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The following sections describe the WebLogic Server EJB container and various aspects of EJB behavior in terms of the features and services that the container provides. For information on the specific topic of container-managed persistence, see WebLogic Server Container-Managed Persistence Service - Basic Features, and WebLogic Server Container-Managed Persistence Service - Advanced Features.
The following sections provide information about how the container supports caching services. WebLogic Server uses a free pool to improve performance and throughput for stateless session EJBs.
The following figure illustrates the WebLogic Server free pool, and the processes by which stateless EJBs enter and leave the pool. This can improve initial response time when clients access EJBs, because initial client requests can be satisfied by activating the bean from the free pool (rather than initializing the bean and then activating it). Note: The maximum size of the free pool is limited either by available memory, or the value of the max-beans-in-free-pool deployment element. When a client calls a method on a stateless session EJB, WebLogic Server obtains an instance from the free pool.
If all instances of an EJB class are active and max-beans-in-free-pool has been reached, new clients requesting the EJB class will be blocked until an active EJB completes a method call. WebLogic Server uses a cache of bean instances to improve the performance of stateful session EJBs. The following figure illustrates the WebLogic Server cache, and the processes by which stateful EJBs enter and leave the cache. To achieve high performance, WebLogic Server reserves the cache for EJBs that clients are currently using and EJBs that were recently in use.
Note: Stateful session EJBs must abide by certain rules to ensure that bean fields can be serialized to persistent storage.
The max-beans-in-cache deployment element in the weblogic-ejb-jar.xml file provides some control over when EJBs are passivated. If max-beans-in-cache is reached and EJBs in the cache are not being used, WebLogic Server passivates some of those beans. Note: When an EJB becomes eligible for passivation, it does not mean that WebLogic Server passivates the bean immediately. You can specify the explicit passivation of stateful EJBs that have reached idle-timeout-seconds by setting the cache-type element in the weblogic-ejb-jar.xml file. If you specify LRU, the container passivates the bean when idle-timeout-seconds is reached. If you specify NRU, the container passivates the bean when there is pressure in the cache and idle-timeout-seconds determines how often the container checks to see how full the cache is.
For cached EJB instances: When WebLogic Server detects that EJB classes are approaching their max-beans-in-cache limit, WebLogic Server takes EJB instances that have not been used for idle-timeout-seconds and removes them from the cache (rather than passivating them to disk). If a stateful session bean is idle for longer than idle-timeout-seconds, WebLogic Server may remove the instance from memory as regular cache maintenance, even if the EJB class is max-beans-in-cache limit has not been reached.
Note: Setting idle-timeout-seconds to 0 stops WebLogic Server from removing EJBs that are idle for a period of time. For passivated EJB instances: After a stateful session EJB instance is passivated, a client must use the EJB instance before idle-timeout-seconds is reached. The EJB developer must ensure that a call to the ejbPassivate() method leaves a stateful session bean in a condition where WebLogic Server can serialize its data and passivate the bean's instance. In general, you should not set the max-beans-in-free-pool element for stateless session beans. Option 2: No instance is available in the pool, but the number of instances in use is less then max-beans-in-free-pool.
Option 3: No instances are available in the pool and the number of instances in use is already max-beans-in-free-pool. Stateless Session Beans: WebLogic Server always creates a new instance for stateless session beans. Message-Driven Beans: Illegal instances of message-driven beans are created and registered as JMS listeners during deployment. EJBs in a WebLogic Server cluster use modified versions of two key structures: the Home object and the EJB object. In a WebLogic Server cluster, the client-side representation of the Home object can be replaced by a cluster-aware "stub." The cluster-aware home stub has knowledge of EJB Home objects on all WebLogic Servers in the cluster. In a WebLogic Server cluster, the server-side representation of the EJBObject can also be replaced by a replica-aware EJBObject stub. Whether or not an EJB can use a replica-aware EJBObject stub depends on the type of EJB deployed and, for entity EJBs, the cache strategy selected at deployment time. This section describes cluster capabilities and limitations for stateful and stateless session EJBs. Stateless session EJBs can have both a cluster-aware home stub and a replica-aware EJBObject stub.
This default behavior ensures that database updates within an EJB method are not "duplicated" due to a failover scenario.
