What is NHibernate.Caches?
NHibernate.Caches namespace contains several second-level cache providers for NHibernate. A cache is place where entities are kept after being loaded from the database; once cached, they can be retrieved without going to the database. This means that they are faster to (re)load.
An NHibernate session has an internal (first-level) cache where it keeps its entities. There is no sharing between these caches - a first-level cache belongs to a given session and is destroyed with it. NHibernate provides a second-level cache system; it works at the session factory level. A second-level cache is shared by all sessions created by the same session factory.
An important point is that the second-level cache does not cache instances of the object type being cached; instead it caches the individual values of the properties of that object. This provides two benefits. One, NHibernate doesn't have to worry that your client code will manipulate the objects in a way that will disrupt the cache. Two, the relationships and associations do not become stale, and are easy to keep up-to-date because they are simply identifiers. The cache is not a tree of objects but rather a map of arrays.
With the session-per-request model, a high number of sessions can concurrently access the same entity without hitting the database each time; hence the performance gain.
Several cache providers have been contributed by NHibernate users:
Uses Bamboo.Prevalence as the cache provider. Open the file Bamboo.Prevalence.license.txt for more information about its license; you can also visit its website.
Uses System.Web.Caching.Cache as the cache provider. This means that you can rely on ASP.NET caching feature to understand how it works. For more information, read (on the MSDN): Caching Application Data.
Similar to NHibernate.Caches.SysCache, uses ASP.NET cache. This provider also supports SQL dependency-based expiration, meaning that it is possible to configure certain cache regions to automatically expire when the relevant data in the database changes.
SysCache2 requires Microsoft SQL Server 2000 or higher and .NET Framework version 2.0 or higher.
Uses memcached. See memcached homepage for more information.
Uses NCache,NCache is a commercial distributed caching system with a provider for NHibernate. The NCache Express version is free for use, see NCache Express homepage for more information.
Here are the steps to follow to enable the second-level cache in NHibernate:
Choose the cache provider you want to use and copy its assembly in your assemblies directory (NHibernate.Caches.Prevalence.dll or NHibernate.Caches.SysCache.dll).
To tell NHibernate which cache provider to use, add in your NHibernate configuration file (can be YourAssembly.exe.config or web.config or a .cfg.xml file, in the latter case the syntax will be different from what is shown below):
"XXX" is the assembly-qualified class name of a class implementing ICacheProvider, eg. "NHibernate.Caches.SysCache.SysCacheProvider, NHibernate.Caches.SysCache".
The expiration value is the number of seconds you wish to cache each entry (here two minutes). This example applies to SysCache only.
Add <cache usage="read-write|nonstrict-read-write|read-only"/> (just after <class>) in the mapping of the entities you want to cache. It also works for collections (bag, list, map, set, ...).
Be careful. Caches are never aware of changes made to the persistent store by another process (though they may be configured to regularly expire cached data). As the caches are created at the session factory level, they are destroyed with the SessionFactory instance; so you must keep them alive as long as you need them.
There is only one configurable parameter: prevalenceBase. This is the directory on the file system where the Prevalence engine will save data. It can be relative to the current directory or a full path. If the directory doesn't exist, it will be created.
As SysCache relies on System.Web.Caching.Cache for the underlying implementation, the configuration is based on the available options that make sense for NHibernate to utilize.
SysCache has a config file section handler to allow configuring different expirations and priorities for different regions. Here's an example:
<?xml version="1.0" encoding="utf-8" ?> <configuration> <configSections> <section name="syscache" type="NHibernate.Caches.SysCache.SysCacheSectionHandler,NHibernate.Caches.SysCache" /> </configSections> <syscache> <cache region="foo" expiration="500" priority="4" /> <cache region="bar" expiration="300" priority="3" /> </syscache> </configuration>
SysCache2 can use SqlCacheDependencies to invalidate cache regions when data in an underlying SQL Server table or query changes. Query dependencies are only available for SQL Server 2005. To use the cache provider, the application must be setup and configured to support SQL notifications as described in the MSDN documentation.
