Advanced ADCs take over where server load balancers leave off

Thursday, 15 July 2010 15:29

Written by Jim Puchbauer

Back in the mid-1990’s, server load balancers were hailed as technological wonders. If you were the person responsible for dealing with server issues, you received amazing benefits from deploying a load balancer. These devices load balanced user traffic among multiple servers to ensure the data center had high-availability. If one server went down, the load balancer directed traffic to the other working servers.

Although, they have been incredibly valuable to the data center, server load balancers are fairly basic in their capabilities, and lack the advanced application-aware functions that today’s data centers require. For example, they are unable to support persistent sessions based on cookies and HTTP headers; they lack application layer routing capabilities; they are unable to perform TCP and HTTP protocol connection management; and they become a performance bottleneck when they multitask the many offloading and acceleration functions.

Fortunately, application delivery controllers (ADCs) have taken over where server load balancers left off.

It’s the destination that matters
Let’s use a comparative analogy that demonstrates how ADCs work within the data center and the networks that connect its users. When a car is disabled in the left lane of an interstate highway, a traffic cop will redirect traffic around the disabled lane. But, the highway, much like the data center, is only a means to the end. What is really important to you is the destination. And every destination is unique, each with its own priority and value to the data center operators and the users accessing the applications. For example, you may take a different route to get to your office than you do to your grocery store. And getting to the office in a timely manner probably has a higher priority. When you get into your car, you want to get to your destination as expediently as possible. You would almost certainly prefer to take a route that only has 2 stop lights, rather than taking a route that has 2 stop signs and 3 stop lights. And, more importantly, you don’t want to take the wrong exit and get lost altogether.

In addition to load balancing traffic, what distinguishes advanced ADCs from server load balancers is their ability to intelligently route users to their application and content destinations efficiently and intelligently, based on business priorities and goals.  Referring to the analogy above, imagine the ADC is the ultimate traffic cop, who would not only redirect you around the disabled lane, but would ask you where you were going, and would take into consideration the time of day, and where the location is within the surrounding city. With that information, he would give you directions that would take you away from the elementary school that lets the children out right when you would be going by, and guides you to the route with the least amount of traffic lights and obstacles. The officer may also get his partner to follow you to make sure you remain safe and secure. This ultimate Robocop would anticipate and provide for your every need until you attain your destination.

Applying this analogy to users requesting applications and content from a data center, an advanced ADC will route users to destination servers based on a variety of criteria that the data center manager implements using policies and advanced application-layer functionality to support business requirements. And, much like our example officer, an advanced ADC will ensure that the users get to the applications based on their specific needs, while protecting the network and applications from security threats.

Much in the same way that a highway commuter lane has fewer cars with higher occupancy to reduce congestion; advanced ADCs offload servers by multiplexing applications to alleviate server and network congestion. ADCs handle transmission turns for chatty protocols such as TCP and HTTP, reducing multiple packets into fewer packets with larger payloads so servers can process more user requests, and reduce network congestion.
Have you ever been driving in your car listening to the directions broadcast by your trusty GPS device, and even though the GPS told you to turn right on 1st Avenue, you accidently turned left? Lucky for you, the GPS began “recalculating” your destination, and got you back on track, without time wasting and unnecessary backtracking.

Similarly, many Web applications require session persistence to keep users connected to specific servers until an entire transaction has been completed. Advanced ADCs use session state with HTTP headers and cookies to ensure that a user is kept persistent with an application. To ensure transaction continuity, the cookie or HTTP header stores session information, such as session ID, user ID, host, etc. A typical application for this is a shopping cart where a single server uses the transaction ID throughout the entire transaction, rather than having the user interactions spread among multiple servers. In a shopping cart transaction, the user needs to persist with one server to complete the transaction.  The ADC uses the cookie within the HTTP header to persist with a specific server where the session state information resides, so that when a subsequent request comes in from that user, the ADC will send them to the same server that has the transaction information. Without this capability, if the user went to a different server, the previous transaction history would be lost, and the user would need to start the transaction over. Most Web applications generate a “session ID” that gets sent within a cookie. Web applications use the cookie’s session ID to find the session on the server. The session ID can be used even after the original session connection has been terminated. This process of using the cookie’s session ID exchange is what maintains persistent state.

Summary
When evaluating an application delivery solution, keep in mind that not all ADCs are capable of advanced policy-driven application routing. If you are considering purchasing these powerful devices, you may find that the flexibility that policy-driven application-layer features provide, will give you greater control to improve how your data center supports your business objectives.

About the author
Jim Puchbauer is Director of Marketing at Coyote Point Systems, a leading provider of Application Delivery Controllers (ADCs) for small-to-medium size enterprises. http://www.coyotepoint.com.

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