Sun Java System Application Server Enterprise Edition 8.1 2005Q2 High Availability Administration Guide

Overview of High Availability

High availability applications and services provide their functionality continuously, regardless of hardware and software failures. Application Server provides high availability for HTTP requests and session data (both HTTP session data and stateful session bean data).

Application Server provides high availability through the following sub-components and features:

Load Balancer Plug-in

The load balancer plug-in accepts HTTP and HTTPS requests and forwards them to application server instances in a cluster. If an instance fails, becomes unavailable (due to network faults), or becomes unresponsive, the load balancer redirects requests to existing, available machines. The load balancer can also recognize when a failed instance has recovered and redistribute the load accordingly. The Application Server Enterprise Edition includes the load balancer plug-in for the Sun Java System Web Server and the Apache Web Server, and Microsoft Internet Information Server.

By distributing workload among multiple physical machines, the load balancer increases overall system throughput. It also provides higher availability through failover of HTTP requests. For HTTP session information to persist, you must configure HTTP session persistence.

For simple, stateless applications a load-balanced cluster may be sufficient. However, for mission-critical applications with session state, use load balanced clusters with HADB.

Server instances and clusters participating in load balancing have a homogenous environment. Usually that means that the server instances reference the same server configuration, can access the same physical resources, and have the same applications deployed to them. Homogeneity assures that before and after failures, the load balancer always distributes load evenly across the active instances in the cluster.

For information on configuring load balancing and failover for see Chapter 4, Configuring Load Balancing and Failover

High Availability Database

Application Server Enterprise Edition provides the High Availability Database (HADB) for high availability storage of HTTP session and stateful session bean data. HADB is designed to support up to 99.999% service and data availability with load balancing, failover, and state recovery. Generally, you must configure and manage HADB independently of Application Server.

Keeping state management responsibilities separated from Application Server has significant benefits. Application Server instances spend their cycles performing as a scalable and high performance Java™ 2 Platform, Enterprise Edition (J2EE™ platform) containers delegating state replication to an external high availability state service. Due to this loosely coupled architecture, application server instances can be very easily added to or deleted from a cluster. The HADB state replication service can be independently scaled for optimum availability and performance. When an application server instance also performs replication, the performance of J2EE applications can suffer and can be subject to longer garbage collection pauses.

For information on planning and setting up your application server installation for high availability with HADB, including determining hardware configuration, sizing, and topology, see Planning for Availability in Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Deployment Planning Guide and Chapter 3, Selecting a Topology, in Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Deployment Planning Guide.

Highly Available Clusters

A cluster is a collection of Application Server instances that work together as one logical entity. A cluster provides a runtime environment for one or more J2EE applications. A highly available cluster integrates a state replication service with clusters and load balancer.

Using clusters provides the following advantages:

All instances in a cluster:

Every cluster in the domain has a unique name; furthermore, this name must be unique across all node agent names, server instance names, cluster names, and configuration names. The name must not be domain. You perform the same operations on a cluster (for example, deploying applications and creating resources) that you perform on an unclustered server instance.

Clusters and Configurations

A cluster's settings are derived from a named configuration, which can potentially be shared with other clusters. A cluster whose configuration is not shared by other server instances or clusters is said to have a stand-alone configuration . By default, the name of this configuration is cluster_name -config, where cluster_name is the name of the cluster.

A cluster that shares its configuration with other clusters or instances is said to have a shared configuration.

Clusters, Instances, Sessions, and Load Balancing

Clusters, server instances, load balancers, and sessions are related as follows:

The cluster thus acts as a safe boundary for session failover for the server instances within the cluster. You can use the load balancer and upgrade components within the Application Server without loss of service.

More Information

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 8.1 2005Q2 Deployment Planning Guide. This manual also provides a high-level introduction to concepts such as:

For more information about developing and deploying applications that take advantage of high availability features, see Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Developer’s Guide.

Tuning High Availability Servers and Applications

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 8.1 2005Q2 Performance Tuning Guide, which discusses topics such as: