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10.2.2. The Object Store

The Object Store provided with JBoss Transaction Service has a fairly restricted interface so that it can be implemented in a variety of ways. For example, object stores may reside in shared memory, in a local file system, or in a remote database. More complete information about the Object Stores available in JBoss Transaction Service can be found in the Appendix.

Note

As with all JBoss Transaction Service classes, the default Object Stores are pure Java implementations. You need to use native methods to access the shared memory and other more complex object store implementations.
All of the object stores hold and retrieve instances of the InputObjectState and OutputObjectState classes, which are named by the Uid and Type of the object that they represent. States are read using the read_committed method and written by the system using the write_uncommitted method. Normally, new object states do not overwrite old object states, but are written to the store as shadow copies. These shadows replace the original only when the commit_state method is invoked. All interaction with the object store is performed by JBoss Transaction Service system components as appropriate, hiding the existence of any shadow versions of objects from the programmer. 
public class ObjectStore
{
    public static final int OS_COMMITTED;
    public static final int OS_UNCOMMITTED;
    public static final int OS_COMMITTED_HIDDEN;
    public static final int OS_UNCOMMITTED_HIDDEN;
    public static final int OS_UNKNOWN;

    /* The abstract interface */
    public abstract boolean commit_state (Uid u, String name)
	throws ObjectStoreException;
    public abstract InputObjectState read_committed (Uid u, String name)
	throws ObjectStoreException;
    public abstract boolean write_uncommitted (Uid u, String name,
					       OutputObjectState os) throws ObjectStoreException;
    . . .
};
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When a transactional object is committing, it needs to make certain state changes persistent, so that it can recover in the event of a failure and either continue to commit, or rollback. When using Transactional Objects for Java, JBoss Transaction Service manages this persistence automatically. To guarantee ACID properties, these state changes are flushed to the persistence store implementation before the transaction commits. Otherwise, the application assumes that the transaction has committed, even though the state changes may still exist within an operating system cache, vulnerable to a system failure. By default, JBoss Transaction Service flushes such state changes. As a trade-off, this behavior can impose a significant performance penalty on the application. To prevent transactional object state flushes, set the com.arjuna.ats.arjuna.objectstore.objectStoreSync variable to OFF.
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