er, sequencer-specific
Your task is to create a sequencer-based multicasting service, using a slight variant of the simple version
of the Amoeba protocol (the first protocol described in www.cdk3.net/coordination), and a small client
application to exercise it. You will use Java RMI and multicast sockets.
The main classes in your implementation will be:
TestSequencer.java – each instance (you will create several) will allow the user to enter
strings and multicast them to a group of instances of TestSequencer. TestSequencer will
also be capable of “stress-testing” – multicasting messages as fast as possible.
Group.java – TestSequencer uses an instance of Group for group communication
services. Group in turn uses a MulticastSocket to receive incoming messages, and uses RMI
to the sequencer for sending messages and other, sequencer-specific operations
Sequencer.java, SequencerImpl.java – the interface to and implementation of a sequencer.
History.java – the implementation of the sequencer's history. History does not need to
have a fixed capacity, but obsolete items should be removed whenever its size grows beyond a
threshold, say 1024 entries.
Attached you will find (i) the RMI interface to an object which will implement the sequencer
(Sequencer.java), (ii) two trivial classes associated with the Sequencer definition
(SequencerJoinInfo.java and SequencerException.java), (iii) outline code for
Group.java, and (iv) some hints on marchalling messages into datagram packets.
You need to write SequencerImpl.java and History.java from scratch.
Note that the scheme you produce will not precisely implement the Amoeba protocol. In that protocol,
messages are sent unreliably to the sequencer in the first instance. In yours, messages to be multicast are
handed to the sequencer using RMI.
Choosing a multicast address and time-to-live
SequencerImpl should take as an argument the IP multicast address that the corresponding Groups are to
use. To avoid collisions with numbers chosen by others, incorporate a random number into the multicast
IP address that you use – e.g. 234.day.month.rand where rand is a random number between 1 and 254.
Don't set your sockets' time-to-live (TTL) to more than 1. That way, your multicast packets will not be
transmitted beyond the local router.
Hints: Marshalling data into a datagram
ByteArrayOutputStream bstream = new
ByteArrayOutputStream(MAX_MSG_LENGTH);
DataOutputStream dstream = new DataOutputStream(bstream);
dstream.writeLong(aLong); // marshals a Long into the byte array underlying bstream
// …
// After marshalling, the data can be obtained from bstream for inclusion in a DatagramPacket:
byte[] theData = bstream.toByteArray();
(For unmarshalling there are corresponding classes ByteArrayInputStream and DataInputStream.)
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