Chapter 12. Hibernate Second Level Cache

Chapter 12. 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 Super Client. Super Client is a member of the cluster, it has socket connection to every member in the cluster and it knows where the data, but does not contain any data.
```
<property name="hibernate.cache.hazelcast.use_super_client">true</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. _(Native Client property takes precedence over Super Client property.)_
```
<property name="hibernate.cache.hazelcast.use_native_client">true</property>
```
To setup Native Client properly, you should add Hazelcastgroup-name, group-password and cluster member hosts properties. Member hosts are comma-seperated addresses. Additionally you can add port number at the end of each address.
```
<property name="hibernate.cache.hazelcast.native_client_hosts">10.34.22.15,127.0.0.1:5703</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 & SuperClient

If you are using one of Hibernate pre-3.3 version, add following property.

<property name="hibernate.cache.provider_class">com.hazelcast.hibernate.provider.HazelcastCacheProvider</property>

If you are using Hibernate 3.3.x (or newer) version, you can choose to use either configuration property above (Hibernate has a built-in bridge to use old-style cache implementations) or following property.
```
<property name="hibernate.cache.region.factory_class">com.hazelcast.hibernate.HazelcastCacheRegionFactory</property>
```

Hazelcast creates a seperate 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 4 cache concurrency strategies:read-only,read-write, nonstrict-read-write andtransactional. 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 not support for transactional strategy yet.

If you are using xml based class configurations, you should add a cache element into your configuration with usage attribute with one ofread-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 {
     ...
}

Accessing underlying HazelcastInstance is possible by using HazelcastAccessor.

   SessionFactory sessionFactory = ...;
    HazelcastInstance hazelcastInstance = HazelcastAccessor.getHazelcastInstance(sessionFactory);

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

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