image In the networking side of the world, vendors often seek to differentiate their solutions not just based on features and functionality, but on form-factor, as well. Using a descriptor to impart an understanding of the deployment form-factor of a particular solution has always been quite common: appliance, hardware, platform, etc… Sometimes these terms come from analysts, other times they come from vendors themselves. Regardless of where they originate, they quickly propagate and unfortunately often do so without the benefit of a clear definition.

A reader recently asked a question that reminded me that we’ve done just that as we cloud computing and virtualization creep into our vernacular. Quite simply the question was, “What’s the definition of a Soft ADC and vADC?” That’s actually an interesting question as it’s more broadly applicable than just to ADCs. For example, the last several years we’ve been hearing about “Soft WOC (WAN Optimization Controller)” in addition to just plain old WOC and the definition of Soft WOC is very similar to Soft ADC. The definitions are, if not well understood and often used, consistent across the entire application delivery realm – from WAN to LAN to cloud.

So this post is to address the question in relation to ADC more broadly, as there’s an emerging “xADC” model that should probably be mentioned as well. Let’s start with the basic definition of an Application Delivery Controller (ADC) and go from there, shall we?

An application delivery controller is a device that is typically placed in a data center between the firewall and one or more application servers (an area known as the DMZ). First- generation application delivery controllers primarily performed application acceleration and handled load balancing between servers.

The latest generation of application delivery controllers handles a much wider variety of functions, including rate shaping and SSL offloading, as well as serving as a Web application firewall.

If you said an application delivery controller was a “load balancer on steroids” (which is how I usually describe them to the uninitiated) you wouldn’t be far from the truth. The core competency of an ADC is load balancing, and from that core functionality has been derived, over time, the means by which optimization, acceleration, security, remote access, and a wealth of other functions directly related to application delivery in scalable architectures can be applied in a unified fashion. Hence the use of the term “Unified Application Delivery.”

If you prefer a gaming metaphor, an application delivery controller is like a multi-classed D&D character, probably a 3e character because many of the “extra” functions available in an ADC are more like skills or feats than class abilities.


So a "Soft ADC" then is simply an ADC in software format, deployed on commodity hardware. That hardware may or may not have additional hardware processing (like PCI-based SSL acceleration) to assist in offloading compute intense processes and the integration of the software with that hardware varies from vendor to vendor.

Soft ADCs are sometimes offered as “softpliances” (many people hate this term) or an “appliance comprised of commodity hardware pre-loaded and configured with the ADC software.” This option allows the vendor to harden and optimize the operating system on which the Soft ADC runs, which can be advantageous to the organization as it will not need to worry about upgrades and/or patches to the solution impacting the functionality of the Soft ADC. This option can also result in higher capacity and better performance for the ADC and the applications it manages, as the operating system’s network stack is often “tweaked” and “tuned” to support the application delivery functions of the Soft ADC.


A "vADC" is a virtualized version of an ADC. The ADC may or may not have first been a "Soft ADC", as in the case of BIG-IP which is not available as a "Soft ADC" but is available as a traditional hardware ADC or a virtual ADC. vADCs are ADCs deployed in a virtual network appliance (VNA) form factor, as an image compatible with modern virtual machines (VMware, Xen, Hyper-V).


There is an additional "type" of ADC emerging mainly because of proprietary virtual image formats in clouds like Amazon, the "ADC as a service" which is offered as a provisionable service within a specific cloud computing environment that is not portable (or usable) outside the environment. In all other respects the “ADC as a service” is indistinguishable from the vADC as it, too, is deployed on commodity hardware and lacks integration with the underlying hardware platform or available acceleration chipsets.


In the general category of application delivery (and most networking solutions as well) we can make the following abstractions regarding these definitions:

“Solution” Soft “Solution” v”Solution” “Solution” as a Service*
A traditional hardware-based “solution”
A traditional hardware-based solution in a software form-factor that can be deployed on an “appliance” or commodity hardware A traditional hardware-based solution in a virtualized form-factor that can be deployed as a virtual network appliance (VNA) on a variety of virtualization platforms. A traditional hardware-based solution in a proprietary form-factor (software or virtual) that is not usable or portable outside the environment in which it is offered.

So if we were to tackle “Soft WOC”, as well, we’d find that the general definition – traditional hardware-based solution in a software form-factor – also fits that category of solution well.

It may seem to follow logically than any version of an ADC (or network solution) is “as good” as the next given that the core functionality is almost always the same regardless of form factor. There are, however, pros and cons to each form-factor that should be taken into consideration when designing an architecture that may take advantage of an ADC. In some cases a Soft ADC or vADC will provide the best value, in others a traditional hardware ADC, and in many cases the highly-scalable and flexible architecture will take advantage of both in the appropriate places within the architecture.

*Some solutions offered “as a service” are more akin to SaaS in that they are truly web services, regardless of underlying implementation, that are “portable” because they can be accessed from anywhere, though they cannot be “moved” or integrated internally as private solutions.