What is CamlGroups?

CamlGroups is a group communication library, implemented using Objective Caml. It offers convenient programming abstractions for the state machine replication. The state machine approach is a general method for implementing a fault-tolerant service by replicating it on independent machines and coordinating client interactions with the server replicas. The CamlGroups can tolerate up to ceil(n/2-1) machine crashes at a time, where n is the number of replicas.

The main programming abstractions of CamlGroups are:

• Reliable Broadcast -- it guarantees that a message will be delivered to all correct processes if at least one process (correct or not) delivers the messages
• Atomic Broadcast -- in addition to the properties of Reliable Broadcast, it also guarantees that all processes deliver all messages in the same total order
• Generic Broadcast -- in addition to the properties of Reliable Broadcast, it also guarantees that all processes deliver messages of a given type in the order specified by a user-defined ordering function
• Membership Service -- it maintains the current view on which processes are in the group; the processes can join or leave the group dynamically, when suspected as crashed by a simple failure detector

For cooperation without centralized control and with the possibility of occasional process crashes, the CamlGroups uses the distributed consensus protocol, which is the main building block of the system architecture. The architecture follows the one developed in the Crystall project.

See a short demo movie explaining the main concepts of group communication using a small example program implemented in CamlGroups.

Distribution

A tarball file with source files is available here:

To compile the files you will need the Objective Caml ver. at least 3.11.1, available from http://caml.inria.fr/.

Team

• Piotr Wysocki
• Paweł Wojciechowski

Documentation

• API documentation in HTML format
• CamlGroups: Projekt, implementacja i walidacja narzędzia do komunikacji grupowej w języku Objective Caml (In Polish). Piotr Wysocki, Paweł T. Wojciechowski. Technical Report TR-ITSOA-OB2-1-PR-09-5, Institute of Computing Science, Poznań University of Technology, November 2009.
• Biblioteka komunikacji grupowej: Architektura systemu dla grup dynamicznych w modelu crash-stop (In Polish). Adam Gawarkiewicz, Jarosław Przybyłowicz, Michał Witkowski, Piotr Wysocki, Paweł T. Wojciechowski Technical Report TR-ITSOA-OB2-1-PR-09-2, Institute of Computing Science, Poznań University of Technology, June 2009.