How Sitecore caching work

This post describes different levels of Sitecore caches. Cache plays very important role in website performance. So, understanding of all Sitecore caches is really important. If we understand all of them, then it would be easy to do performance tuning using cache settings.

This post contains just theoretical overview of cache, will be posting about practical usage and performance tuning of caches soon :)

We can check how different cache are allocated and cleared, we have a tool given by Sitecore: http://mysite.com/sitecore/admin/cache.aspx. Even a great tool available to Sitecore Market Place - Sitecore Cache Admin, which describes how actual cache is managed by Sitecore.

Different Database Cache:

Prefetch Cache

Prefetch caches contain items that Sitecore accesses during and immediately after initialization and items with children that Sitecore often accesses as a group. Sitecore maintains those caches over the life of the application.

Each database prefetch cache entry represents an item in a database. Database prefetch cache entries include all field values for all versions of that item, and information about the parent and children of the item.

Read How to configure prefetch cache and how it affects application startup.

Data Cache

Data caches are dependent on database prefetch caches, which operate at a lower level. Like database prefetch caches, each entry in a database data cache represents a single item in a database, including parent/child relationships and field values for all versions in all languages of that item. Sitecore does not pre-populate database data caches.

The Caching.DefaultDataCacheSize setting in the web.config file specifies the default size for database data caches.

The purpose of this cache is to minimize the amount of requests to the database. This is extremely important for performance, as requesting items from the database is rather expensive.

Item Cache

Item caches store items. Database item caches are dependent on database data caches, which operate at a lower level. Each entry in a database item cache represents a single version of an item in a single language. Sitecore does not pre-populate database item caches.

The Caching.DefaultDataCacheSize setting in the web.config file specifies the default size for database data caches.

Database item caches contain objects of type Sitecore.Data.Items.Item.

The Caching.DefaultltemCacheSize setting in the web.config file specifies the default size for database item caches.

It would be best to have the average size of an item in Caching.AverageltemSize configuration attribute.

Standard Value Cache

Standard values caches contain standard values for data templates in the database. Sitecore does not pre-populate database standard values caches. Database standard values caches do not depend on any other caches.

The Caching.StandardValues.DefaultCacheSize setting in the web.config file specifies the default size for database standard values caches.

Sitecore uses the Caching.StandardValues.AverageValueSize setting in the web.config file to estimate the amount of memory consumed by the database standard values cache.

Different Website Cache

If you don't specify cache at a level then it gets its values from default website cache in the web.config. We can customize all these cases site-wise.

<cacheSizes>
 <sites>
         <website>
            <html>10MB</html>
            <registry>0</registry>
            <viewState>0</viewState>
            <xsl>5MB</xsl>
         </website>
       </sites>
</cacheSizes>

HTML Cache

The HTML cache (also known as the output cache) associated with each managed Web site contains the output generated by individual renderings under different conditions.

Sitecore provides caching options which allow the rendered data to be retrieved from cache if the data source, device, authentication status, user, rendering parameters and/or query string parameters are the same as the previous request.

Sitecore allows developers to define output cache criteria in three places:

In the Caching section of the sublayout and rendering definition item. (Global)
In the properties of the presentation component when you statically bind it to a layout or sublayout. (Static Controls)
In the Caching section of the Control Properties dialog when you bind a presentation component to a placeholder in layout details. (Dynamic)

HTML cache is disabled in the preview, webedit and debug modes.

Filtered Item Cache

The filtered items cache associated with each managed Web site contains information about versions of items relevant to different users.

The filteredItemsCacheSize attribute of each /configuration/Sitecore/sites/siteelement in the web.config file specifies the size of the filtered items cache for that managed Web site.

The Caching.DefaultFilteredItemsCacheSize setting in the web.config file specifies the default size of the size filtered items caches.

Registry Cache

The registry cache associated with each managed Web site contains data used primarily by the Sitecore user interfaces.

The registryCacheSize attribute of each /configuration/Sitecore/sites/site element in the web.config file specifies the size of the registry cache for that managed Web site.

The Caching.DedaultRegistryCacheSize setting in the web.config file specifies the default size for the registry caches.

Media Cache

Sitecore stores all media files to physical file system. All other cache are stored in RAM actually.

When publishing is done, Sitecore does not clear Media Cache like it does for other caches. Sitecore clears these media cache periodically.

User Cache

The client data store cache stores information about each authenticated user, such as the username or other user properties.

The Caching.DefaultClientDataCacheSize setting in the web.config file specifies the size of the client data store cache.

The disableClientData attribute of each /configuration/Sitecore/sites/site element in the web.config file enables or disables client data caching for that managed Web site.

Proxy Cache

Sitecore has a proxy item feature that allows items in one area of the content tree to appear in another.

These proxy items behave in the same way as normal items but have unique IDs to distinguish them from the original items.

The proxy cache keeps track of these IDs and how they map back to the original items.

How cache clearing works?

HTML cache

On publishing of any item, HTML cache is cleared. If we are using multisite module, the we can rewrite HTML cache module to clear HTML cache for item's related site. HTML cache for each page is built from multiple items, so publishing a single item, we cannot judge at how many items it would affect. That's why we have to clear full site HTML cache.

Item cache

Whenever any item is published, its Item Cache is also updated. If you publish an item which is linked to other items, then these items are cleared as well.

If you publish standard values or a template, it will clear all items based on that template.

If you delete/recycle/restore any item, its parent item's cache also updated, similarly while doing sorting, their siblings cache might get updated.

Data cache

Data cache is updated incrementally when changes take effect after a publish. It is rebuild incrementally when the items are requested again.

Prefetch cache

By default, items/templates specified in the config file are cached in Prefetch Cache when application initiated. Prefetch cache are updated same way of Data Cache.

Note: An ASP.NET application server restarts effectively removes all entries from all caches, except media cache. If we are using ASP.NET caching based on some items' values, we must clear ASP.NET cache too on publishing.

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