Use Python 3.8, ayncio and grpc.

1. Run "python3.8 server.py"
2. Run "python3.8 client.py"

For the grpc implementation, use "grpc_server.py" and "grpc_client.py" instead.

The client application supports the following commands, via stdin.

1. CREATE: "create user" creates a new user named "user". Error will be returned if "user" already exists.
2. CONNECT: "connect user" receives any queued up messages for "user". After calling this, "user" will immediately receive any messages sent to it.
3. DELETE: "delete user" deletes the user named "user", if it exists. Unread messages to "user" will be deleted as well.
4. LIST: "list pattern" lists all users matching the regex "pattern".
5. SEND: "send user msg" sends "msg" to user "user", if it exists. If "user" doesn't exist, error is returned. If "user" is logged in and has invoked "connect user" already, then "msg" will be delivered immediately. If "user" is not logged in, but exists, "msg" will be added to a queue. The next time "user" logs in and invokes "connect user", all messages in the queue will be delivered.

1. Open 3 terminals.
2. On Terminal 1, run "python3.8 server.py". On Terminals 2 and 3, run "python3.8 client.py".
3. On Terminal 2, enter "create user2". On Terminal 3, enter "create user3".
4. On Terminal 2, enter "list" to see the existing users.
5. On Terminal 2, enter "send user3 hello" to send the message "hello" to user3.
6. Note that on Terminal 3, the message has not arrived yet. We need to connect first. On Terminal 3, enter "connect user3". The message "hello" should now arrive.
7. On Terminal 2, enter "send user3 helloagain". Note that the message "helloagain" should appear immediately on Terminal 3.

This protocol is only relevant for part 1. In part, these details are abstracted away by using gRPC.

This protocol permits transmission of strings of the form "command args data", where "command" is one of {create, connect, delete, list, send}, "args" denotes a list of arguments (such as usernames), and "data" is a string (e.g. message). To encode "command args data":

• First 15 bytes correspond to UTF8 encoding of "command", padded with null bytes as needed. So, null byte(s) will separate the command part from the arguments/data part.
• Next bytes correspond to UTF8 encoding of "args"
• Next bytes correspond to UTF8 encoding of "data"
• If "args" or "data" contain multiple space-separated strings, they get replaced by UTF8 encoding of those comma-separated strings.
• Buffer size is 1024 bytes.