Chapter 13. Hibernate Second Level Cache

Chapter 13. Hibernate Second Level Cache
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Hazelcast provides distributed second level cache for your Hibernate entities, collections and queries. Hazelcast has two implementations of Hibernate 2nd level cache, one for hibernate-pre-3.3 and one for hibernate-3.3.x versions. In your Hibernate configuration file (ex: hibernate.cfg.xml), add these properties:

To enable use of second level cache

<property name="hibernate.cache.use_second_level_cache">true</property>

To enable use of query cache

<property name="hibernate.cache.use_query_cache">true</property>

And to force minimal puts into cache

<property name="hibernate.cache.use_minimal_puts">true</property>

To configure Hazelcast for Hibernate, it is enough to put configuration file named hazelcast.xml into root of your classpath. If Hazelcast can not find hazelcast.xml then it will use default configuration from hazelcast.jar.

You can define custom named Hazelcast configuration xml file with one of these Hibernate configuration properties.

<property name="hibernate.cache.provider_configuration_file_resource_path">
     hazelcast-custom-config.xml
</property>

<property name="hibernate.cache.hazelcast.configuration_file_path">
     hazelcast-custom-config.xml
</property>

You can set up Hazelcast to connect cluster as Native Client. Native client is not member and it connects to one of the cluster members and delegates all cluster wide operations to it. When the relied cluster member dies, client will transparently switch to another live member.
```
<property name="hibernate.cache.hazelcast.use_native_client">true</property>
```
To setup Native Client properly, you should add Hazelcast group-name, group-password and cluster member address properties. Native Client will connect to defined member and will get addresses of all members in the cluster. If the connected member will die or leave the cluster, client will automatically switch to another member in the cluster.
```
<property name="hibernate.cache.hazelcast.native_client_address">10.34.22.15</property>
<property name="hibernate.cache.hazelcast.native_client_group">dev</property>
<property name="hibernate.cache.hazelcast.native_client_password">dev-pass</property>
```
To use Native Client you should add hazelcast-client-<version>.jar into your classpath.
Read more about NativeClient

To define Hibernate RegionFacyory, add following property.

<property name="hibernate.cache.region.factory_class">
     com.hazelcast.hibernate.HazelcastCacheRegionFactory
</property>

Or as an alternative you can use HazelcastLocalCacheRegionFactory which stores data in local node and sends invalidation messages when an entry is updated on local.

<property name="hibernate.cache.region.factory_class">
     com.hazelcast.hibernate.HazelcastLocalCacheRegionFactory
</property>

Hazelcast creates a separate distributed map for each Hibernate cache region. So these regions can be configured easily via Hazelcast map configuration. You can define backup, eviction, TTL and Near Cache properties.

Hibernate has four cache concurrency strategies: read-only, read-write, nonstrict-read-write and transactional. But Hibernate does not forces cache providers to support all strategies. And Hazelcast supports first three (read-only, read-write, nonstrict-read-write) of these four strategies. Hazelcast has no support fortransactional strategy yet.

If you are using xml based class configurations, you should add a cache element into your configuration with usage attribute with one of read-only, read-write, nonstrict-read-write.

<class name="eg.Immutable" mutable="false">
    <cache usage="read-only"/>
    .... 
</class>

<class name="eg.Cat" .... >
    <cache usage="read-write"/>
    ....
    <set name="kittens" ... >
        <cache usage="read-write"/>
        ....
    </set>
</class>

If you are using Hibernate-Annotations then you can add class-cache or collection-cache element into your Hibernate configuration file with usage attribute with one of read only, read/write, nonstrict read/write.
```
<class-cache usage="read-only" class="eg.Immutable"/>
<class-cache usage="read-write" class="eg.Cat"/>
<collection-cache collection="eg.Cat.kittens" usage="read-write"/>
```

Alternatively, you can put Hibernate Annotation's @Cache annotation on your entities and collections.

@Cache(usage = CacheConcurrencyStrategy.READ_WRITE)
public class Cat implements Serializable {
     ...
}

The last thing you should be aware of is to drop hazelcast-hibernate-<version>.jar into your classpath.

Additional Properties:

Accessing underlying HazelcastInstance
Using com.hazelcast.hibernate.instance.HazelcastAccessor you can access the underlying HazelcastInstance used by Hibernate SessionFactory.
```
SessionFactory sessionFactory = ...;
HazelcastInstance hazelcastInstance = HazelcastAccessor.getHazelcastInstance(sessionFactory);        
```
Changing/setting lock timeout value of read-write strategy
Lock timeout value can be set using hibernate.cache.hazelcast.lock_timeout_in_seconds Hibernate property. Value should be in seconds and default value is 300 seconds.
Using named HazelcastInstance
Instead of creating a new HazelcastInstance for each SessionFactory, an existing instance can be used by setting hibernate.cache.hazelcast.instance_name Hibernate property to HazelcastInstance's name. For more information see Named HazelcastInstance.
Disabling shutdown during SessionFactory.close()
Shutting down HazelcastInstance can be disabled during SessionFactory.close() by setting hibernate.cache.hazelcast.shutdown_on_session_factory_close Hibernate property to false. (In this case Hazelcast property hazelcast.shutdownhook.enabled should not be set to false.) Default value is true.

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