Unshareable and shareable connections

Operating Systems: i5/OS
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Unshareable and shareable connections

WebSphere Application Server supports both unshareable and shareable connections. An unshareable connection is not shared with other components in the application. The component using this connection has full control of this connection.

Access to a resource marked as unshareable means that there is a one-to-one relationship between the connection handle a component is using and the physical connection with which the handle is associated. This access implies that every call to the getConnection method returns a connection handle solely for the requesting user. Typically, choose unshareable if you might do things to the connection that could result in unexpected behavior occurring in another application that is sharing the connection (for example, unexpectedly changing the isolation level).

Marking a resource as shareable allows for greater scalability. Instead of creating new physical connections on every getConnection() invocation, the physical connection (that is, managed connection) is shared through multiple connection handles, as long as each getConnection request has the same connection properties. However, sharing a connection means that each user must not do anything to the connection that could change its behavior and disrupt a sharing partner (for example, changing the isolation level). The user also cannot code an application that assumes sharing to take place because it is up to the run time to decide whether or not to share a particular connection.

Connection property requirements

To permit sharing of connections used within the same transaction, the following data source properties must be the same:

Java Naming and Directory Interface (JNDI) name. While not actually a connection property, this requirement simply means that you can only share connections from the same data source in the same server.
Resource authentication
In relational databases:
- Isolation level (corresponds to access intent policies applied to CMP beans)
- Readonly
- Catalog
- TypeMap

Additionally, the following connection factories properties must be the same for the sharing of connections within the same transaction:

JNDI name. While not actually a connection property, this requirement simply means that you can only share connections from the same resource adapter in the same server.
Resource authentication

In addition, the ConnectionSpec object used to get the connection must also be the same. For more information on sharing a connection with a CMP bean, see Sharing a connection with a CMP bean.

Java Message Service (JMS) connections cannot be shared with non-JMS connections.

Sharing a connection with a CMP bean

WebSphere Application Server allows you to share a physical connection among a CMP bean, a BMP bean, and a JDBC application to reduce the resource allocation or deadlock scenarios. There are several ways to ensure that all of these entity beans and the JDBC applications are sharing the same physical connection.

Sharing a connection between CMP beans or methods
When all CMP bean methods use the same access intent, they all share the same physical connection. A different access intent policy triggers the allocation of a different physical connection. For example, a CMP bean has two methods; method 1 is associated with wsPessimisticUpdate intent, whereas method 2 has wsOptimisticUpdate access intent. Method 1 and method 2 cannot share the same physical connection within a transaction. In other words, an XA data source is required to run in a global transaction.
You can experience some deadlocks from a database if both methods try to access the same table. Therefore, sharing a connection is determined by the access intents that are defined in the CMP methods.
Sharing a connection between CMP and BMP beans
Remember to first verify that the getConnection methods of both the BMP bean and the CMP bean set the same connection properties. To match the authentication type of the CMP bean resource, set the authentication type of the BMP bean resource to container-managed, which is designated in the deployment descriptor as res-auth = Container. Additionally, use one of the following options to ensure connection-sharing between the bean types:
- Define the same access intent on both CMP and BMP bean methods. Because both use the same access intent, they share the same physical connection. The advantage to using this option is that the backend is transparent to a BMP bean. However, this option also makes the BMP non-portable because it uses the WebSphere extended API to handle the isolation level. For more information, refer to the code example in Example: Accessing data using IBM extended APIs to share connections between container-managed and bean-managed persistence beans.
- Determine the isolation level that the access intent uses on a CMP bean method, then use the corresponding isolation level that is specified on the resource reference to look up a data source and a connection. This option is more of a manual process, and the isolation level might be different from database to database. For more information refer to the isolation level and access intent mapping table: Access intent isolation levels and update locks and the Isolation level and resource reference section.
Sharing a connection between CMP and a JDBC application that is used by a servlet or a session bean
Determine the isolation level that the access intent uses on a CMP bean method, then use the corresponding isolation level specified on the resource reference to look up a data source and a connection. For more information refer to Access intent isolation levels and update locks and Isolation level and resource reference.