At the bean level, if you set stateless-bean-methods-are-idempotent to "true", WebLogic Server assumes that the method is idempotent and will provide failover services for the EJB method, even if a failure occurs during a method call. The following figure illustrates stateless session EJBs in a WebLogic Server clustered environment. To enable stateful session EJBs to use cluster-aware home stubs, set home-is-clusterable to "true." This provides failover and load balancing for stateful EJB lookups. Note: Load balancing and failover are discussed extensively in Using WebLogic Server Clusters. Each time a client of the EJB commits a transaction that modifies the EJB's state, WebLogic Server replicates the bean's state to the secondary server instance.
Should the primary server instance fail, the client's next method invocation is automatically transferred to the EJB instance on the secondary server. Clients of a stateful session EJB are therefore guaranteed to have quick access to the latest committed state of the EJB, except under the special circumstances described in Limitations of In-Memory Replication.
To replicate the state of a stateful session EJB in a WebLogic Server cluster, make sure that the cluster is homogeneous for the EJB class.
By default, WebLogic Server does not replicate the state of stateful session EJB instances in a cluster. By replicating the state of a stateful session EJB, clients are generally guaranteed to have the last committed state of the EJB, even if the primary WebLogic Server instance fails.
A client commits a transaction involving a stateful EJB, but the primary WebLogic Server fails before the EJB's state is replicated. A client creates an instance of a stateful session EJB and commits an initial transaction, but the primary WebLogic Server fails before the EJB's initial state can be replicated. Note: In the preceding figure, the set of three arrows for both home stubs refers to the EJBHome on each server. When you configure a cluster, you supply a cluster address that identifies the Managed Servers in the cluster.
The following sections provide information on how the EJB container supports transaction management services. The current version of WebLogic Server supports Java Transaction API (JTA), which you can use to implement distributed transactional applications. Session EJBs can rely on their own code, their client's code, or the WebLogic Server container to define transaction boundaries. In container-managed transactions, the WebLogic Server EJB container manages the transaction demarcation. The sequence of transaction events differs between container-managed and bean-managed transactions. To define transaction boundaries in EJB or client code, you must obtain a UserTransaction object and begin a transaction before you obtain a Java Transaction Service (JTS) or JDBC database connection. If you start a transaction after obtaining a database connection, the connection has no relationship to the new transaction, and there are no semantics to "enlist" the connection in a subsequent transaction context. Once you create a database connection within a transaction context, that connection becomes "reserved" until the transaction either commits or rolls back. Note: You can associate only a single database connection with an active transaction context.
There are two ways to begin a transaction: explicitly with a user transaction or automatically using the EJB container. You set the isolation level for container-managed transactions in the transaction-isolation element of the weblogic-ejb-jar.xml deployment file.
Many datastores provide limited support for detecting serialization problems, even for a single user connection.
Even if you use the TransactionSerializable setting for an EJB, you may receive exceptions or rollbacks in the EJB client if contention occurs between clients for the same rows.
In addition, use WebLogic Server's optimistic concurrency strategy with a ReadCommitted isolation level.
You specify the locking mechanism that the EJB uses by setting the concurrency-strategy deployment parameter in weblogic-ejb-jar.xml.
The following excerpt from weblogic-ejb-jar.xml shows how to set an optimistic concurrency strategy for an EJB. WebLogic Server does support transactions that are distributed over multiple datasources; a single database transaction can span multiple EJBs on multiple servers. In the following code fragment, a client application obtains a UserTransaction object and uses it to begin and commit a transaction.


In the above code fragment, updates performed by the "account1" and "account2" EJBs occur within the context of a single UserTransaction. The only requirement for wrapping EJB calls in this manner is that both "account1" and "account2" must support the client transaction. The "wrapper" EJB can explicitly obtain a transaction context before invoking other EJBs, or WebLogic Server can automatically create a new transaction context, if the EJB's trans-attribute element is set to Required or RequiresNew.
WebLogic Server provides additional transaction performance benefits for EJBs that reside in a WebLogic Server cluster. In some cases, a transaction can use EJBs that reside on multiple WebLogic Server instances in a cluster.
However, for best performance, the cluster should be homogeneous — all EJBs should reside on all available WebLogic Server instances.
To insert multiple entries into the database in one SQL statement, as described in Batch Operations.
