Solution:
Apparently, I am too naive and I think too few. Here‘s the detailed solution.

What is our chat server?
This is something you should discuss with your interviewer, but let’s make a couple of as- sumptions: imagine we’re designing a basic chat server that needs to support a small number of users. People have a contact list, they see who is online vs offline, and they can send text-based messages to them. We will not worry about supporting group chat, voice chat, etc. We will also assume that contact lists are mutual: I can only talk to you if you can talk to me. Let’s keep it simple.

What specific actions does it need to support?

• User A signs online
• User A asks for their contact list, with each person’s current status
• Friends of User A now see User A as online
• User A adds User B to contact list
• User A sends text-based message to User B
• User A changes status message and/or status type
• User A removes User B
• User A signs offline

What can we learn about these requirements?
We must have a concept of users, add request status, online status, and messages.

What are the core components?
We’ll need a database to store items and an "always online" application as the server. We might recommend using XML for the communication between the chat server and the clients, as it’s easy for a person and a machine to read.

What are the key objects and methods?
We have listed the key objects and methods below. Note that we have hidden many of the details, such as how to actually push the data out to a client.

enum StatusType {    online, offline, away;} class Status {    StatusType status_type;    String status_message;} class User {    String username;    String display_name;    User[] contact_list;    AddRequest[] requests;     boolean updateStatus(StatusType stype,             String message) {    };     boolean addUserWithUsername(String name);     boolean approveRequest(String username);     boolean denyRequest(String username);     boolean removeContact(String username);     boolean sendMessage(String username,             String message);} // Holds data that from_user would like to add to_userclass AddRequest {    User from_user;    User to_user;} class Server {    User getUserByUsername(String username);}

What problems would be the hardest to solve (or the most interesting)?

1. How do we know if someone is online—I mean, really, really know?
   While we would like users to tell us when they sign off, we can’t know for sure. A user‘s connection might have died, for example. To make sure that we know when a user has signed off, we might try regularly pinging the client to make sure it’s still there.
2. How do we deal with conflicting information?
   We have some information stored in the computer’s memory and some in the database. What happens if they get out of sync? Which one is "right"?
3. How do we make our server scale?
   While we designed out chat server without worrying -too much- about scalability, in real life this would be a concern. We’d need to split our data across many servers, which would increase our concern about out of sync data.
4. How we do prevent denial of service attacks?
   Clients can push data to us—what if they try to DOS us? How do we prevent that?