WebSphere Application Server - Express V6 Developers Guide and Development Examples

Chapter 3. Product overview 103

WebSphere Application Server supports the <resource-env-ref> mechanism by

providing administration objects for the following:

򐂰 Resource environment provider

This provider defines an administrative object that groups together the

referenceable, resource environment entry administrative objects and any

required custom properties.

򐂰 Referenceable

The Referenceable provider defines the class name of the factory class that

returns object instances implementing a Java interface.

򐂰 Resource environment entry

This entry defines the binding target (JNDI name), factory class and return

object type (with the link to the referenceable) of the resource environment

entry.

3.16 Workload management

Clustering application servers that host Web containers automatically enables

plug-in workload management for the application servers and the servlets they

host. Routing of servlet requests occurs between the Web server plug-in and the

clustered application servers using HTTP or HTTPS.

Figure 3-14 Plug-in (Web container) workload management

This routing is based on weights associated with the cluster members. If all

cluster members have identical weights, the plug-in sends equal requests to all

members of the cluster assuming no strong affinity configurations. If the weights

are scaled in the range from zero to twenty, the plug-in routes requests to those

cluster members with the higher weight value more often. A rule of thumb

formula for determining routing preference would be:

% routed to Server1 = weight1 / (weight1+weight2+...+weightn)

App Server

Web

Container

App Server

Web

Container

Servlet

Requests

HTTP

Server

Plug-in

104 WebSphere Application Server - Express V6 Developers Guide and Development Examples

There are n cluster members in the cluster.

The Web server plug-in temporarily routes around unavailable cluster members.

Workload management for EJB containers can be performed by configuring the

Web container and EJB containers on separate application servers. Multiple

application servers with the EJB containers can be clustered, enabling the

distribution of EJB requests between the EJB containers.

Figure 3-15 EJB workload management

In this configuration, EJB client requests are routed to available EJB containers

in a round robin fashion based on assigned server weights. The EJB clients can

be servlets operating within a Web container, stand-alone Java programs using

RMI/IIOP, or other EJBs.

The server-weighted, round-robin routing policy ensures a distribution based on

the set of server weights that have been assigned to the members of a cluster.

For example, if all servers in the cluster have the same weight, the expected

distribution for the cluster would be that all servers receive the same number of

requests. If the weights for the servers are not equal, the distribution mechanism

sends more requests to the higher weight value servers than the lower weight

value servers. The policy ensures the desired distribution, based on the weights

assigned to the cluster members. In V6, the balancing mechanism for weighted

round-robin had been enhanced to ensure more balanced routing distribution

among servers.

You can also choose to have requests sent to the node on which the client

resides as the preferred routing. In this case, only cluster members on that node

will be chosen (using the round-robin weight method). Cluster members on

remote nodes will only be chosen if a local server is not available.

EJB

Requests

App Server

EJB

Container

App Server

EJB

Container

App Server

Web

Container

EJB

Requests

Java

Client