In order to address multi-user scenarios successfully, several aspects of the framework should be noted.
A SessionContext is an expensive-to-create, threadsafe object intended to be shared by all application threads. It is created once, usually on application startup, from a Configuration instance. A BeanPool is an inexpensive, non-threadsafe object that should be used once, for a single request (single unit of work) and then discarded. The CurrentBeanPoolContext interface defines the contract for implementations which knows how to scope the notion of a current bean pool. ThreadLocalCurrentBeanPoolContext, which maintains current bean pools for the given execution thread, is provided as an example implementation of this interface.
The specification of JAXR does not support transactions or locking. CSAF and CentraSite's implementation extend the API with some locking and transaction capabilities. Here are some points to note:
Transactions are handled internally and control over them (including
isolation, demarcation, etc.) is not exposed through CSAF. There is only
support for bulk operations by using the
BeanPool.delete(java.util.Collection) and
BeanPool.update(java.util.Collection) methods. These
methods guarantee the atomicity of the performed operation. There is also a
BeanPool.flush() which performs one bulk operation for
the deleted beans and one for the created and updated beans.
Each modification to a registry bean (RegistryBean instance) leads to obtaining an exclusive lock for writing on the whole registry object in the database. This is a pessimistic locking strategy, as the lock is obtained when the object is modified and not when it is actually persisted.
Whenever a lock on a registry object cannot be obtained (because it is taken by another client), a com.softwareag.centrasite.appl.framework.persistence.LockNotAvailableException is thrown.
The notion of an outdated object denotes a registry object whose database representation has been changed since it was read. This is usually caused by a different client modifying the same instance. Trying to modify an outdated object leads to a com.softwareag.centrasite.appl.framework.persistence.ObjectOutdatedException. CSAF supports automatic re-reading of outdated objects; this forces a re-read of the object from the database before applying the changes. See Configuration for more details on how this can be configured.
In general, the application should minimize the time a registry object is kept locked in the database, i.e., the time during which there are ongoing modifications on it.
