Distributed Chat Server with Java NIO

Project roadmap

1. 01

   Set up the selector loop

   ~2h

   Create a ServerSocketChannel in non-blocking mode, register it with a Selector for accept events, and write the event loop: select, go through the ready keys, and handle accept (registering the new client's SocketChannel for reads, with a per-client state object attached to its key). Done when: several netcat clients can connect, and the server logs each connection.

2. 02

   Read and write with buffers

   ~2.5h

   Give each client a read ByteBuffer and a queue of outgoing buffers. On a read event, read into the buffer, split it into complete lines (keeping any partial line for the next read), and decode them as UTF-8. To send, add to the client's queue and register interest in write events; on a write event, write as much as possible and remove interest in writes when the queue is empty. Done when: an echo test works, even for long messages sent in several pieces.

3. 03

   Define the chat protocol

   ~1.5h

   Define text commands: /nick NAME (required first, unique, and 2 to 20 allowed characters), /join ROOM, /leave, /rooms (list rooms with their number of users), /who (users in the current room), /quit, and plain lines as messages. Parse them into commands with clear error replies for invalid input, and limit line length. Done when: every command works from netcat, and invalid ones get clear errors.

4. 04

   Route messages to rooms

   ~1.5h

   Keep a map of rooms to their clients, send each message to everyone else in the sender's room with the sender's name and time, announce joins and leaves, and remove empty rooms. Done when: messages reach only the people in the same room.

5. 05

   Handle edge cases

   ~2h

   Handle clients that disconnect (read returns -1 or an IOException) by cancelling their key, closing the channel and announcing that they left. Protect against slow clients: if a client's outgoing queue grows past a limit, disconnect it instead of using unlimited memory. Close idle clients after a timeout. Shut down cleanly on Ctrl+C. Done when: sudden disconnects and slow clients never affect the server or other clients.

6. 06

   Write a client and load test

   ~2h

   Write a simple terminal client (with one thread reading the keyboard and one reading the server), and a load test program (run only against your own server) that opens thousands of connections spread across rooms, sends messages at a steady rate, and measures delivery time. Record the connections, messages per second and latency in README.md. Done when: the server handles thousands of clients on one thread, and README.md shows the results.

7. 07

   Test it

   ~1.5h

   Write JUnit tests that start the server on a free port and connect real client sockets: nicknames, rooms, message routing, partial lines, disconnects and slow client handling. Test the protocol parsing separately. Done when: all tests pass.

Resources

• DocsJava NIO: Selector (Java API)
• DocsJava NIO: ByteBuffer (Java API)
• DocsJava NIO: SocketChannel (Java API)
• ArticleThe C10K problem
• DocsJUnit 5 user guide