Some of those conscientious companies noticed there was more work to be done and, consequently, deployed Application Delivery Controllers between the Web Servers and the Application Servers. Often, the plug-in still exists but it only see a single IP Address for the Application Server Tier (see below). The ADC virtualises the Application.
Virtualising each tier separately is much better than the previous designs when it comes to service resilience and availability but we are not there yet. We have not delivered Service Agility. In the diagram above we have virtualised. Yes. However, we are still at the mercy of a Web Server Plug-in. And this is not a place I would want to be.
Once you have virtualised a service you can then deliver:
*) rapid response to market demand: add a new server to the ADC pool without change to the Application
*) Separate In-band monitoring of each tier
*) Reduction in devices traversed, and latency incurred, between the users and the Application
*) Datacentre migration of a live applications: this is a must see - http://devcentral.f5.com/weblogs/nojan/archive/2010/02/02/introducing-long-distance-vmotion-with-vmware.aspx
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