2.3. Lock management and concurrency control

Concurrency control information within TxCore is maintained by locks. Locks which are required to be shared between objects in different processes may be held within a lock store, similar to the object store facility presented previously. The lock store provided with TxCore deliberately has a fairly restricted interface so that it can be implemented in a variety of ways. For example, lock stores are implemented in shared memory; on the Unix file system (in several different forms); and as a remotely accessible store. More information about the object stores available in TxCore can be found in the Appendix.

Note

As with all TxCore classes the default lock stores are pure Java implementations; to access the shared memory and other more complex lock store implementations it is necessary to use native methods.
public class LockStore
{
    public abstract InputObjectState read_state (Uid u, String tName)
    throws LockStoreException;
    
    public abstract boolean remove_state (Uid u, String tname);
    public abstract boolean write_committed (Uid u, String tName,
    OutputObjectState state);
    };

2.3.1. Selecting a lock store implementation

TxCore comes with support for several different object store implementations. If the object model being used is SINGLE, then no lock store is required for maintaining locks, since the information about the object is not exported from it. However, if the MULTIPLE model is used, then different run-time environments (processes, Java virtual machines) may need to share concurrency control information. The implementation type of the lock store to use can be specified for all objects within a given execution environment using the com.arjuna.ats.txoj.lockstore.lockStoreType property variable. Currently this can have one of the following values:
BasicLockStore
This is an in-memory implementation which does not, by default, allow sharing of stored information between execution environments. The application programmer is responsible for sharing the store information.
BasicPersistentLockStore
This is the default implementation, and stores locking information within the local file system. Therefore execution environments that share the same file store can share concurrency control information. The root of the file system into which locking information is written is the LockStore directory within the TxCore installation directory. This can be overridden at runtime by setting the com.arjuna.ats.txoj.lockstore.lockStoreDir property variable accordingly, or placing the location within the CLASSPATH:
java -D com.arjuna.ats.txoj.lockstore.lockStoreDir=/var/tmp/LockStore myprogram
or
java –classpath $CLASSPATH;/var/tmp/LockStore myprogram
If neither of these approaches is taken, then the default location will be at the same level as the etc directory of the installation.