Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Developer's Guide

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# Value Added Features

The Application Server provides a number of value additions that relate to EJB development. These capabilities are discussed in the following sections (references to more in-depth material are included):

- Read-Only Beans

- pass-by-reference

- Pooling and Caching

- Bean-Level Container-Managed Transaction Timeouts

- Priority Based Scheduling of Remote Bean Invocations

- Immediate Flushing

## Read-Only Beans

Another feature that the Application Server provides is the read-only bean, an entity bean that is never modified by an EJB client. Read-only beans avoid database updates completely. A read-only bean is not portable.

A read-only bean can be used to cache a database entry that is frequently accessed but rarely updated (externally by other beans). When the data that is cached by a read-only bean is updated by another bean, the read-only bean can be notified to refresh its cached data.

The Application Server provides a number of ways by which a read-only bean’s state can be refreshed. By setting the `refresh-period-in-seconds` element in the `sun-ejb-jar.xml` file and the `trans-attribute` element in the `ejb-jar.xml` file, it is easy to configure a read-only bean that is (a) always refreshed, (b) periodically refreshed, (c) never refreshed, or (d) programmatically refreshed.

Read-only beans are best suited for situations where the underlying data never changes, or changes infrequently. For further information and usage guidelines, see Using Read-Only Beans.

## pass-by-reference

The `pass-by-reference` element in the `sun-ejb-jar.xml` file allows you to specify the parameter passing semantics for colocated remote EJB invocations. This is an opportunity to improve performance. However, use of this feature results in non-portable applications. See pass-by-reference.

## Pooling and Caching

The EJB container of the Application Server pools anonymous instances (message-driven beans, stateless session beans, and entity beans) to reduce the overhead of creating and destroying objects. The EJB container maintains the free pool for each bean that is deployed. Bean instances in the free pool have no identity (that is, no primary key associated) and are used to serve the method calls of the home interface. The free beans are also used to serve all methods for stateless session beans.

Bean instances in the free pool transition from a Pooled state to a Cached state after `ejbCreate` and the business methods run. The size and behavior of each pool is controlled using pool-related properties in the EJB container or the `sun-ejb-jar.xml` file.

In addition, the Application Server supports a number of tunable parameters that can control the number of “stateful” instances (stateful session beans and entity beans) cached as well as the duration they are cached. Multiple bean instances that refer to the same database row in a table can be cached. The EJB container maintains a cache for each bean that is deployed.

To achieve scalability, the container selectively evicts some bean instances from the cache, usually when cache overflows. These evicted bean instances return to the free bean pool. The size and behavior of each cache can be controlled using the cache-related properties in the EJB container or the `sun-ejb-jar.xml` file.

Pooling and caching parameters for the `sun-ejb-jar.xml` file are described in bean-cache.

### Pooling Parameters

One of the most important parameters of Application Server pooling is `steady-pool-size`. When `steady-pool-size` is set to greater than 0, the container not only pre-populates the bean pool with the specified number of beans, but also attempts to ensure that there is always this many beans in the free pool. This ensures that there are enough beans in the ready to serve state to process user requests.

This parameter does not necessarily guarantee that no more than `steady-pool-size` instances exist at a given time. It only governs the number of instances that are pooled over a long period of time. For example, suppose an idle stateless session container has a fully-populated pool with a `steady-pool-size` of `10`. If 20 concurrent requests arrive for the EJB component, the container creates 10 additional instances to satisfy the burst of requests. The advantage of this is that it prevents the container from blocking any of the incoming requests. However, if the activity dies down to 10 or fewer concurrent requests, the additional 10 instances are discarded.

Another parameter, `pool-idle-timeout-in-seconds`, allows the administrator to specify, through the amount of time a bean instance can be idle in the pool. When `pool-idle-timeout-in-seconds` is set to greater than 0, the container removes or destroys any bean instance that is idle for this specified duration.

### Caching Parameters

Application Server provides a way that completely avoids caching of entity beans, using commit option C. Commit option C is particularly useful if beans are accessed in large number but very rarely reused. For additional information, refer to Commit Options.

The Application Server caches can be either bounded or unbounded. Bounded caches have limits on the number of beans that they can hold beyond which beans are passivated. For stateful session beans, there are three ways (LRU, NRU and FIFO) of picking victim beans when cache overflow occurs. Caches can also passivate beans that are idle (not accessed for a specified duration).

## Bean-Level Container-Managed Transaction Timeouts

The default transaction timeout for the domain is specified using the Transaction Timeout setting of the Transaction Service. A transaction started by the container must commit (or rollback) within this time, regardless of whether the transaction is suspended (and resumed), or the transaction is marked for rollback.

To override this timeout for an individual bean, use the optional `cmt-timeout-in-seconds` element in `sun-ejb-jar.xml`. The default value, `0`, specifies that the default Transaction Service timeout is used. The value of `cmt-timeout-in-seconds` is used for all methods in the bean that start a new container-managed transaction. This value is not used if the bean joins a client transaction.

## Priority Based Scheduling of Remote Bean Invocations

You can create multiple thread pools, each having its own work queues. An optional element in the `sun-ejb-jar.xml` file, `use-thread-pool-id`, specifies the thread pool that processes the requests for the bean. The bean must have a remote interface, or `use-thread-pool-id` is ignored. You can create different thread pools and specify the appropriate thread pool ID for a bean that requires a quick response time. If there is no such thread pool configured or if the element is absent, the default thread pool is used.

## Immediate Flushing

Normally, all entity bean updates within a transaction are batched and executed at the end of the transaction. The only exception is the database flush that precedes execution of a finder or select query.

Since a transaction often spans many method calls, you might want to find out if the updates made by a method succeeded or failed immediately after method execution. To force a flush at the end of a method’s execution, use the flush-at-end-of-method element in the `sun-ejb-jar.xml` file. Only non-finder methods in the Local, Local Home, Remote, and Remote Home interfaces of an entity bean can be flush-enabled.

Upon completion of the method, the EJB container updates the database. Any exception thrown by the underlying data store is wrapped as follows:

- If the method that triggered the flush is a `create` method, the exception is wrapped with `CreateException`.

- If the method that triggered the flush is a `remove` method, the exception is wrapped with `RemoveException`.

- For all other methods, the exception is wrapped with `EJBException`.

All normal end-of-transaction database synchronization steps occur regardless of whether the database has been flushed during the transaction.

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