You must specify that the EJB Container delaying the database insert until after ejbPostCreate when a cmr-field is mapped to a foreign-key column that does not allow null values. Note: You may not set the cmr-fields during a ejbCreate method call, before the primary key of the bean is known. Multiple instances of same type of container-managed persistence (cmp) entity beans are often changed in a single transaction. Application developers often have to take the performance impact or use SQL statements directly to update entries in the database; neither solution is desirable.
The EJB batch operations features solves this problem by updating multiple entries in a database table in one SQL statement. To permit batch database inserts, updates or deletes, set the enable-batch-operations element in the weblogic-cmp-rdbms-jar.xml file to True. The batch operations feature includes database operation ordering functionality that can prevent constraint errors by sorting database dependency between batch inserts, updates and deletes. With database ordering feature enabled, the EJB container sorts out these dependencies, and sends batch operations to the database in such way that does not cause any database exceptions. For more information on the order-database-operations element, see order-database-operations.
When using batch operations, you must set the boundary for the transaction, as batch operations only apply to the inserts, updates or deletes between transaction begin and transaction commit. Note: Batch operations only work with drivers that support the addBatch() and executeBatch() methods. The total number of entries created in a single batch operation cannot exceed the max-beans-in-cache setting, which is specified in the weblogic-ejb-jar.xml file. If you set the dbms-column-type element in the weblogic-cmp-rdbms-jar.xml file to either OracleBlob or OracleClob, batch operation automatically turns off because you will not save much time if a Blob or Clob column exist in the database table.
The following sections provide information on how the EJB container supports resource services.
Using resource factories enables the EJB to map a resource factory reference in the EJB deployment descriptor to an available resource factory in a running WebLogic Server. The following sections explain how to bind JDBC datasource and URL resources to JNDI names in WebLogic Server.
With a non-transactional data source, the JDBC connection operates in auto commit mode, committing each insert and update operation to the database immediately, rather than as part of a container-managed transaction. With a transactional data source, multiple insert and update operations in a method can be submitted as a single, container-managed transaction that either commits or rolls back as a logical unit. Note: Entity beans that use container-managed persistence should always use a transactional data source, rather than a non-transactional data source, to preserve data consistency.
Select JDBC Data Source Factory and click the Configure a New JDBC Data Source Factory option in the right pane. Enter values in the Name, User Name, URL, Driver Class Name, and Factory Name, attribute fields. Specify the link to the EJB using the optional ejb-link deployment descriptor element of the ejb-ref element of the referencing application component. Because ejb-names are not required to be unique across EJB JAR files, you may need to provide the qualified path for the link. Use the following syntax to provide the path name for the EJBs within the same J2EE application.
This reference provides the path name of the EJB JAR file that contains the referenced EJB with the appended ejb-name of the target bean separated from the path by "#". For instructions on specifying deployment descriptors, see Specifying and Editing the EJB Deployment Descriptors.
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For each session and entity bean deployed in a container, the container provides a home interface. They describe the life cycle of EJB instances in WebLogic Server, from the perspective of the server. If the transaction times out (or, for non-transactional calls, if five minutes elapse), WebLogic Server throws a RemoteException for a remote client or an EJBException for a local client. The cache stores active EJB instances in memory so that they are immediately available for client requests. As clients look up and obtain references to individual beans, WebLogic Server initializes new instances of the EJB class and stores them in the cache.
Removing, rather than passivating, the instance ensures that "inactive" EJBs do not consume cache or disk resources in WebLogic Server. During passivation, WebLogic Server attempts to serialize any fields that are not declared transient.
The only reason to set max-beans-in-free-pool is to limit access to an underlying resource. You wait until either your transaction times out or a bean instance that already exists in the pool becomes available. Essentially, it means that Option 3 should never happen because you will always just allocate a new bean instance. It describes the behavior of EJBs and their associated transactions in a WebLogic Server cluster, and explains key deployment descriptors that affect EJB behavior in a cluster. In a single server (unclustered) environment, a client looks up an EJB through the EJB's home interface, which is backed on the server by a corresponding Home object. The clustered home stub provides load balancing by distributing EJB lookup requests to available servers. Whether or not a cluster-aware home stub is created is determined by the home-is-clusterable deployment element in weblogic-ejb-jar.xml. This stub maintains knowledge about all copies of the EJBObject that reside on servers in the cluster. By default, WebLogic Server provides failover services for EJB method calls, but only if a failure occurs between method calls. For example, if a client calls a method that increments a value in a datastore and WebLogic Server fails over to another server before the method completes, the datastore would be updated twice for the client's single method call.