To configure cache regions with SqlCacheDependencies a syscache2 config section must be defined in the application's configuration file. See the sample below.
<configSections> <section name="syscache2" type="NHibernate.Caches.SysCache2.SysCacheSection, NHibernate.Caches.SysCache2"/> </configSections>
A table-based dependency will monitor the data in a database table for changes. Table-based dependencies are generally used for a SQL Server 2000 database but will work with SQL Server 2005 or superior as well. Before you can use SQL Server cache invalidation with table based dependencies, you need to enable notifications for the database. This task is performed with the aspnet_regsql command. With table-based notifications, the application will poll the database for changes at a predefined interval. A cache region will not be invalidated immediately when data in the table changes. The cache will be invalidated the next time the application polls the database for changes.
To configure the data in a cache region to be invalidated when data in an underlying table is changed, a cache region must be configured in the application's configuration file. See the sample below.
Table-based Dependency Configuration Properties
A command-based dependency will use a SQL command to identify records to monitor for data changes. Command-based dependencies work only with SQL Server 2005.
Before you can use SQL Server cache invalidation with command-based dependencies, you need to enable the Service Broker for query notifications. The application must also start the listener for receiving change notifications from SQL Server. With command based notifications, SQL Server will notify the application when the data of a record returned in the results of a SQL query has changed. Note that a change will be indicated if the data in any of the columns of a record change, not just the columns returned by a query. The query is a way to limit the number of records monitored for changes, not the columns. As soon as data in one of the records is modified, the data in the cache region will be invalidated immediately.
To configure the data in a cache region to be invalidated based on a SQL command, a cache region must be configured in the application's configuration file. See the samples below.
Example 25.4. Stored Procedure
<cacheRegion name="Product" priority="High" > <dependencies> <commands> <add name="price" command="ActiveProductsStoredProcedure" isStoredProcedure="true"/> </commands> </dependencies> </cacheRegion>
Example 25.5. SELECT Statement
<cacheRegion name="Product" priority="High"> <dependencies> <commands> <add name="price" command="Select VideoTitleId from dbo.VideoTitle where Active = 1" connectionName="default" connectionStringProviderType="NHibernate.Caches.SysCache2.ConfigConnectionStringProvider, NHibernate.Caches.SysCache2"/> </commands> </dependencies> </cacheRegion>
Command Configuration Properties
Multiple cache dependencies can be specified. If any of the dependencies triggers a change notification, the data in the cache region will be invalidated. See the samples below.
Example 25.6. Multiple commands
<cacheRegion name="Product"> <dependencies> <commands> <add name="price" command="ActiveProductsStoredProcedure" isStoredProcedure="true"/> <add name="quantity" command="Select quantityAvailable from dbo.VideoAvailability"/> </commands> </dependencies> </cacheRegion>
Example 25.7. Mixed
<cacheRegion name="Product"> <dependencies> <commands> <add name="price" command="ActiveProductsStoredProcedure" isStoredProcedure="true"/> </commands> <tables> <add name="quantity" databaseEntryName="Default" tableName=" VideoAvailability" /> </tables> </dependencies> </cacheRegion>
In addition to data dependencies for the cache regions, time based expiration policies can be specified for each item added to the cache. Time based expiration policies will not invalidate the data dependencies for the whole cache region, but serve as a way to remove items from the cache after they have been in the cache for a specified amount of time. See the samples below.
Example 25.8. Relative Expiration
<cacheRegion name="Product" relativeExpiration="300" priority="High" />
Example 25.9. Time of Day Expiration
<cacheRegion name="Product" timeOfDayExpiration="2:00:00" priority="High" />
Additional Configuration Properties
Number of seconds that an individual item will exist in the cache before being removed.
24 hour based time of day that an item will exist in the cache until. 12am is specified as 00:00:00; 10pm is specified as 22:00:00. Only valid if relativeExpiration is not specified. Time of Day Expiration is useful for scenarios where items should be expired from the cache after a daily process completes.
There is a known issue where some SQL Server 2005 notifications might not be received when an application subscribes to query notifications by using ADO.NET 2.0. To fix this problem install SQL hotfix for kb 913364.