How load balancing works in case of WebSphere XD/ virtual enterprise

The On Demad Router (ODR) is an intelligent routing engine that forms the core of WebSphere XD topologies. It does many of the features of Web Server Plugin and the edge-of-network routers, like the WebSphere Edge Server Network dispatcher

It does following things

• Listens for requests that it proxies
  
• Classifies the requests according to the configuration of the WebSphere application server for which the requests are destined


• Prioritizes the requests and uses its knowledge of the status of the application server to determine which application server the request should be routed to


• Queues and issues the requests to the application server that are servicing them according to the weigth assigned to the request


• Communicates with other parts of the XD system to coordinate its activities with other components and driver features like dynamic placement

There are a number of advantages that this topology holds over the standard Network Deployment topology:

• 
  Here, the routing information used by the ODR is dynamic; as applications are deployed (or undeployed), or as application servers are added to (or removed from) the topology, the ODR routing tables are automatically updated. This eliminates the need for a plugin-xml.cfg file, since the communication between the deployment manager and the ODR is direct.


• Since the ODR is an active application server instance in its own right, it can use additional information from the application servers to make dynamic determinations about request weighting.