Legion
From P2P Wiki
Overview
Legion provides an object based service model so that objects can be replicated and located arbitrarily transparently. Legion‘s scheme is similar to the one used by Globe for separating the object name from its address. However, the main difference between both systems is the way objects are considered. In Globe, objects are assumed to be physically distributed over many resources in the system. However, in Legion, objects can be physically distributed over multiple physical resources, but are expected to physically reside in a single address space. These conflicting views of objects result in different mechanisms for object communication: Globe loads part of the object (called a local object) into the caller‘s address space, whereas Legion sends a message of a specified format from the caller to the callee. Further, although Legion may provide support for data streams, its purpose is broader than standalone support for the communication paradigm.
Legion does not comply with the dynamicity requirement, which is not targeted to the edges of the Internet. Legion‘s main aim is to provide a worldwide supercomputer, composed of relatively static powerful servers, much like the target of Grid Computing. It provides fault tolerance mechanisms, but keeps in mind that failures will not be as frequent as in a highly dynamic environment. The high availability requirement is partly implemented, since it provides object replication primitives, but the semantics of group communication are left to the developer. It also provides fault tolerance mechanisms, and programming Legion applications is not a trivial task.
Finally, Legion does not provide any kind of load balancing mechanisms since, as stated by the authors, an object could easily become a bottleneck and limit application performance. Our idea of load balancing would perfectly suit this scenario. Therefore, a vast number of requests should be dealt with by being distributed among active object replicas transparently by the middleware. Legion does not do so in a transparent way, and this responsibility is left to the application developer.
See also
References
- M. Lewis and A. Grimshaw, "The Core Legion Object Model," link in 5th IEEE International Symposium on High Performance Distributed Computing, 1996.
