Question

Explain how you would design a chat server. In particular, provide details about the various back end components, classes, and methods.

What would be the hardest problems to solve?

Solution

First, decide the objects and methods. Here we’ll focus on the core user management and conversation aspects.

Class

1. UserMgmt (business logic)
2. User (includes basic info, UserStatus, a map of conversation, a map of requests)
3. UserStatus (on/offline, status message)
4. Conversation Abstract Class (a list of user and a list of messages)
   1. PrivateChat (private conversation)
   2. GroupChat
5. Message (a string and a date/time)
6. Request (add request and delete request, involves 2 Users)

Functions

1. Sign in and log off (update availability)
2. update status message
3. add/delete request
4. send/accept/reject a request
5. create a conversation (group or private)
6. add a new message (group or private)

Code

The most important classes is User and UserMgmt. The others are simply data containers.

UserManager.java

public class UserManager {
    private static UserManager instance;
    private HashMap<Integer, User> usersById = new HashMap<Integer, User>();
    private HashMap<String, User> usersByAccountName = new HashMap<String, User>();
    private HashMap<Integer, User> onlineUsers = new HashMap<Integer, User>();

    public static UserManager getInstance() {
        if (instance == null) {
            instance = new UserManager();
        }
        return instance;
    }

    public void addUser(User fromUser, String toAccountName) {
        User toUser = usersByAccountName.get(toAccountName);
        AddRequest req = new AddRequest(fromUser, toUser, new Date());
        toUser.receivedAddRequest(req);
        fromUser.sentAddRequest(req);
    }

    public void approveAddRequest(AddRequest req) {
        req.status = RequestStatus.Accepted;
        User from = req.getFromUser();
        User to = req.getToUser();
        from.addContact(to);
        to.addContact(from);
    }

    public void rejectAddRequest(AddRequest req) {
        req.status = RequestStatus.Rejected;
        User from = req.getFromUser();
        User to = req.getToUser();
        from.removeAddRequest(req);
        to.removeAddRequest(req);       
    }

    public void userSignedOn(String accountName) {
        User user = usersByAccountName.get(accountName);
        if (user != null) {
            user.setStatus(new UserStatus(UserStatusType.Available, ""));           
            onlineUsers.put(user.getId(), user);
        }
    }

    public void userSignedOff(String accountName) {
        User user = usersByAccountName.get(accountName);
        if (user != null) {
            user.setStatus(new UserStatus(UserStatusType.Offline, ""));
            onlineUsers.remove(user.getId());
        }
    }   
}

User.java

Property:

1. id and name
2. a map of conversations
3. a map of sent request
4. a map of received request
5. a map of friends list

Methods:

1. sendMessageToUser(User)
2. addContact(User)
3. receivedAddRequest(Request)
4. sentAddRequest(Request)
5. removeAddRequest(Request)
6. addConversation(Conversation)

Note that all user actions are controlled by the UserManager Class. For example, when adding a friend:

1. User A clicks “add user” on the client.
2. User A calls requestAddUser (User B).
3. This method calls UserManager.addUser(User a, userBid).
4. UserManager calls both User A.sentAddRequest() and User B.receivedAddRequest().

Code:

public class User {
    private int id;
    private UserStatus status = null;
    private HashMap<Integer, PrivateChat> privateChats = new HashMap<Integer, PrivateChat>();
    private ArrayList<GroupChat> groupChats = new ArrayList<GroupChat>();
    private HashMap<Integer, AddRequest> receivedAddRequests = new HashMap<Integer, AddRequest>();
    private HashMap<Integer, AddRequest> sentAddRequests = new HashMap<Integer, AddRequest>();

    private HashMap<Integer, User> contacts = new HashMap<Integer, User>();
    private String accountName;
    private String fullName;

    public User(int id, String accountName, String fullName) {
        this.accountName = accountName;
        this.fullName = fullName;
        this.id = id;
    }

    public boolean sendMessageToUser(User toUser, String content) {
        PrivateChat chat = privateChats.get(toUser.getId());
        if (chat == null) {
            chat = new PrivateChat(this, toUser);
            privateChats.put(toUser.getId(), chat);
        }
        Message message = new Message(content, new Date());
        return chat.addMessage(message);
    }

    public boolean sendMessageToGroupChat(int groupId, String content) {
        GroupChat chat = groupChats.get(groupId);
        if (chat != null) {
            Message message = new Message(content, new Date());
            return chat.addMessage(message);
        }
        return false;
    }

    public void setStatus(UserStatus status) {
        this.status = status;
    }

    public UserStatus getStatus() {
        return status;
    }

    public boolean addContact(User user) {
        if (contacts.containsKey(user.getId())) {
            return false;
        } else {
            contacts.put(user.getId(), user);
            return true;
        }
    }

    public void receivedAddRequest(AddRequest req) {
        int senderId = req.getFromUser().getId();
        if (!receivedAddRequests.containsKey(senderId)) {
            receivedAddRequests.put(senderId, req);
        }       
    }

    public void sentAddRequest(AddRequest req) {
        int receiverId = req.getFromUser().getId();
        if (!sentAddRequests.containsKey(receiverId)) {
            sentAddRequests.put(receiverId, req);
        }       
    }

    public void removeAddRequest(AddRequest req) {
        if (req.getToUser() == this) {
            receivedAddRequests.remove(req);
        } else if (req.getFromUser() == this) {
            sentAddRequests.remove(req);
        }
    }

    public void requestAddUser(String accountName) {
        UserManager.getInstance().addUser(this, accountName);
    }

    public void addConversation(PrivateChat conversation) {
        User otherUser = conversation.getOtherParticipant(this);
        privateChats.put(otherUser.getId(), conversation);
    }

    public void addConversation(GroupChat conversation) {
        groupChats.add(conversation);
    }   

    public int getId() {
        return id;
    }

    public String getAccountName() {
        return accountName;
    }

    public String getFullName() {
        return fullName;
    }
}