Websphere Cluster Vs Load balance

When we talk about scalability and Workload Management (WLM) then cluster comes to picture. A WebSphere  cluster  consists  of  having  multiple  application servers (cluster member  or  clones)  across a machine  (Vertical Cluster)  or  across  several  machines (Horizontal Cluster).

When looking a a clustered web application deployed to a single cluster we  can  say  the application is load balanced as long as we use IBM HTTP Server (IHS) to provide workload management via a plugin. When WLM is enabled by using IHS  the  client  HTTP  requests are distributed across the application servers by configured web  server  definitions in the administration console and mapping of application to these server.

Clustering without IHS WLM still allows for high availability and workload management for all other protocols like RMI etc. This is dependant on the design of  Enterprise application. Bad programming can invalidate a cluster  design.  It  is  all  about  how  IIOP is used or CorbaLoc.

There are several modes of load balancing using software i.e. handled using WebSphere plugin, which we can call HTTP WLM and cluster load balance for other Java protocols i.e RMI and we can also use the IBM Edge server software which can become a software load balancer which sprays IP traffic to IHS (if web WLM is being used)  or  spray IP  traffic  to WAS Clusters. Remember the application design must be design correctly  to  benefit the clustering model. You can also use a hardware load balancer  such as  Nortel Application Switches e.g Alteon.

Below is  a  diagram that  shows a simple  diagram  of  what we mean  by  vertical  and horizontal clusters.

                             

You will notice in this diagram that the Deployment Manager is on the same node as Node 01, but this doesn't have to be the case. Often it is  because  of budget  constraints  and ease of use. in this diagram IBM HTTP Server (IHS) is also installed on the same node as Node 01,  but  could  be  separate machine.  

If  IHS  is  not  on  the  same  machine as a WebSphere node it  makes  administration  and  maintenance  more  complex. A physical Load Balancer would exist between the client and the web server.

An example of a possible Load balanced Workload Management (WLM) design is shown below.

                                

The diagram  above  depicts  a  possible  configuration  for  load  balancing and workload management for IBM WebSphere Network Deployment  (ND). The load balancers could be appliances  or  software.  IHS (IBM HTTP Server)  is  used for  workload  management to spray HTTP  requests  in  a  round  robin fashion  across the applications on clones. The WebSphere Plug-in is used by IHS and allows requests to spray across application clones in a cluster.