Aims

$$Develop a networked application using TCP sockets

$$Implement a protocol according to a given specification from both the client and server side

$$Implement client$-$side checking to ensure only valid messages are sent to the server

$---$Introduction$---$

In this assignment, you will be creating a network messaging system that allows users to leave messages for other users. Your task is to use TCP sockets to create a server process that can store messages and allow them to be retrieved, and a client process that can be used to interact with the server. The protocol that your solution must implement is described below.

Protocol

Each command and response in the protocol consists of a string of ASCII characters followed by the line feed character $($ASCII code $10).$ All commands and responses are case sensitive$.$

An interaction begins when a client sends a LOGIN command to the server $($where is replaced with the name of the user to log in$).$ Usernames must not include space $($ASCII code $32)$ characters. If a LOGIN command is sent with the username containing a space, the server should see this message as an error.

The server responds to a LOGIN command with the number of messages that are currently stored on the system for the user with the given username. Thus, if there are no messages for the user, the server will respond with $0.$

After this response, the client can choose to either COMPOSE a message to be sent to a user, READ a message that was previously sent to the logged$-$in user, or EXIT the system.

To compose a message, the client sends a COMPOSE command $($where is the name of the user they wish to send a message to$),$ followed by a command $($where is the single$-$line message to be sent$),$ to the server. If the server successfully stores the message, the server must respond with MESSAGE SENT. Otherwise, the server must respond with MESSAGE FAILED. Your server must be able to store at least $100$ unread messages for each user before failing $($though it would be better if unlimited messages were supported$).$

To read a message, the client sends a READ command to the server. If there are no messages for the logged$-$in user, the server must respond with READ ERROR. Otherwise, the server must respond with $,$ followed by the response $($where is the sender of the earliest unread message for the logged$-$in user and is the content of that message$).$ The message must then be removed from the server so that any further READ command would result in a different message being delivered to the user $($or a READ ERROR response if there were no more messages$).$

To exit, the client sends an EXIT command and then disconnects from the server. When the server receives an EXIT command it must close the connection.

After a COMPOSE or READ interaction, the client can choose to either COMPOSE a message to be sent to a user, READ a message that was previously sent to the logged$-$in user, or EXIT the system.

Any other message sent to or from the client is considered an error, and should result in the receiving party dropping the connection.

Example

These examples show the protocol only. This is not how interactions with the program should look to the end user. Note, also, that the protocol does not include the "Client: $"$ or "Server: $"$ strings $-$ they are just included to make these examples clearer.

Example $1$

Client: LOGIN my username contains the space character

Note that the server drops the connection after an error.

Example $2$

Client: LOGIN bob

Server: $0$

Client: COMPOSE alice

Client: Hi Alice!

Server: MESSAGE SENT

Client: READ

Server: READ ERROR

Client: COMPOSE alice

Client: I guess you're not online

Server: MESSAGE SENT

Client: COMPOSE bob

Client: At least you'll talk to me

Server: MESSAGE SENT

Client: READ

Server: bob

Server: At least you'll talk to me

Client: EXIT

Both client and server exit normally.

Example $3$

Client: LOGIN alice

Server: $1024$

Client: COMPOSE alice

Client: Note to self: Read my messages

Server: MESSAGE FAILED

Client: READ

Server: bob

Server: Hi Alice!

Client: READ

Server: bob

Server: I guess you're not online

Client: EXIT

Both client and server exit normally.

Example $4$

Client: LOGIN carol

Server: $1$

Client: read

Note that the server drops the connection after an error.

Details:

required to support multiple consecutive clients $($in particular, this will require you to synchronise access to each user's message queue$).$ In either case, having a client disconnect should not cause the server to exit $-$ the server should instead wait for a new client to connect unless explicitly forced to exit.