Using NCache as NHibernate Second Level Cache

NHibernate has a first level or session level cache which is used by default. Each session contains its own session level cache which is not shared with other sessions. This cache is disposed whenever the session is closed. However, NHibernate allows to use second level, across session cache which spans across sessions’ life time and can be shared by multiple sessions.

NCache provides a second level or process level cache for NHibernate by implementing NHibernate's ICacheProvider and ICache interfaces. This enables NCache to be used as NHibernate second cache in existing NHibernate application without any code change. All that’s needed is to change NHibernate configuration file to plug-in NCache as cache provider and distributed caching can be used.

Second level cache can be configured by enabling the use of second level cache and configuring second level cache provider in NHibernate configurations.

Configuring NHibernate Application to use NCache as Second Level Cache

Important

Use NHibernate Version 2.2.3 in order to use NCache as NHibernate Second Level Cache.

Create NHibernate region in Web.config

Add the <hibernate-configuration> in Web.config under the <configSections> tag:

<configSections>
  <section name="hibernate-configuration" type="NHibernate.Cfg.ConfigurationSectionHandler, NHibernate" />
  <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
</configSections>

Specify Configurations for NHibernate Region

Intall NCache NuGet pacakge for application without NCache installation. Execute any of the following commands in Package Manager Console inside Visual Studio:
- For Enterprose Edition:
```
Install-Package NHibernate.NCache
```
- For Professional Edition:
```
Install-Package NHibernate.NCache.Professional
```
- For OpenSource:
```
Install-Package NHibernate.NCache.OpenSource
```
Configure NCache as NHibernate's second level cache provider by adding the cache.provider_class property under the specified region (<hibernate-configuration>).
Enable the use of second level cache in application by setting the cache.use_second_level_cache property to true.

<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
   <session-factory>
     <property name="cache.provider_class">Alachisoft.NCache.Integrations.NHibernate.Cache.NCacheProvider, Alachisoft.NCache.Integrations.NHibernate.Cache</property>
     <property name="cache.use_second_level_cache">true</property>
   </session-factory>
 </hibernate-configuration>

Specify Application ID for NCache NHibernate Provider

NCache Provider for NHibernate identifies each application by an application ID. This ID is used to find the appropriate configuration for that application from NCache configuration file for NHibernate (NCacheNHibernate.xml). This application id must be specified in application's configuration file (Web.config):

  <appSettings>
    <add key="ncache.application_id" value="myapp" />
  </appSettings>

Configuring Cacheable Objects

Enabling the use of second level cache does not cache each class object by default. Instead classes needed to be cached are to be marked cacheable in class mapping (.hbm.xml) file. To mark a class cacheable, add the following tag in class's configuration:

<cache usage="read-write" region ="AbsoluteExpirationRegion"/>

In the above tag, properties usage and region can be changed.

Members	Description
`region`	Specifies the name of second level cache region to be used for this class objects. If no region is specified, Fully Qualified Name (FQN) will be used as region name with default region configurations.
`usage`	Specifies caching concurrency strategy to be used for this class. NHibernate allows following three concurrency strategies to be used for caching: `Read-Write` `Nonstrict-Read-Write` `Read-Only`

Configuring Cache Regions

NHibernate uses cache regions to store objects. NCache allows NHibernate cache regions to be configured with different properties. For this purpose, NCache has a configuration file named NCacheNHibernate.xml, which contains all regions’ configurations used by NHibernate Second Level Cache. This file can be placed at root directory of application or in %NCHOME%/config (Windows) or opt\ncache\config (Linux).

 <configuration>
   <application-config application-id="myapp" enable-cache-exception="true" default-region-name="default" key-case-sensitivity="false">
   <cache-regions>
        <region name="AbsoluteExpirationRegion" cache-name="mycache"
        priority="Default" expiration-type="sliding" expiration-period="180" />
        <region name="default" cache-name="mycache" priority="default"
        expiration-type="none" expiration-period="0" />
    </cache-regions>
  </application-config>
 </configuration>

Multiple application configurations can be specified in NCacheNHibernate.xml. Any NHibernate application will use one of the specified configurations based on the application-id specified in applications App.config/Web.config file.

Following are the configurable options in application-config section.

Member	Description
`application-id`	This option lets the user specify a unique id for current `application-config`. This id is used to select appropriate configuration for a particular application.
`enable-cache-exception`	Identifies whether the exceptions if occurred in NCache will be propagated to NHibernate provider.
`default-region-name`	Allows to specify a default region for the application. If a particular region's configurations are not found, default region configurations will be used. Specified default region configuration must exist in `cache-regions` section.
`key-case-sensitivity`	This option allows to specify whether cache keys will be case sensitive or not. This option has to be configured according to database used. If database is case-sensitive set this option true, otherwise false.
`cache-regions`	This section lets the user configure multiple cache regions for NHibernate application. Each region's configuration is specified in region tag.
`region`	This tag contains the configurations for one particular region. Following options can be configured for any particular region. `name`: Each region is identified by its name. Name of the region should be unique. `cache-name:` This option allows specifying NCache’s cache name to be used for this region. `priority:` The priority the user wants to use for cached items. The possible values are: `Default` `Low` `BelowNormal` `Normal` `AboveNormal` `NotRemovable` `expiration-type:` This options allows to specify type of expiration for any item in this region. Possible values are `absolute`/`sliding`/`none`. `expiraion-period:` Allows to specify expiration period if absolute/sliding expiration is configured. Its value should be greater than 0.
`database-dependencies`	It lets the user specify database dependencies for current NHibernate application. `dependency`: Each dependency is configured in a dependency tag. Multiple tags can be configured for multiple dependencies. The following options can be configured for each dependency. `entity-name:` Dependencies are added based on the Fully Qualified Name (FQN) of classes. This option allows to specify name of class for which current dependency is to be added. Any entity can have maximum one dependency. `type:` Allows to specify database dependency type. Possible values are sql/oracle/oledb. Since NCache provider uses connection string specified in NHibernate's configuration, use dependency type appropriate to connection string. `sql-statement:` Allows to specify sql statement to be used for building NCache's database dependency. `cache-key-format:` This option is used to configure cache key format for any item of current entity type. Cache key format should include "[pk]" in it, which will be replaced with the primary key of record. cache-key-format may also include `[en]` which will be replaced with entity name. Default value of cache key format is"`NHibernateNCache:[en] #[pk]`. Same cache key format should be used while writing database trigger in case of oledb database dependency. `composite-key-separator:` If the records of current entity have composite key as primary key, `cache-key` is formed by combining all keys separated by composite-key-separator. composite-key-separator must be of one-character length. Its default value is "$".