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Taming wild EJBs: performance and scalability
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You can either cache entities or queries in the PersistenceUnit cache. When you retrieve some entities using a query, those entities will be cached. If you try to retrieve a cached entity by using the EntityManager s find method, then the entity instance will be returned from the cache. Typically, persistence providers store the entities in the cache using their identities, so you must use the find method to retrieve an entity by its primary key. If your EJB 3 persistence provider supports caching of entities in a PersistenceUnit cache, it is probably done with a vendor-specific name-value pair of properties in a configuration file. In the following example we are trying to cache 5000 instances of the Category entity in TopLink Essentials:
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<persistence> <persistence-unit name = "actionBazaar"> ... <properties> <property name = "toplink.cache.type.Category" value = "CacheType.Softweak"/> <property name = "toplink.cache.size.Category" value = "5000"/> </properties> </persistence-unit> </persistence>
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If you want to cache a query result, then you probably want to do it on a per-query basis, either stored in an external configuration or as a QueryHint for the query. Check your persistence provider documentation to determine how it supports the caching of queries. Here is an example of how TopLink Essentials can be used for caching in a named query using @QueryHint:
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@NamedQuery( name = "findActiveCategory", query = "SELECT c FROM Category c WHERE c.status = Active", hints = {@QueryHint(name = "toplink.cache-usage", value = "CheckCacheOnly")} )
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You can also provide a hint to refresh the cache from the database with a query like this:
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Set<Category> category = (Category) em.createNamedQuery("findActiveCategory") .setHint("toplink.refresh", "true") .getResultList();
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Now that you have seen some examples of caching, let s discuss some caching best practices.
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Improving performance of EJB 3 components
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Read-only entities You can significantly improve application performance by using read-only entities. Examine your applications to determine whether any entities may be made read-only. For example, in our ActionBazaar system, office locations change very rarely and we can make the ShippingType entity read-only. Unfortunately, like caching features, the read-only feature is a vendor extension and you have to depend on either a vendor-specific API or configuration to mark an entity as readonly. Normally, read-only entity instances will be loaded into the PersistenceUnit cache and never discarded. Read-only entities will significantly improve performance because persistence providers won t calculate the change set for the read-only entity, and no clone or merge operations will be performed against a read-only entity. Caching best practices Understanding your applications and checking your caching strategy usually makes sense for your applications. The cache is best used for entities that are not frequently updated or that are read only. Make sure that table data is updated only by the application that is using the cache, because if any external applications update the same cache, someone working with the cache will end up with stale data. Check your persistence provider for details on how they support caching entities and queries; the caching mechanism varies from provider to provider. For example, one provider may follow this rule: if an entity is updated in a transaction, then the entity will be updated in the cache when the transaction is committed to the database. However, some persistence providers may choose to expire the cache instead. Stress-test your applications with and without a cache. It s the only way to determine if using the cache, and more specifically the particular cache configuration, will improve or degrade your applications performance. That concludes our discussion on improving EJB 3 entity performance. Are you feeling faster yet Even if you re not, your entities should be. Next, let s look at how you can improve the performance of session and message-driven beans.
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13.3 Improving performance of EJB 3 components
Most application servers provide the ability to generate usage and utilization statistics of EJB components such as session beans and MDBs grouped by application. You have to read your vendor documentation about the parameters they provide,
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