Within hours of Application Request Routing Version 2.5 release (see announcement here), I've been asked why ARRv2.5 is available only via WebPI.
Well, a simple response would be that WebPI is the way to consume web technology applications and components, including IIS Extensions, such as ARR.
A more detailed response actually involves more understanding of what makes up ARR. In previous versions of ARR, it was shipped with a packaged bootstapper, which includes all its dependent IIS Extensions. We shipped it this way because 1) it was before WebPI was available and 2) we needed a way to manage the dependencies. As you may know, "ARR" is more than just one IIS Extension:
It is built on top of Web Farm Framework.
It uses URL rewrite to manage routing rules.
It uses External Cache to share ARR states when there are multiple ARR servers (only needed if hostname affinity feature is used.)
To simplify the install experience, we built a stand-alone bootstrapper that hid much of the complexities that go into setting up ARR correctly. Now with WebPI, it does that for us.
For those customers who have already installed all other components and only need ARR to upgrade to ARRv2.5, you can just download the ARRv2.5 installers. They can be found at: