> I wasn't aware that JMS could
>
> a) allow you to modify/customize the protocol stack
> b) provide notification of members that joins, leaves or crashes
> c) atomicity
Exactly. Furthermore, it may also give you synchronous view group
communication i.e.
"- Agreement: correct processes deliver the same set of messages in any
given view
- Integrity: if a process P delivers message m, then it will not deliver m
again
- Validity: correct processes always deliver the messages that they send.
If the system fails to deliver a message to any process q, then it notifies
the suviving processes by delivering a new view with q excluded, immediately
after the view in which any of them delivered the message.
In a view-synchronous system, the delivery of a new view draws a conceptual
line across the system and every message that is delivered at all is
consistently devlivered one side or the other of that line. This enables the
programmer to draw userful conclusions about the set of messages that other
correct processes have delivered when it delivers a new view, based only on
the local ordering of message delivery and view delivery events." (from
Distributed systems, concepts and design, Coulouris-Dollimore-Kindbert, 3rd
ed., Addison-Wesley)
Some approximate definitions:
- a group is a set of processes
- a view is a set of process
- delivering a message means receiving it
- delivering a view means receiving the new set of processes that are
corrects (i.e. alive, ...)
Consequently, it is possible to exchange information in a group (new JNDI
name binding for example...) and be sure that everyone correct will receive
it (in the same order if necessary). As each view change (v0, v1, v2, ...,
vn) and message (m1, m2, ..., mn) is numbered, it is possible for a process,
when it wakes up, to ask the group to get all updates > mi (i is the last
message he got before collapsing). Furthermore, such a system needs to
detect when a system has failed (to deliver a new view), which why such a
subsystem is present in javagroups to detect these potential failures. The
algorithm that determines which view is valid, ... is distributed and
fault-tolerant in itself : if the "master" dies, an election automatically
takes place to determine the new master.
I understand it is a bit a quick introduction... ask if you have some
questions!
Cheers,
Sacha
