Introduction

This chapter provides some background information on how HTTP session state is managed by the Sun ONE Application Server.

Sun ONE Application Server uses a high-availability database (HADB), to make the state of HTTP session objects highly available and resilient against failures. All serializable objects stored in HttpSession are persisted and made available to other server instances in the event a server instance fails. Since some EJB references and references to other serializable J2EE components such as InitialContexts and DataSources are serializable, the application developer must understand how these references behave when a failover occurs.

This document provides guidelines for working with these reference types in the Sun ONE Application Server.



Note	Although reference failover is supported for a particular object type, it is not recommended that the reference be stored in a session. More information on the working of each reference type are provided in the specific reference types section of this document.

The following table lists various J2EE object types and provides information on whether reference failover is supported.

The left column lists the EJB reference types, the second column indicates if Home reference is supported or not, the third column indicates if the Object reference is supported or not, and the right-most column indicates if it is recommended or not.

Table 1-1 EJB Reference Types
EJB Reference Types (Local or Remote1)	Home Reference Supported	Object Reference Supported	Recommended
Stateless Session Bean	Yes	Yes	No
Stateful Session Bean	Yes	No	No2
Entity Bean	Yes	Yes	Home - No Object - Yes

1In the Sun ONE Application Server, remote EJB references are co-located in the same virtual machine as EJB references.

2Stateful Session bean object reference types would be recommended for HttpSession if stateful session bean reference failover was supported by the Sun ONE Application Server.

The left column lists the JMS reference types, the second column indicates if the JMS reference type is supported or not, the third column indicates if it is recommended or not.

Table 1-2 JMS Reference Types
JMS References	Supported	Recommended
ConnectionFactory	No	No
Connection	No	No
Session	No	No
Destination	No	No
MessageProducer	No	No
MessageConsumer	No	No

The left column lists the J2EE reference types, the second column indicates if the J2EE reference type is supported or not, the third column indicates if it is recommended or not.

Table 1-3 J2EE References Types
Other J2EE Reference Types	Supported	Recommended
InitialContext	Yes	No
UserTransaction	Yes. If the instance that fails is never restarted, any global transactions are lost and may not be correctly rolled back or committed.	No
JDBC	No	No
J2EE Connector	Depends on the Connector	Only if required by the connector.

So of all the reference types listed above, only references to stateful session beans and entity beans should be stored in the session. All the other referenced object types are stateless in nature and must not be held in a user’s session from one web request to the next. Since the Sun ONE Application Server does not provide failover support for stateful session beans, if failover is a requirement in your application, the only reference type that should be stored in HttpSession is an entity bean reference.

Chapter 1 Introduction

Chapter 1
Introduction