See these three sections: "EJB and RMI Objects", "Load Balancing for EJBs and RMI Objects" and "Replication and Failover for EJBs and RMIs". When a stateful session EJB is deployed, WebLogic Server creates a cluster-aware EJBHome stub and a replica-aware EJBObject stub for the stateful session EJB.
Replication of the bean's state occurs directly in memory, for best performance in a clustered environment. The secondary server becomes the primary WebLogic Server for the EJB instance, and a new secondary server handles possible additional failovers. In other words, deploy the same EJB class to every WebLogic Server instance in the cluster, using the same deployment descriptor. In this case, the client's next method invocation works against the previous committed state.
The client's next method invocation fails to locate the bean instance, because the initial state could not be replicated. For example, failover is automatically supported if there is a failure after a method completes, or if the method fails to connect to a server. The cluster address is used in entity and stateless beans to construct the host name portion of URLs. The two-phase commit protocol is a method of coordinating a single transaction across two or more resource managers. EJBs can use container- or client-demarcated transaction boundaries, but they cannot define their own transaction boundaries unless they observe certain restrictions. For EJBs that use container-managed transactions (or EJBs that mix container and bean-managed transactions) you can use several deployment elements to control the transactional requirements for individual EJB methods.
If a JTS connection is not associated with a transaction context, it operates similarly to a standard JDBC connection that has autocommit equal to true, and updates are automatically committed to the datastore.
To maintain performance and throughput for your applications, always ensure that your transaction completes quickly, so that the database connection can be released and made available to other client requests. Therefore, even if you set transaction-isolation to TransactionSerializable, you may experience serialization problems due to the limitations of the datastore. As a consequence, even with a setting of TransactionSerializable, Oracle does not detect serialization problems until commit time.


To avoid these problems, make sure that the code in your client application catches and examines the SQL exceptions, and that you take the appropriate action to resolve the exceptions, such as restarting the transaction. You set concurrency-strategy at the individual EJB level, so that you can mix locking mechanisms within the EJB container. When set, every SELECT query from that point on will have FOR UPDATE added to acquired locks on the selected rows. When set, every SELECT query from that point on will have FOR_UPDATE_NOWAIT added to acquire locks on the selected rows. You can explicitly enable support for these types of transactions by starting a transaction and invoking several EJBs. When a single transaction utilizes multiple EJBs, WebLogic Server attempts to use EJB instances from a single WebLogic Server instance, rather than using EJBs from different servers. This can occur in heterogeneous clusters, where all EJBs have not been deployed to all WebLogic Server instances.
In this case, set the cmr-field to a non-null value in ejbPostCreate before the bean is inserted into the database. Creating these additional instances may make delaying the database insert impossible if database constraints prevent related beans from referring to a bean that has not yet been created. Each cmp entity bean instance is often an entry in a database table, and the EJB container will make a database update for every cmp entity bean instance. Batch operations support increases the performance of container-managed persistence (CMP) bean creation by enabling the EJB container to perform multiple database inserts, deletes or updates for CMP beans in one SQL statement, thereby economizing network roundtrips. For example, performing an update before an insert or after a delete triggers a constraint error.
To enable this database ordering, set the order-database-operations element of weblogic-cmp-rdbms-jar.xml to True.
If the EJB container detects unsupported drivers, it reports that batch operations are not supported and disables batch operations.
In WebLogic Server, EJBs can access JDBC connection pools by directly instantiating a JDBC pool driver.
Although the resource factory reference must define the type of resource factory to use, the actual name of the resource is not specified until the bean is deployed. Note that you can set up either a transactional or non-transactional JDBC datasource as necessary. See Managing JDBC Connectivity in the Administration Console Online Help for more information. See Specifying and Editing the EJB Deployment Descriptors for instructions on editing deployment descriptors. You can link an EJB reference that is declared in one application component to an enterprise bean that is declared in the same J2EE application.
The target EJB can be in any EJB JAR file in the same J2EE application as the referencing application component.
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During the entire life cycle of an EJB object, from its creations to removal, it lives in the container. The home interface allows a client to create, find, and remove entity objects that belong to the entity bean as well as to execute home business methods which are not specific to a particular entity bean object.
These sections use the term EJB instance to refer to an actual instance of the EJB bean class. Unbound EJB instances are instances of a stateless session EJB class that are not processing a method call.
