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
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
App Server
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.
App Server
App Server
App Server

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