High availability applications and services provide their functionality continuously, regardless of hardware and software failures. Such applications are sometimes referred to as providing five nines of reliability, because they are meant to be available 99.999% of the time.
Application Server provides the following high availability features:
High Availability Session Persistence
High Availability Java Message Service
RMI-IIOP Load Balancing and Failover
Application Server provides high availability of HTTP requests and session data (both HTTP session data and stateful session bean data).
J2EE applications typically have significant amounts of session state data. A web shopping cart is the classic example of a session state. Also, an application can cache frequently-needed data in the session object. In fact, almost all applications with significant user interactions need to maintain session state. Both HTTP sessions and stateful session beans (SFSBs) have session state data.
Preserving session state across server failures can be important to end users. For high availability, Application Server provides the capability to persist session state in the HADB. If the Application Serverinstance hosting the user session experiences a failure, the session state can be recovered, and the session can continue without loss of information.
For a detailed description of how to set up high availability session persistence, see Chapter 9, Configuring High Availability Session Persistence and Failover
The Java Message Service (JMS) API is a messaging standard that allows J2EE applications and components to create, send, receive, and read messages. It enables distributed communication that is loosely coupled, reliable, and asynchronous. The Sun Java System Message Queue (MQ), which implements JMS, is tightly integrated with Application Server, enabling you to create components that rely on JMS, such as message-driven beans (MDBs).
JMS is made highly available through connection pooling and failover and MQ clustering. For more information, see Chapter 10, Java Message Service Load Balancing and Failover.
Application Server supports JMS connection pooling and failover. The Application Server pools JMS connections automatically. By default, Application Server selects its primary MQ broker randomly from the specified host list. When failover occurs, MQ transparently transfers the load to another broker and maintains JMS semantics.
For more information about JMS connection pooling and failover, see Connection Pooling and Failover.
MQ Enterprise Edition supports multiple interconnected broker instances known as a broker cluster. With broker clusters, client connections are distributed across all the brokers in the cluster. Clustering provides horizontal scalability and improves availability.
For more information about MQ clustering, see Using MQ Clusters with Application Server.
With RMI-IIOP load balancing, IIOP client requests are distributed to different server instances or name servers, which spreads the load evenly across the cluster, providing scalability. IIOP load balancing combined with EJB clustering and availability also provides EJB failover.
When a client performs a JNDI lookup for an object, the Naming Service essentially binds the request to a particular server instance. From then on, all lookup requests made from that client are sent to the same server instance, and thus all EJBHome objects will be hosted on the same target server. Any bean references obtained henceforth are also created on the same target host. This effectively provides load balancing, since all clients randomize the list of target servers when performing JNDI lookups. If the target server instance goes down, the lookup or EJB method invocation will failover to another server instance.
IIOP Load balancing and failover happens transparently. No special steps are needed during application deployment. However, adding or deleting new instances to the cluster will not update an existing client's view of the cluster. To do so, you must manually update the endpoints list on the client.
For more information on RMI-IIOP load balancing and failover, see Chapter 11, RMI-IIOP Load Balancing and Failover.
For information about planning a high-availability deployment, including assessing hardware requirements, planning network configuration, and selecting a topology, see Sun Java System Application Server Enterprise Edition 8.2 Deployment Planning Guide. This manual also provides a high-level introduction to concepts such as:
Application server components such as node agents, domains, and clusters
IIOP load balancing in a cluster
Message queue failover
For more information about developing applications that take advantage of high availability features, see Sun Java System Application Server Enterprise Edition 8.2 Developer’s Guide.
For information on how to configure and tune applications and Application Server for best performance with high availability, see Sun Java System Application Server Enterprise Edition 8.2 Performance Tuning Guide, which discusses topics such as:
Tuning persistence frequency and persistence scope
Checkpointing stateful session beans
Configuring the JDBC connection pool
Tuning HADB disk use, memory allocation, performance, and operating system configuration
Configuring load balancer for best performance