However, if you want inactive instances of the EJB to exist in WebLogic Server when it is started, specify how many in the initial-beans-in-free-pool deployment descriptor element, in the weblogic-ejb-jar.xml file. Active EJBs consist of instances that are currently in use by a client, as well as instances that were recently in use, as described in the following sections. Passivation is the process by which WebLogic Server removes an EJB from cache while preserving the EJB's state on disk. Passivation occurs only when the EJB is eligible for passivation and there is pressure on server resources, or when WebLogic Server performs regular cache maintenance.
This means that you must ensure that all non-transient fields represent serializable objects, such as the bean's remote or home interface.
For example, if you use stateless session EJBs to implement a legacy connection pool, you do not want to allocate more bean instances than the number of connections that can support your legacy system. After referencing the bean, the client interacts with the bean's methods through the remote interface, which is backed on the server by an EJB object.
It can also provide failover support for lookup requests, because it routes those requests to available servers when other servers have failed.
For example, failover is automatically supported if a failure occurs after a method completes, or if the method fails to connect to a server.
For more information on in-memory replication for stateful session EJBs, see In-Memory Replication for Stateful Session EJBs. Whereas in WebLogic Server 5.1 only the EJBHome object is clustered for stateful session EJBs, the EJB container can also replicate the state of the EJB across clustered WebLogic Server instances. The EJBObject stub maintains a list of the primary WebLogic Server instances on which the EJB instance runs, as well as the name of a secondary WebLogic Server to use for replicating the bean's state.
Should the EJB's secondary server fail, WebLogic Server enlists a new secondary server instance from the cluster. To enable replication, set the replication-type deployment parameter in the weblogic-ejb-jar.xml deployment file to InMemory. The client needs to recreate the EJB instance, using the clustered EJBHome stub, and restart the transaction. EJBs that engage in distributed transactions (transactions that make updates in multiple datastores) guarantee that all branches of the transaction commit or roll back as a logical unit. It guarantees data integrity by ensuring that transactional updates are committed in all participating databases, or are fully rolled back out of all the databases, reverting to the state prior to the start of the transaction. The isolation level defines how concurrent transactions accessing a persistent store are isolated from one another for read purposes. The behavior of the transaction depends both on the EJB's isolation level setting and the concurrency control of the underlying persistent store. Consequently, if Oracle cannot lock the rows affected by the query immediately, then it waits until the rows are free.
Consequently, if Oracle cannot lock the rows affected by the query immediately, then Oracle terminates the query before completion.
Or, a single EJB can invoke other EJBs that implicitly work within the same transaction context. The wrapper EJB responds by starting a new transaction and invoking one or more EJBs to do the work of the transaction. All EJBs invoked by the wrapper EJB must be able to support the transaction context (their trans-attribute elements must be set to Required, Supports, or Mandatory).
In these cases, WebLogic Server uses a multitier connection to access the datastore, rather than multiple direct connections.
Setting either of these options specifies the precise time at which the EJB Container inserts a new bean that uses RDBMS CMP into the database. Sometimes, a transaction needs to update thousands of cmp entity bean instances, so it will cause thousands of database roundtrips.
However, it is recommended that you instead bind a JDBC datasource resource into the WebLogic Server JNDI tree as a resource factory. The EJB container provides a standard set of services, including caching, concurrency, persistence, security, transaction management, locking, environment, memory replication, and clustering for the EJB objects that live in the container. A client can look up the entity bean's home interface through the Java Naming and Directory Interface (JNDI) or by following an EJB reference, which is preferred.
EJB instance does not refer to the logical instance of the EJB as seen from the point of view of a client.
Because WebLogic Server unbinds stateless session beans from clients after each method call, the actual bean class instance that a client uses may be different from invocation to invocation. The cache is unlike the free pool insofar as stateful session beans in the cache are bound to a particular client, while the stateless session beans in the free pool have no client association. While passivated, EJBs use minimal WebLogic Server resources and are not immediately available for client requests (as they are while in the cache). For example, if a client invokes an EJB method call on a particular WebLogic Server and the server goes down, the EJBObject stub can failover the method call to another, running server. When failures occur while an EJB method is in progress, WebLogic Server does not automatically fail over from one server to another.
For more information on setting container-managed transaction isolation levels, see Programming WebLogic JTA. The container is responsible for making the entity bean's home interface available in the JNDI name space.
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