in the cache section of the dispatcher.any configuration file. For example, to cache only GIF files, you would specify *.gif in the cache section.

1. How does the Dispatcher determine if a document is up-to-date?

If the document is not subject to auto-invalidation, the Dispatcher considers it up-to-date. If the document is subject to auto-invalidation, the Dispatcher checks whether it is the latest version. If not, the Dispatcher will request the current version from the AEM instance.

1. Can you implement multiple Dispatchers in a setup?

Yes, you can implement multiple Dispatchers in a setup. However, ensure that both Dispatchers can access the AEM website directly. A Dispatcher cannot handle requests coming from another Dispatcher.

1. Can a Dispatcher and an AEM Publish instance reside on the same physical machine?

Yes, a Dispatcher and an AEM Publish instance can reside on the same physical machine if the machine is sufficiently powerful. However, it is recommended to set up the Dispatcher and the AEM Publish instance on different machines.

1. Can you use sticky connections and caching in tandem?

For most pages that use sticky connections (maintaining user sessions on a specific server instance), you should turn off caching. Otherwise, the same instance of the page will be displayed to all users, regardless of session content.

However, for some applications, it can be possible to use both sticky connections and caching. For example, if you display a form that writes data to a session, you can use sticky connections and caching in tandem.

1. Can you run an AEM website without a Dispatcher?

Yes, you can run an AEM website without a Dispatcher. However, this is generally not recommended, as the Dispatcher provides critical caching, load balancing, and security features for your website.

1. Can you have a Dispatcher for an AEM author instance?

Yes, you can have a Dispatcher for an AEM author instance.

1. How do you delete files from the cache?

You can delete files from the AEM cache by using an HTTP request. When the HTTP request is received, the Dispatcher deletes the files from the cache. The Dispatcher will cache the files again only when it receives a client request for the page. The HTTP request has the following syntax:

POST /dispatcher/invalidate.cache HTTP/1.1CQ-Action: ActivateCQ-Handle: path-patternContent-Length: 0

The Dispatcher deletes the cached files and folders that have names matching the value of the CQ-Handle header.

1. How do you prevent a particular component from being cached while caching the entire page in the Dispatcher?

To prevent a particular component from being cached while caching the entire page in the Dispatcher, you need to configure the component in the Sling Dynamic Include (SDI) configuration. SDI is an OSGi service that can automatically replace configured components with include tags (SSI, ESI, or AJAX). You can enter the list of component resource types into the SDI configuration, choose the include tag type, and SDI will handle the rest.

1. What types of documents are cached by the Dispatcher?

The caching behavior is configured inside the /cache property in the dispatcher.any file. Inside the /rules property, you can specify the list of cacheable documents.

/cache {    /rules {        # List of cacheable documents    }    /invalidate {        # List of auto-invalidated documents    }}

1. What types of documents are subject to auto-invalidation for the Dispatcher?

The documents subject to auto-invalidation are configured inside the /cache property in the dispatcher.any file. Inside the /invalidate property, you can specify the list of documents subject to auto-invalidation.

/cache {    /rules {        # List of cacheable documents    }    /invalidate {        # List of auto-invalidated documents    }}

1. What is the /renders property?

The /renders property defines the URL to which the Dispatcher sends requests to render a document. For example, the following /renders section distributes render requests equally among two AEM instances:

/renders {    /myFirstRenderer {        /hostname "myCompany.com"        /port "4503"    }    /mySecondRenderer {        /hostname "127.0.0.1" # runs on the same computer as Dispatcher        /port "4503"    }}

1. How do you set up the Dispatcher in AEM?

You can find detailed instructions on setting up the Dispatcher in AEM in the following tutorial: Set up Dispatcher in AEM.

By familiarizing yourself with these AEM Dispatcher interview questions and their explanations, you’ll be better prepared to showcase your knowledge and expertise during job interviews. Remember, the key to success lies in understanding the fundamentals, staying up-to-date with the latest developments, and practicing your responses. Good luck!