You may be aware that Microsoft has recently announced its change of licensing around how frequently an ‘application’ can be moved between servers in a farm*.

As it stands, at least until the 1st September 2008, one of these ‘applications’ can only be reassigned from one server to another every 90 days.

In times gone by this may have seemed reasonable as why, generally, would you have wanted to go to all the effort of moving an application from one physical server to another on a regular basis?

With the introduction of virtual machines and rather groovy virtualization products such as VMware ESX it is now possible to move a virtualized server running various applications (eg: Ms Exchange, SQL, etc) between physical servers with ease – manually or automated (eg: DRS. Live Migration).  This however does bring with it potential licensing implications as the regular moving of virtual machines back and forth between physical hosts would breach Microsoft’s 90 day mobility rule.

Under Microsoft’s new revised licensing terms applications will be able to be moved between servers in a farm an unlimited amount of times with no minimum time restrictions.  Here is Microsoft’s announcement of these licensing changes.  It is worth reading a  Microsoft released document called, ‘Application Server License Mobility‘ as this outlines those application products included in this licensing change and gives some good scenario examples.

Also, have a look at this blog posting from vmblog.com which gives a good follow-up to this announcement.

*Microsoft’s Server Farm Definition: A server farm consists of up to two data centers each physically located in the following areas:

· In a time zone that is within four hours of the local time zone of the other (Coordinated Universal Time [UTC] and not DST), and/or

· Within the European Union (EU) and/or European Free Trade Association (EFTA)

Each data center may be part of only one server farm. You may reassign a data center from one server farm to another, but not on a short-term basis (that is, not within 90 days of the last assignment).