The varieties of single system image

A single system image is the illusion that a collection of otherwise independent computing engines is a single computational resource. But any single system image has a boundary: Within it one perceives unity; outside it there is multiplicity. So many single system images can co-exist in one system. The types of single systems images can be arranged in a hierarchy, ranging from application-level through operating system to hardware. An eight-level hierarchy of single system images is presented and described.