19.2.2. Strategy: read only
		If your application needs to read, but not modify, instances of a persistent class, a 
read-only cache can be used. This is the simplest and optimal performing strategy. It is even safe for use in a cluster.
	<class name="eg.Immutable" mutable="false">
    <cache usage="read-only"/>
    ....
</class>
<class name="eg.Immutable" mutable="false">
    <cache usage="read-only"/>
    ....
</class>