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Output caching is the most effective way to increase page performance. The output cache stores the full source code of pages, i.e. the HTML and client script that the server sends to browsers for rendering. When a visitor views a page, the server caches the output code in the application's memory. Until the cache expires, the system displays the page using the cached output. As a result, the application can quickly serve requests without running the page code or communicating with SQL servers.
We recommend using output caching for all possible pages. The drawback is that cached output can become outdated on dynamic pages, or if the website's content is updated. Output caching is not suitable for pages that need to refresh frequently (for example pages that update based on user input, or have randomly generated content). You cannot disable output caching for parts of pages.
However, the system provides several solutions that allow you to cache output even for pages with dynamic content:
Caching page output on the IIS level
You can also configure IIS Output caching (opens in a new tab) for your web application. IIS Output caching is completely separate from the caching mechanisms in Kentico. When clients request a page whose valid output is cached on the IIS level, the server returns the response without requiring any processing from Kentico.
Note: Kentico only supports the User-mode policy for IIS output caching. You cannot use Kernel-mode caching.
To use output caching for your website's pages:
Allow output caching globally:
Enable output caching for individual pages:
To enable output caching for all pages on your site, edit the properties of the root page. The underlying pages need to have the Use output cache property set to Inherit (this is the default state).
When a visitor requests the page, the system saves the full output code into the cache. Until the cache expires, the system displays the cached output without processing the page.
The output cache of pages can become outdated when users update the website's pages and objects. Dependencies allow the system to automatically delete cached content whenever related objects are modified. The output cache provides default dependencies that clear the cache for pages:
By default, pages do NOT delete their output cache when you modify other pages or objects whose data is displayed on the page. For example, if a page contains a list of news articles and you update one of the news pages, the page displays the old content until the output cache expires.
To ensure that pages are always up-to-date, you need to set your own output cache dependencies:
The system clears the page's output cache whenever an object specified by the dependencies is modified. With the cache cleared, the system fully processes the page on the next request, and caches the new output.
Note: If the page uses a shared page template, the dependencies apply to all pages with the given template.
By default, the application stores the output cache of pages only in the memory. Websites lose all cached content if the application restarts. You can configure the system to save the output cache in the server's file system, which provides more persistent storage. The output cache then stores the content of the selected pages in both the application's memory and file system. The file system cache is used if the output of a requested page is not found in the memory cache.
Warning: Enabling the file system output cache may cause your website to serve outdated content. When clearing invalid output cache due to changes made to pages or other dependencies, the system can only remove cache files if the corresponding cache item also exists in the memory. The file system cache remains outdated if the memory cache is not available when the invalidation occurs (for example after an application restart or if you have different expiration times set for the two caching types).
As a workaround, you can manually clear the file system output cache by clicking Clear cache on the General tab of the System application.
To allow storage of output cache in the file system:
To enable persistent output caching for pages:
To enable file system output caching for all pages on your site, edit the properties of the root page. The underlying pages need to have the Allow file system cache property set to Inherit (this is the default state).
To manually clear the output cache of a page:
The page's output is removed from the cache in the server's memory. The next time a visitor opens the page, the system runs the page's code and saves the output into the cache again.
Note: The Clear output cache action does not delete the file system output cache. To manually clear the file system output cache, you need to delete all data cached by the application – open the System application and click Clear cache on the General tab.
If you cannot use full output caching on a page, consider enabling Partial caching for sections of the page. The partial cache stores the HTML output of individual web parts (components that make up the content of pages). Partial caching allows the system to refresh pages on every request, but still improves performance by caching at least portions of the page output. You can set up partial caching for any number of web parts on a single page.
Note: Partial caching is not available on precompiled websites.
Web parts do not use partial caching by default. You need to allow partial caching globally for your website, and then enable it for individual web part instances.
To allow partial caching:
To enable partial caching for web part instances:
When a visitor opens the page, the system saves the HTML output generated by the web part into the cache. Until the cache expires, the system renders the page without processing the web part, and displays the cached output instead.
Automatic clearing of partial cache
By default, the system deletes the partial cache of web part instances:
You can set additional cache dependencies for web parts through the Partial cache dependencies property (in the Performance category). The system uses the dependencies to automatically clear the partial cache when the related objects are modified.
To disable the default dependencies listed above, uncheck Use default cache dependencies and enable the Preserve partial cache on postback property.
Tip: If you wish to use partial caching on pages that are not based on the portal engine, you can leverage standard ASP.NET output caching for controls (opens in a new tab).
Web pages often display different content based on variable parameters, such as user permissions, language preferences, or the device and browser used to view the page. To compensate, the output cache stores multiple versions of pages. The system then automatically serves the appropriate cache version to visitors.
The output cache always creates separate cache items for pages according to the following variables:
You can configure the conditions that determine when the system uses separate or shared output cache:
Open the Settings application.
Select the System -> Performance category.
Click Edit next to the Output cache variables setting.
Enable or disable the following output cache variables:
Cache key variable | Ensures separate output cache for |
|---|---|
username | Every logged in user. Public users share the same output cache. |
sitename | Every site in the system. Important: Leave the sitename variable checked unless your sites all have identical content on pages with the same path. |
lang | Each language version of pages. |
browser | Different types of web browsers. |
cookielevel | Different cookie level preferences of visitors. |
deviceprofile | Different device profiles detected for visitors. |
domain | Website domain names. Check to disable output cache sharing between domain aliases of sites. |
viewmode | The page view modes used by Kentico, such as design, edit, preview or live site. Currently, the system only uses caching for the live site view mode. |
Click Save.
The system adds the selected variables and their values into the names of output cache keys. As a result, the cache keys are unique for all combinations of the required conditions.
For example, the username variable ensures that the system stores a separate version of each page's output cache for every logged in user. If you disable the username option, the output cache does not distinguish between users — if a page's output is cached for one user, the system may load the same content for all other users (depending on the remaining output cache variables).
Tip: You can configure how the system shares partial output cache through the Partial cache variables setting. The variables apply globally to the partial cache of all web parts.
Personalized content (for example content contained in web part zones or personalized web parts) is cached per personalization variant.
For example, when a page containing a web part zone with multiple personalization variants is served, the system caches only the personalization variant fitting the current request. When another request, meeting different personalization criteria, is made, the system reuses cached data for the static segments of the page (if still available) and caches only the different personalization variant of the web part zone. This way, the system avoids storing the static segments of the page multiple times.