java qr code reader app Implementing conversations in Java

Printer QR-Code in Java Implementing conversations

Implementing conversations
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// Notify seller and winner ... s.getTransaction().commit(); } }
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The client calls the endAuction() method and passes back the detached item instance this is the same instance returned in the first step. The update() operation on the Session reattaches the detached object to the persistence context and schedules an SQL UDPATE. Hibernate must assume that the client modified the object while it was detached. (Otherwise, if you re certain that it hasn t been modified, a lock() would be sufficient.) The persistence context is flushed automatically when the second transaction in the conversation commits, and any modifications to the once detached and now persistent object are synchronized with the database. The saveOrUpdate() method is in practice more useful than upate(), save(), or lock(): In complex conversations, you don t know if the item is in detached state or if it s new and transient and must be saved. The automatic state-detection provided by saveOrUpdate() becomes even more useful when you not only work with single instances, but also want to reattach or persist a network of connected objects and apply cascading options. Also reread the definition of the merge() operation and when to use merging instead of reattachment: Merging the state of a detached object in chapter 9, section 9.3.2. So far, you ve solved only one of the conversation implementation problems: little code was required to implement the conversation. However, the application user still expects that the unit of work is not only isolated from concurrent modifications, but also atomic. You isolate concurrent conversations with optimistic locking. As a rule, you shouldn t apply a pessimistic concurrency-control strategy that spans a long-running conversation this implies expensive and nonscalable locking. In other words, you don t prevent two operators from seeing the same auction item. You hope that this happens rarely: You re optimistic. But if it happens, you have a conflict resolution strategy in place. You need to enable Hibernate s automatic versioning for the Item persistent class, as you did in Enabling versioning in Hibernate in chapter 10, section 10.2.2. Then, every SQL UPDATE or DELETE at any time during the conversation will include a version check against the state present in the database. You get a StaleObjectStateException if this check fails and then have to take appropriate action. In this case, you present an error message to
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Conversations with Hibernate
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the user ( Sorry, somebody modified the same auction! ) and force a restart of the conversation from step one. How can you make the conversation atomic The conversation spans several persistence contexts and several database transactions. But this isn t the scope of a unit of work from the point of view of the application user; she considers the conversation to be an atomic group of operations that either all fail or all succeed. In the current conversation this isn t a problem, because you modify and persist data only in the last (second) step. Any conversation that only reads data and delays all reattachment of modified objects until the last step is automatically atomic and can be aborted at any time. If a conversation reattaches and commits modifications to the database in an intermediate step, it s no longer atomic. One solution is to not flush the persistence contexts on commit that is, to set a FlushMode.MANUAL on a Session that isn t supposed to persist modifications (of course, not for the last step of the conversation). Another option is to use compensation actions that undo any step that made permanent changes, and to call the appropriate compensation actions when the user aborts the conversation. We won t have much to say about writing compensation actions; they depend on the conversation you re implementing. Next, you implement the same conversation with a different strategy, eliminating the detached object state. You extend the persistence context to span the whole conversation.
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