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Improving entity performance
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Keep these three levels in mind when evaluating possible options to put in your cache. Try moving items from one cache to another if possible, to determine which configuration works best for your specific application and runtime usage patterns. Transactional cache Transactional caching ensures that the object from the cache is returned when the same object is requested again. A typical example is that you run a query that returns an Item entity, and that entity will be cached in the transactional cache. When you use the EntityManager.find method to retrieve the Item again, the persistence provider will return the same Item instance from the transactional cache. The other benefit of a transactional cache is that all updates to an entity are deferred to the end of the transaction. Imagine what would happen if you did the following in the same transaction:
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Item item = new Item(); item.setTitle(title); item.setInitialPrice(initialPrice); Seller seller = entityManager.find(Seller.class, sellerId); item.setSeller(seller); entityManager.persist(item); item.setInitialPrice(newInitialPrice);
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If your persistence provider doesn t use a cache and doesn t defer the commit until the end of the transaction, then it probably will perform what translates to at least two SQL statements. First, it will execute a SQL INSERT to persist the Item. This will be followed by a SQL UPDATE to modify the initialPrice. Most persistence providers will make use of a cache and then execute a single INSERT that will take the new price into account. Hibernate calls this cache its first level or session cache, and TopLink calls it the UnitOfWork cache. Both of these products enable these caches by default. Check whether your persistence provider supports this type of cache. We aren t aware of any reason you d want to disable the transactional cache. Using an extended persistence context The transaction cache will demarcate a single transaction. This could be a problem if your application needs to maintain conversational state between different method calls, since it will require multiple round-trips to the database to retrieve the same entity. You can avoid this situation by using an extended persistence context. You may remember from our discussions in chapter 9 that only stateful session beans support extended persistence contexts. They allow you to keep entity
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Taming wild EJBs: performance and scalability
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instances managed beyond single method calls and transaction boundaries. You can use extended persistence context as a cache for your stateful bean instance during the state of conversation, which can reduce the number of round-trips to your database. Additionally, all database updates resulting from persist, merge, remove, and so forth are queued until the end of the persistence context, reducing the number of database operations. Listing 13.1 shows how you can use an extended persistence context for caching entities between method calls of your application.
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Listing 13.1 Using an extended persistence context to cache objects across method calls
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@Stateful @TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED) public class PlaceOrderBean implements PlaceOrder { @PersistenceContext(unitName = "ActionBazaar", type = PersistenceContextType.EXTENDED) EntityManager em; private private private private Bidder bidder; List<Item> items; ShippingInfo shippingInfo; BillingInfo billingInfo;
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Instance variable to hold entities
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public PlaceOrderBean() { items = new ArrayList<Item>(); } public void setBidder(String bidderId) { this.bidder = em.find(Bidder.class,bidderId); } public Bidder getBidder(){ return this.bidder; } public void addItem(Long itemId) { Item item = em.find(Item.class,itemId); items.add(item); } public void setShippingInfo(ShippingInfo shippingInfo) { this.shippingInfo = shippingInfo; } public void setBillingInfo(BillingInfo billingInfo) { this.billingInfo = billingInfo; em.merge(billingInfo); Method to merge detached entity }
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Improving entity performance
@Remove Remove method @TransactionAttribute(TransactionAttributeType.REQUIRED) public Long confirmOrder() { Transactional Order order = new Order(); method order.setBidder(bidder); order.setBillingInfo(billingInfo); ... em.persist(order); return order.getOrderId(); } }
The PlaceOrderBean uses an extended EntityManager by setting the PersistenceContextType to EXTENDED. The persistence context will live during the entire state of conversation and will be destroyed when the bean instance is destroyed or removed e. You can store entities as the instance variables of the stateful session bean b and values for the instances are set by different methods c, and the entities are managed during the lifespan of the extended EntityManager and can be used without having to be detached at the end of the method call. Entity operations such as merge d can be performed outside the scope of a transaction since we have set the default transaction attribute for the bean to NOT_SUPPORTED. The database updates resulting from these operations are queued up and performed when the persistence context is associated with a transaction f. This reduces the number of round-trips to the database. However, you have to be careful when your stateful session bean invokes another EJB such as a stateless session bean there are several limitations related to the propagation of extended persistence contexts. Persistence unit level cache The transactional and persistence context caching mechanisms can only be used with a single client and cannot be shared. You ll probably see a real performance boost when entity instances in the cache are shared by multiple clients, thus reducing trips to the database for all of them. You can call this an application cache, but we call it a PersistenceUnit cache because entities are scoped in a persistence unit. Hibernate calls this a second-level cache, and you need to configure an external cache provider to take advantage of this second level or session factory level cache. TopLink refers to it as a session cache, and it s integrated with the persistence provider. In addition, the TopLink implementation provides several options for configuring this cache. Review your vendor documentation to see whether it provides the ability to cache objects out of the box.
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