Protocol
A protocol describes the steps of exchanging
information. This is usually done by a finite state machine. Our protocol is
simple, so we just proceed using a less formal line diagram.
|
time |
step |
server |
client |
|
1 |
c.1 |
|
client connects to server by opening a socket |
|
2 |
s.1 |
server sends the server ready signal, by writing the character ! on the socket |
|
|
3 |
c.2 |
|
client reads the ! character, so it knows that the server is ready |
|
4 |
c.3 |
|
client sends the request (see below how) |
|
5 |
s.2 |
server reads the request |
|
|
6 |
s.3 |
server performs the request |
|
|
7 |
s.4 |
server transmits a status code to the client (see below how) |
|
|
8 |
c.4 |
|
client reads the status code and reports it |
|
8 |
c.4 |
|
client says good bye, by writing a single ! character on the socket and then closes it |
|
9 |
c.5 |
server reads the good bye and closes the socket |
|
This is a very simple protocol, no recovery, retransmission and other
complications.
- s.1: the server writes
first, because the client socket is not ready for writing until the accept() method
returns the client's peer socket on the server side. To be safe, do not
print a single !,
but print 3 white spaces and then a !.
- c.2: the client is in a
read loop, until it sees the ! character
- c.3: the client must
send the request. This request contains the following information:
- user
- logevent
e.g. !sam/login!
- s.2: the server waits
for the !
character and reads until the trailing ! character (skipping all escaped !
characters).
- s.3: the server
- chops the request into its tokens
- checks the user
- checks the logevent
- adds the entry into the statistics database
(with the current date)
- s.4: the server sends a
status code enclosed in a pair of ! characters.
The status codes are:
0: success
1: logevent unknown
2: transmission error (the request could not be
parsed)
3: database error
4: any other error that prohibited the server to
fulfill the request
So, if all went well, the
server would write !0! and if the log event was unknown, it would write !1!. If more
than one error occurred, only one has to be reported.
- c.4: client reads the response , if there is an error an exception is raised.
- c.5: client says good
bye, by writing !
on the socket
- s.5: server reads the ! character and closes the socket. This may not actually
happen, depending on the speed of the communication line. This is why you
should set a timeout on the socket (setSoTimeout).