Kevin Saitta, a solution consultant, has a nice blog post on architecting a Microsoft BizTalk 2006 R2 solution. Unfortunately, amidst the goodness, is a statement regarding load balancers that needs to be corrected. Kevin is not alone in his beliefs regarding load balancers, unfortunately, I've seen a lot of posts lately that seem to indicate that folks out there still have a circa 1999 knowledge set regarding the capabilities of load balancers.
It is the "NOTE" that needs to be addressed, especially given the reference to BIG-IP, which is absolutely capable of understanding applications that run on the servers and, in fact, does understand the applications and their protocols and is capable of optimizing, accelerating, and securing those applications.
BIG-IP is capable of recognizing the difference between the server being available, and the application being available and has the ability to understand when the application is serving the right content or not. BIG-IP can even dig into the payload, the application messages, and apply policies based on the application-specific nuggets of information, rather than just HTTP headers or even application(HTTP)/transport layer (TCP) protocols. Add in iRules and BIG-IP becomes an application delivery platform: extensible, dynamic, and extremely intelligent.
The problem is that load balancers aren't just load balancers any more. They have evolved into application delivery controllers, and are capable of much more intelligence and integration with the applications they are tasked to deliver and are just as capable of intelligent monitoring of applications to ensure they are actually available.
Kevin's assertion that a load balancer will deliver requests to the server even if it's down is wrong, at least in the case of BIG-IP. While Windows NLB is certainly incapable of this particular functionality (it's called network load balancing for a reason), application delivery controllers like BIG-IP and its competitors are capable of intelligent monitoring, dynamic routing, and recognizing when an application is no longer available or responding correctly.
As Deb pointed out in response to a previous post, there are plenty of examples and samples for BIG-IP which illustrate the powerful, flexible, application aware monitoring capabilities of BIG-IP.
To be fair to Kevin, who wrote a really informative and interesting post, this is not the first blog or forum post I've seen in the last two weeks that improperly tags modern load balancers as "dumb" network devices incapable of understanding application layer protocols and messages. There's a perception - and a wrong one at that- that load balancers are still just simple layer 4 devices that add very little value to the network or application architecture.
As it's part of my job to educate folks on topics like this, I guess I'm not doing very well, am I? I'll get to work on that, pronto, after my upcoming mini-vacation.
For starters I'd like to say to everyone out there interested in load balancing: welcome to the 21st century, where application delivery controllers have replaced load balancers, and where intelligent, application aware devices are capable of much, much more than their predecessors.
Stay tuned, there's much, much more to come.