The following is a clip from VD-Interfaces specification paper.
“Virtual Device Interfaces (VDI) provide a standard way to publish interfaces of virtual devices by a software component. This enables other software components to interact with these devices. Going forward, the first component will be called the back-end and the second component will be called the front-end. An example for using Virtual Device Interfaces is as part of a virtual machine system, where the back-end will be the hardware emulation layer. The back-end will expose interfaces like display port, mouse input etc. The front-end will plug into the display output and will render its output according to it's specific implementation. The front-end will also plug into the mouse input and send mouse events to be processed by the back-end. In addition many other interface types can be exposed by the back-end. Another example of back-end is a remote display system in a physical machine environment. Here, the back-and is implemented using known techniques for interacting with the native OS for the purpose of receiving display updates and pushing inputs. The back-end exposes interfaces like display output, mouse input etc. The front-end can be exactly the same as in the previous example.
By using VDI one back-end can use many types of front-ends without any special code modification. It is also possible for the back-end to dynamically switch front-ends, and improve back-end usability and flexibility. The use of front-ends by many back-ends allows for a better sharing of development, maintenance, and overall product quality.”
Spice server is implemented as a VDI front-end and the Spiced-QEMU provides back-end interfaces. The specification is in draft form and requires some minor changes. Spice project plans to make an effort to push this specification forward in order to benefit all.
Download the complete specification.