220 Epilogue be employed on an as-required basis,
220 Epilogue be employed on an as-required basis, although in most cases the default fetch group will remain adequate. In the interim, look to the JDO vendors to implement their own enhancements ahead of any standardization in this area. 13.1.5 Optimistic transactions Optimistic transactions are an optional feature of JDO 1.0, although this feature is under-specified, especially in the areas of isolation and failure recovery. In particular, there is no straightforward and portable way of discerning the instances for which optimistic concurrency assumptions have failed. Furthermore, the application remains responsible for refreshing all instances associated with failed optimistic transactions, a tedious task for which additional support should be provided. As users of JDO employ optimistic transactions and vendors provide proprietary enhancements to make the task easier, the JDO Expert Group will be working to standardize the necessary features. 13.1.6 Standardized O-R mapping JDO 1.0 does not state how the persistent fields of an instance are mapped to the rows and columns of pre-existing database schemas. This work is being undertaken independently by each RDBMS-based JDO vendor in order to achieve competitive superiority. In due course the description of these mappings may be standardized, which would improve the portability of such mappings between implementations. 13.1.7 Event-driven reactive instances Enterprise applications are making significant use of JMS for inter-component asynchronous communication. Increasingly, JMS is being employed in the business-to-business arena. The content of such messages typically needs to be persisted. A message-driven bean can be constricted that will persist incoming data through JDO. This was illustrated in Chapter 11. However, this approach mandates the presence of a J2EE application server. It is easy to conceptualize a JDO instance which itself reacts to incoming JMS messages, through a direct JMS/JDO connection which is independent of J2EE and the message-driven bean architecture. It will be interesting to see how JDO evolves in this area. 13.2 Sources of further information Further information about JDO is available in a number of online forums that are updated regularly.
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