Distributed and Concurrent Systems 		25.4.2008
Exercise 7

Last lectures 28.-29.4. Last exercises 6.5  10:30 - 12 T/D106.
Course exam 6.5. 08:00-10:00.

We shall implement the time agreement with a (overly simple)
server-based TCP-protocol. The server application has a port to init
agreement. The agreement is initiated by giving own applicable times and
the addresses of the other requested clients.
Each functioning client listens a TCP port to accept agreement requests.
When the server receives a new agreement init request, it connects to
all requested clients, asks for applicable times, receives them, makes
the decision and announces the time to all clients (and closes
connections).

For simplicity, we use simple integers as applicable times. The first
integer common to all will be chosen. If some client is unreachable (or
gives badly formed answers), it will be ignored. Message lines are
separated by a newline.

The initial request by the first client to server is of form:

    AGREE_INIT
    AGREE_MY_TIMES     1 3 4 5 8 9 10 11 ...
    AGREE_MY_FRIENDS   ip_addr1:port1 ip_addr2:port2 ...

where "1 3 4 5 8 9 10 11 ..." is the applicable time for the client, and
"ip_addr1:port1 ..." is the list of addresses of the other requested
clients. The server replies

    AGREE_INIT_WAIT

After this, the servers connects to each client given in
AGREE_MY_FRIENDS -field, and send message

    AGREE_GIVE_TIMES

Client should reply

    AGREE_MY_TIMES   3 4 5 6 ...

After the server has got all replies (or a reasonable timeout), it makes
the decision, and sends to all connections:

    AGREE_DECISION 3

where 3 is the common time. If no time is suitable to all, server
replies by -1. After reply, server closes all connections.

37) What weaknesses (or defects) the protocol has? Augment the protocol
    to fix these weaknesses. How much the augmentations would increase
    programming work in client and server? The follow tasks are made
    using the original protocol.

The following X3 exercise is obligatory for all students. The answers to
X-exercises have to be unique for every student. No copies of the same
answer are allowed. The answer has to be sent by Thursday 2:00 pm (the
previous day). You will receive an acknowledgment upon successful
processing. Answers will be graded. The answer must also contain a short
self-evaluation (as a program comment) in which you describe whether the
program works, nearly works, or does not probably work; how efficient it
is, etc. A correct and proper self-evaluation is worth one point (in
case of a proper answer).

Send your answer to using cs to user sjuva with a subject HS_X3_username
and the answer (with self-evaluation) as the body of the message (no
attachments). At simplest using program mail at cs:

        /usr/ucb/mail -s HS_X3_username sjuva < username.java

Where username is your cs username and username.java is a text file
containing your answer. To make program compilable, the main class name
must be exactly the same as your user name. No need to include
AgreeProto class to your answer. 

X3) Implement the client for the above protocol. This consists of two
parts. Firstly, the part that initiates the agreement and connects to the
server. Secondly, the part that listens for new agreements and replies
to the requests coming from server. Make The listening part as a
separate thread. For half points, make at least one part.

40-41) Implement the server for above protocol.

42) Make the server concurrent so that it will concurrently connect each
    client.

To help the implementation, in waw-page is AgreeProto -helper class, and
skeletons of client and server. For testing purposes, there are
jar-files of both client and server.