Change Control Stablity/Support as a Design Criteria

Post date: Oct 29, 2012 4:08:46 PM

In most network designs cost and redundancy seem to be the primary drivers in the design. Unfortunately, when most designers talk about redundancy they really mean availability. Redundancy is just one of the tools that can be used to achieve high availability, the other important tools is reliability.

Another interesting item that many designers fail to include in there design criteria is now much down time will need to be scheduled for change to the infrastructure. When people talk about downtime in terms such as 5 9's they are referring to unscheduled downtime. So it is entirely possible that the systems might now be available for large blocks of time needed to make changes to the infrastructure. For many applications and infrastructures such as E-Commerce or communications service providers any type of downtime whether scheduled or unscheduled is not acceptable. This means that change controls and the ability to make changes without effecting availability of services must be considered in the design.

The consideration of change control and change mechanisms in the design of services leads to an interesting paradox. The more redundancy is introduced into a system the more complex making changes can become. In the past many systems have overcome this problem by very tightly controlling changes and only introducing them at a very slow and deliberate pace. Unfortunately, this approach runs into problems with the business requirements for fast time to make and the need to make changes to many environments for security reasons.

The other approach is to take a different approach to the use of redundancy to help achieve availability. The main idea of this approach is that as much as possible all elements should be active during normal operations. When changes need to be made to the systems they are made to one "side" of the system while the other "side" of the system is up and running. This is one of the key elements in the Stack Architecture that I have been working as part of a engagement with a large insurance firm.