Haystack

Comet

Introduced the concept of distributed shared memory (DSM). In a DSM, RAMs from multiple servers would appear as if they are all belonging to one server, allowing better scalability for caching.
Comet model works by offloading the computation intensive process from the mobile to only one server.
The offloading process works by passing the computation intensive process to the server and hold it on the mobile device. Once the process on the server completes, it returns the results and the handle back to the mobile device. In other words, the process does not get physically offloaded to the server but instead it runs on the server and stopped on the mobile device.

Represents a building block towards building this global distributed systems. The main critique to it is that it didn’t present a specific use case upon which their design is built upon.
Sapphire does not show their scalability boundaries. There is no such distributed system model that can be “one size fits all”, most probably it will break in some large scale distributed application.
Reaching this global distributed system that address all the distributed OS use cases will be a cumulative work of many big bodies and building it block by block and then this system will evolve by putting all these different building blocks together. In other words, reaching a global distributed system will come from a “bottom up not top down approach” [Somayaji, 2015].