Factors that determine sharing

The listing here is not an exhaustive one. The product might or might not share connections under different circumstances.

Only connections acquired with the same resource reference (resource-ref) that specifies the res-sharing-scope as shareable are candidates for sharing. The resource reference properties of res-sharing-scope and res-auth and the IBM extension isolationLevel help determine if it is possible to share a connection. IBM extension isolationLevel is stored in IBM deployment descriptor extension file; for example: ibm-ejb-jar-ext.xmi.
You can only share connections that are requested with the same properties.
Connection Sharing only occurs between different component instances if they are within a transaction (container- or user-initiated transaction).
Connection Sharing only occurs within a sharing boundary. Current sharing boundaries include Transactions and LocalTransactionContainment (LTC) boundaries.
Connection Sharing rules within an LTC Scope:
- For shareable connections, only Connection Reuse is allowed within a single component instance. Connection reuse occurs when the following actions are taken with a connection: get, use, commit/rollback, close; get, use, commit/rollback, close. Note that if you use the LTC resolution-control of ContainerAtBoundary then no start/commit is needed because that action is handled by the container.
  The connection returned on the second get is the same connection as that returned on the first get (if the same properties are used). Because the connection use is serial, only one connection handle to the underlying physical connection is used at a time, so true connection sharing does not take place. The term "reuse" is more accurate.
  More importantly, the LocalTransactionContainment boundary enclosing both get actions is not complete; no cleanUp() method is invoked on the ManagedConnection object. Therefore the second get action inherits all of the connection properties set during the first getConnection() call.
Shareable connections between transactions (either container-managed transactions (CMT), bean-managed transactions (BMT), or LTC transactions) follow these caching rules:
- In general, setting properties on shareable connections is not allowed because a user of one connection handle might not anticipate a change made by another connection handle. This limitation is part of the J2EE 1.3 standard.
- General users of resource adapters can set the connection properties on the connection factory getConnection() call by passing them in a ConnectionSpec.
  However, the properties set on the connection during one transaction are not guaranteed to be the same when used in the next transaction. Because it is not valid to share connections outside of a sharing scope, connection handles are moved off of the physical connection with which they are currently associated when a transaction ends. That physical connection is returned to the free connection pool. Connections are cleaned before going in the free pool. The next time the handle is used, it is automatically associated with an appropriate connection. The appropriateness is based on the security login information, connection properties, and (for the JDBC API) the isolation level specified in the extended resource reference, passed in on the original request that returned the current handle. Any properties set on the connection after it was retrieved are lost.
- For JDBC users, WebSphere Application Server provides an extension to enable passing the connection properties through the ConnectionSpec.
  Use caution when setting properties and sharing connections in a local transaction scope. Ensure that other components with which the connection is shared are expecting the behavior resulting from your settings.
You cannot set the isolation level on a shareable connection for the JDBC API using a relational resource adapter in a global transaction. The product provides an extension to the resource reference to enable you to specify the isolation level. If your application requires the use of multiple isolation levels, create multiple resource references and map them to the same data source or connection factory.

Connection sharing violations

There is a new exception, the SharingViolation exception, that the resource adapter can issue whenever an operation violates sharing requirements. Possible violations include changing connection attributes, security settings, or isolation levels, among others. When such a mutable operation is performed against a managed connection, the SharingViolation exception can occur when both of the following conditions are true:

The number of connection handles associated with the managed connection is more than one.
The managed connection is associated with a transaction, either local or XA.

Both the component and the J2C run time might need to detect this SharingViolation exception, depending on when and how the managed connection becomes unshareable. If the managed connection becomes unshareable because of an operation through the connection handle (for example, you change the isolation level), then the component needs to process the exception. If the managed connection becomes unshareable without being recognized by the application server (due to some component interaction with the connection handle), then the resource adapter can reject the creation of a connection handle by issuing the SharingViolation exception.