The JGroups project
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// $Id: README,v 1.7 2006/08/09 13:08:02 belaban Exp $ JGroups - A Framework for Group Communication in Java ======================================================== March 3, 1998 Bela Ban 4114 Upson Hall Cornell University Ithaca, NY 14853 firstname.lastname@example.org email@example.com JGroups is a Java package for reliable group communication. It consists of 3 parts: (1) a socket-like API for application development, (2) a protocol stack, which implements reliable communication, and (3) a set of building blocks, which give the application/protocol programmer high-level abstractions (e.g. DistributedHashtable, derived from java.util.Hashtable, which is similar to Linda/JavaSpaces). The API (a channel) looks like a socket: there a methods for joining and leaving groups, sending and receiving messages to/from members, getting the shared group state, and registering for notifications when a member joins, or an existing member leaves or crashes. The protocol stack is a linked list of protocols, through which each message has to be passed. Each protocol implements an up() and down() method, and may modify, reorder, encrypt, fragment/unfragment, drop a message, or pass it up/down unchanged. The protocol stack is created according to a specification given when a channel is created. New protocols can be plugged into the stack easily. Building blocks hide the channel and provide a higher abstraction. Example: DistributedHashtable is a subclass of java.util.Hashtable and overrides all methods that change the hashtable (clear, put, remove). Those methods are invoked on all hashtables in the same group simultaneously, so that all hashtables have the same state. A new hashtable uses a state transfer protocol to initially obtain the shared group state from an existing member. This enables replication of data structures across process boundaries. Group communication is important in the following situations: - A service has to be replicated for availability. As long as at least one of the servers remains operational, the service itself remains operational - Service requests have to be balanced between a set of servers - A large number of objects have to be managed as one entity (e.g. a management domain) - Notification service / push technology: receivers subscribe to a channel, senders send data to the channels, channels distribute data to all receivers subscribed to the channel (see iBus, CastaNet etc.). Used for example for video distribution, videoconferencing JGroups deliberately models a rather low-level message-oriented middleware (MOM) model. The reason is that we don't want to impose a one-size-fits-all model on the programmer, who usually will want to extend the model in various (previously unconceived) ways anyway. Providing low level Java classes allows the programmer to extend/replace classes at will, as the granularity of the system is finer. JGroups can also be used for the construction of higher level toolkits/frameworks. Such frameworks should provide a certain transparency, without, however, preventing extensions to be made. The principle of creating partly 'opened-up' black boxes is called Open Implementation (OI, http://www.parc.xerox.com/spl/projects/oi/) and will be applied both to JGroups and to a further higher level toolkit.