problemhardood

Object-Oriented Design for an Online Chat Server

HardUpdated: Oct 7, 2025

Problem

Explain how you would design a chat server. In particular, provide details about the various backend components, classes, and methods. What would be the hardest problems to solve?

Solution

We certainly need a user class. We need to handle concurrency.

User class:


public class User {

   private Long userId;
   private String nickname;

   public User(Long userId, String nickname) {
    this.userId = userId;


## Problem

Design an online chat server. Describe the backend components, classes, and method-level responsibilities required to support multiple users exchanging messages (directly or in sessions), and discuss the hardest problems to solve (concurrency, scaling, durability).

## Solution

### 1. Requirements Analysis

Functional requirements:
- Represent users and messages, with message timestamps and optional metadata.
- Support sessions (chat rooms) where multiple participants can join, send, and receive messages.
- Allow registration/unregistration of listeners (clients) that receive new messages for a session.
- Provide an API or CLI for sending messages and subscribing to message streams.

Non-functional requirements:
- Concurrency: support many concurrent users and sessions with low-latency message delivery.
- Scalability: ability to scale out (multiple servers or a message broker) to handle high fan-out.
- Durability/Retention: optional retention policy for messages (TTL) and capacity to trim old messages.
- Loose coupling: decouple producers and consumers (observer vs broker patterns).

Edge cases and constraints (inferred from the original note):
- Large fan-out (e.g., 1000 listeners) where synchronous observer notifications become expensive.
- Ordering guarantees, at-most-once vs at-least-once delivery (not fully specified in source; left as an implementation decision).

### 2. Use Case Diagram

Actors: User (client), System Administrator (optional), Message Broker / ChatServer.

Use case summary: A User registers/listens to a Session, posts messages, and receives messages published to that Session.

```mermaid
graph TD
    subgraph Chat System
        UC1(Send Message)
        UC2(Receive Message)
        UC3(Join Session)
        UC4(Leave Session)
    end
    User --> UC1
    User --> UC2
    User --> UC3
    User --> UC4
    Admin --> UC3

3. Class Diagram

Core classes and brief responsibilities (all inferred from file content):

  • User: identifier and user metadata (userId, nickname).
  • Message: interface/contract for messages (value, timestamp, sender).
  • SimpleMessage: a basic Message implementation.
  • ChatListener: observer interface that receives new Message events.
  • ChatWindow / Client: a listener implementation that holds a local message list for a user.
  • ChatSession (or Session): manages registered listeners and publishes incoming messages to them.
  • Optionally, MessageBroker / ChatServer: scalable component that handles distribution (e.g., via a queue or pub/sub system).
classDiagram
    class User {
        +Long userId
        +String nickname
        +getUserId()
        +getNickname()
    }

    class Message {
        <<interface>>
        +String getValue()
        +void setValue(String)
        +Date getTimestamp()
        +void setTimestamp(Date)
        +User getUser()
    }

    class SimpleMessage {
        +String value
        +User user
        +Date timestamp
    }

    class ChatListener {
        <<interface>>
        +void notify(Message)
    }

    class ChatWindow {
        +User user
        +List~Message~ messageList
        +void notify(Message)
    }

    class ChatSession {
        +List~ChatListener~ registeredChatListeners
        +void register(ChatListener)
        +void incomingMessage(Message)
        +void publish(Message)
    }

    ChatSession "1" -- "0..*" ChatListener : registers
    ChatWindow ..|> ChatListener
    SimpleMessage ..> User : sender

4. Activity Diagrams

Activity: Sending a Message

graph TD
    U[User types message] --> S[Client sends message to ChatSession or Broker]
    S --> P[Session/Broker validates and enqueues message]
    P --> N[Publish message to registered listeners]
    N --> L[Each listener.enqueueOrDeliver(message)]

Activity: Registering a Listener (subscribe)

graph TD
    U[Client requests subscribe] --> S[ChatSession.register(listener)]
    S --> A[Add listener to registered list]
    A --> R[Return success]

5. High-Level Code Implementation

Below are cleaned skeletons adapted from the original Java examples and converted to Python equivalents. These are intentionally minimal: class signatures, attributes, and key method signatures only.

Java

// User.java
public class User {
        private Long userId;
        private String nickname;

        public User(Long userId, String nickname) {
                this.userId = userId;
                this.nickname = nickname;
        }

        public Long getUserId() { return userId; }
        public String getNickname() { return nickname; }
}

// Message.java
public interface Message {
        String getValue();
        void setValue(String value);
        Date getTimestamp();
        void setTimestamp(Date ts);
        User getUser();
}

// SimpleMessage.java
public class SimpleMessage implements Message {
        private String value;
        private User user;
        private Date timestamp;

        // constructors, getters, setters
}

// ChatListener.java
public interface ChatListener {
        void notify(Message newMessage);
}

// ChatWindow.java (client-side listener)
public class ChatWindow implements ChatListener {
        private User user;
        private List<Message> messageList;

        public void notify(Message newMessage) {
                if (messageList == null) messageList = new ArrayList<>();
                messageList.add(newMessage);
        }
}

// ChatSession.java
public class ChatSession {
        private List<ChatListener> registeredChatListeners;

        public void register(ChatListener listener) {
                if (registeredChatListeners == null) registeredChatListeners = new ArrayList<>();
                registeredChatListeners.add(listener);
        }

        public void incomingMessage(Message message) {
                publish(message);
        }

        protected void publish(Message messageToPublish) {
                if (registeredChatListeners != null) {
                        for (ChatListener eachListener : registeredChatListeners) {
                                eachListener.notify(messageToPublish);
                        }
                }
        }
}

Python

from __future__ import annotations
from dataclasses import dataclass, field
from typing import List, Protocol
from datetime import datetime

@dataclass
class User:
        user_id: int
        nickname: str

class Message(Protocol):
        def get_value(self) -> str: ...
        def set_value(self, value: str) -> None: ...
        def get_timestamp(self) -> datetime: ...
        def get_user(self) -> User: ...

@dataclass
class SimpleMessage:
        value: str
        user: User
        timestamp: datetime = field(default_factory=datetime.now)

class ChatListener(Protocol):
        def notify(self, message: Message) -> None: ...

class ChatWindow:
        def __init__(self, user: User) -> None:
                self.user = user
                self.message_list: List[Message] = []

        def notify(self, message: Message) -> None:
                self.message_list.append(message)

class ChatSession:
        def __init__(self) -> None:
                self.registered_listeners: List[ChatListener] = []

        def register(self, listener: ChatListener) -> None:
                self.registered_listeners.append(listener)

        def incoming_message(self, message: Message) -> None:
                self.publish(message)

        def publish(self, message: Message) -> None:
                for listener in self.registered_listeners:
                        listener.notify(message)

References

Comments