Fleet is a middleware system
implementing a distributed repository for persistent objects based on Byzantine
quorum systems for a better system performance. Fleet is primarily target for
supporting highly critical applications: in particular the objects it stores
maintain the correct semantics despite the arbitrary failure (including hostile
corruption) of a limited number of Fleet servers. It is designed to be highly
available, dynamically extensible with new object types and highly scalable to
large numbers of servers and clients. Below are sketched briefly some of its
features:
- Fault tolerance: the
correctness of the services provided and the semantics of the object stored is guaranteed
despite any benign failures and a limited number of Byzantine server
failures.
- Fault detection: Fleet
continually monitors servers to detect when the number of faulty servers is approaching that
tolerable limit
- High scalability: this
is achieved by means of Byzantine quorum system techniques. This resulted also in a better load balancing
across servers and lower access cost.
- Concurrent semantics:
Fleet provides a generic protocol for turning any object into a shared
object with linearizable concurrent semantics,
in addition to protocols for implementing safe and atomic varibles.
- Extensibility: Fleet provide a mechanism to turn at run-time any application-defined
object into Fleet objects. This mechanism is completely transparent to the
user.
- Liveness: during periods of stability operations between clients and
servers terminate with probability one.
- Autonomy: each Fleet object
is replicated at a set of Fleet servers designed by the application
depending on the level of fault-tolerance desired by the creator of the
object.
- Isolation: to support
possible overlapping universes of Fleet servers, each Fleet Object is
created with a name space and that allows only the creator of the name
space to place new object within it.
A prototype of a voting
application using the Fleet package was implemented and demonstrated at DARPA
Exposition in July 2001.