The Desktop Delivery Controller is just like it sounds — it manages user access to desktop VMs. Pools of desktops can be defined and linked to specific Active Directory groups. In this way, you could give all HR users a desktop VM with 512MB of RAM and a specific CPU share, while all Engineering users get a desktop with 1024MB of RAM and a more powerful CPU. Obviously, you could also deliver Windows XP to one group and Vista to another.
The DDC is also where time-based resource management is handled. It's possible to create rules that will keep a minimum number of desktop VMs ready and waiting for a login between 8.30am and 9.30am, and then reduce the number of idle VMs throughout the rest of the day, finally keeping a small number active after business hours. This reduces the load on the VM infrastructure and helps handle the morning ramp-up. It's also possible to assign desktop VMs to specific users, rather than pooling them for a group of users.
The transient nature of VDI requires some method of delivering per-user profiles to the desktops, which is generally handled via roaming profiles, much like in the traditional terminal server world. Windows administrators have typically weathered bristly relationships with roaming profiles, but the reality is that they're not going away, and their benefits outweigh their detriments, at least for now.
However, Citrix is heading toward a better world after licensing code from Sepago to address profile management issues. Citrix will be leveraging sepagoPROFILE to hopefully ease this particular burden in the future.
XenDesktop's management methods also allow for quick updating of VMs, as it's possible to modify the baseline VM image that will then be used to boot all new VMs. You can update that image in the middle of the day, and every VM will pick up those changes the next time it reboots.
As with any enterprise-scale virtualisation infrastructure, shared storage is a must. In order to migrate running VMs from one host to another, especially with the write-cache nature of XenDesktop, all the hypervisor hosts need to be playing in the same sandbox.
Apps in the stream
But what about the applications? This is where Citrix brings its app streaming tech to the table. A baseline VM image can be built that links to any number of streamed applications, such as Microsoft Office apps. The user who logs into that VM sees normal application launch icons, but these icons link to an application stream from a Citrix XenApp server. Thus, the application isn't installed on the VM at all, but is pulled into the VM when needed from the network. This reduces the footprint of the VDI infrastructure significantly, since you only need a single installation of Office 2007, rather than one installation per desktop. The apps run like they were natively installed, and users won't notice a difference.