In the previous decade and the beginning of this one, Server Load Balancing (SLB) reigned supremely in the web data center. Lately, a new class of products is replacing the older load balancers. These products are known as Application Delivery Controllers (ADC) and in the following paragraphs I will share my thoughts on the reasons for that.

Server Load Balancing. When a web site exceeds the capacity of a single server, the obvious solution is to share the load among several servers. This used to be done by DNS load balancing where client’s request to resolve a host name would be rotated among several servers. There are two issues with this method; First is that the clients don’t resolve a host name for every transaction – actually with DNS caching they rarely do. This leads to severe imbalance on the servers. Another problem is that the actual load on the servers is not known to the DNS load balancer, resulting again in imbalance. The solution was a new family of products known as Server Load Balancers. An appliance would front several (perhaps even hundreds) of servers and balance the load across them.  All incoming traffic would pass through the device and get routed to the ‘most appropriate’ server according to the algorithms employed by the SLB device. The IT manager has a simple strategy now – the more traffic needs to be handled, the more servers I add, up to the capacity of the SLB device.

The world is changing. Over the last decade, web data centers experienced a significant growth in load. This can be attributed to many factors, among them the proliferation of web based applications and the move from LAN based local IT infrastructure to a global WAN based infrastructure. Instead of growing linearly, the load on the web servers grows exponentially, forcing IT managers to deploy ever-increasing number of servers in their farms. This situation led to a new class of products – one that not only load balances but also offloads the servers. These new machines, called Application Delivery Controllers (ADC), take huge burden off the servers – concentrating on areas that are peripheral to server’s main function of delivering web and application content. In addition, ADC’s employ several acceleration functions aimed at reducing the amount of traffic over the WAN. By employing an Application Delivery Controller it is possible to avoid the exponential growth in server numbers (and perhaps even lower the server count) while improving end user experience.

What’s in an ADC? As a new product, the definition continues to evolve, but these elements exist in most products on the market:

• SLB -Since the appliance replaces a traditional load balancer, it has to take its basic functionality of load balancing among several servers. This is neither offload nor acceleration but is required as a legacy feature.
• TCP connection management and multiplexing—Even a strong server can be brought to its knees by making it deal with tens of thousands of TCP connections opening and closing rapidly. By handling all the myriad TCP issues on the client side and multiplexing the client requests into few well behaved TCP connections towards the server, a significant offload is achieved.
• Compression - Compressing web content is an established way of reducing bandwidth and reducing response time. Doing compression on the ADC offloads this task from the server.
• SSL - Secure Socket Layer is widely used for content encryption. Being computationally intensive, offloading it to the ADC can free up server resources.
• L7 advanced features - An advanced ADC allows the enforcement of policies such as traffic and content control based on L7 information. This allows complex data center behavior without burdening the servers.
• Caching - Most ADC’s consolidate a web caching feature in the device. This reduces load at the servers as cached content is served directly from the ADC.
• GSLB - Global Server Load Balancing allows balancing geographically separated sites based on their load.

What’s the secret sauce? How can a single device offload dozens or even hundreds of servers? There is more than one answer but most products fall into one of two categories:

1. Standard server architecture using streamlined standard OS or completely rewritten OS optimized for ADC functionality. Such architecture promotes rapid feature development but suffers from inherent bottlenecks, especially when several features are turned on simultaneously.
2. A mix of standard processors with dedicated hardware. The mix ratio can range from mostly General Purpose (GP) to mostly hardware based solution. The development cycle may be longer but feature concurrency is maintained.

To summarize, modern web data centers experience exponential growth in server load, requiring a non-linear solution. In these environments, IT managers migrate from legacy SLB solutions to ADC products offering offload and acceleration on top of load-balancing.