Features Required:

User Authentication:
- Users should be able to sign up, log in, and manage their profiles.
Code Editor:
- Users should be able to create code sessions.
- Real-time collaborative code editing.
Video and Voice Communication:
- Users should be able to start audio and video sessions.
- Screen sharing for collaborative coding.
Session Management:
- Managing active code sessions.
- Inviting other users to join a session.
Code Version Control:
- Recording and managing different versions of code during a session.
- Ability to roll back to previous code states.
Collaborative Whiteboard:
- A shared whiteboard for drawing diagrams, taking notes, etc.
User Roles:
- Distinction between driver and navigator roles.
Real-Time Chat:
- Users should be able to communicate via chat.
Code Review:
- Review and comment on the shared code.

Design Patterns Involved or Used:

Singleton Pattern:
- Use a singleton pattern for managing user sessions and active code sessions to ensure there is a single point of access.
Factory Pattern:
- Create instances of user sessions and code sessions using factories to encapsulate their creation.
Observer Pattern:
- Implement real-time features like collaborative code editing, chat, and video sessions using the observer pattern to notify participants of changes.
State Pattern:
- Manage different states of a code session such as "editing," "in session," "paused," using state classes.
Command Pattern:
- Implement commands for actions like "share code," "start video," "undo," and "redo" during a code session.
Composite Pattern:
- Organize components of a code session, such as the code editor, whiteboard, and chat, using a composite pattern.
Strategy Pattern:
- Use the strategy pattern for video and voice communication, allowing users to select different communication strategies.

Code: Detailed Implementation of Classes Based on Each Design Pattern Mentioned Above:

// Singleton Pattern
class SessionManager {
    private static SessionManager instance;
    private Map<String, UserSession> activeUserSessions;

    private SessionManager() {
        activeUserSessions = new ConcurrentHashMap<>();
    }

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

    public UserSession createUserSession(User user) {
        UserSession session = new UserSession(user);
        activeUserSessions.put(user.getId(), session);
        return session;
    }

    public UserSession getUserSession(String userId) {
        return activeUserSessions.get(userId);
    }
}

class UserSession {
    private User user;
    private List<CodeSession> activeCodeSessions;
    private List<Collaborator> collaborators;

    public UserSession(User user) {
        this.user = user;
        this.activeCodeSessions = new ArrayList<>();
        this.collaborators = new ArrayList<>();
    }

    public void startCodeSession(CodeSession codeSession, List<User> participants) {
        activeCodeSessions.add(codeSession);
        codeSession.addCollaborators(participants);
    }

    // Other methods for managing code sessions and collaborators
}

class CodeSession {
    private String sessionId;
    private User creator;
    private List<Collaborator> collaborators;
    private CodeEditor codeEditor;

    public CodeSession(User creator, CodeEditor codeEditor) {
        this.sessionId = UUID.randomUUID().toString();
        this.creator = creator;
        this.codeEditor = codeEditor;
        this.collaborators = new ArrayList<>();
    }

    public void addCollaborators(List<User> participants) {
        // Add participants as collaborators
    }

    public void shareCode(String code) {
        codeEditor.addCode(code);
        // Notify collaborators about code changes
    }

    // Other methods for managing code sessions
}

// Observer Pattern
interface Collaborator {
    void update(String message);
}

class User implements Collaborator {
    private String id;
    private String name;

    public User(String id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public void update(String message) {
        // Receive and display messages from the code session
    }
}

// State Pattern
class CodeEditor {
    private CodeSessionState state;

    public void setState(CodeSessionState state) {
        this.state = state;
    }

    public void editCode(String code) {
        state.editCode(code);
    }

    public void undo() {
        state.undo();
    }

    // Other methods for managing code editing
}

interface CodeSessionState {
    void editCode(String code);
    void undo();
}

class EditingState implements CodeSessionState {
    @Override
    public void editCode(String code) {
        // Handle code editing state
    }

    @Override
    public void undo() {
        // Undo code changes
    }
}

// Command Pattern
class CommandExecutor {
    public void execute(Command command) {
        command.execute();
    }
}

interface Command {
    void execute();
}

class ShareCodeCommand implements Command {
    private CodeSession codeSession;
    private String code;

    public ShareCodeCommand(CodeSession codeSession, String code) {
        this.codeSession = codeSession;
        this.code = code;
    }

    @Override
    public void execute() {
        codeSession.shareCode(code);
    }
}


// Composite Pattern
class CodeSessionComponent {
    List<CodeSessionComponent> children;

    public void addComponent(CodeSessionComponent component) {
        children.add(component);
    }

    // Other methods
}

// Strategy Pattern
interface CommunicationStrategy {
    void startCommunication(User user);
}

class VideoCommunication implements CommunicationStrategy {
    @Override
    public void startCommunication(User user) {
        // Start a video session
    }
}

Please note that this code is an outline and doesn't cover the complete implementation of the pair programming platform. You'll need to add more classes and methods to handle features like video and voice communication, chat, and other functionalities. Additionally, you should consider security and error handling aspects as well as the integration of real-time communication libraries for video and voice chats.

Issues in Above Design

1️⃣ Singleton `SessionManager` Is a Scalability Bottleneck

Current

class SessionManager (Singleton)
Map<String, UserSession> activeUserSessions

Problems

Single JVM bottleneck
No horizontal scaling
Memory-bound
No failover
No session ownership transfer

📌 Interview Expectation

Session management must be distributed, not singleton.

💥 Real-world: Sessions live in Redis / etcd / CRDT-backed stores, not JVM memory.

Solution

will be covered in our premium course.

2️⃣ UserSession Is Not Thread-Safe

Current

List<CodeSession> activeCodeSessions;
List<Collaborator> collaborators;

Issues

Non-concurrent collections
Race conditions on join/leave
Concurrent modifications during notifications

📌 Concurrency Bug

Two users joining at the same time → undefined behavior.

Solution

will be covered in our premium course.

3️⃣ Observer Pattern Is Insufficient for Real-Time Editing

Current

Collaborator.update(String message)

Why This Fails

No ordering guarantees
No conflict resolution
No causality tracking
No replay
No late join support

📌 Critical Interview Insight

Real-time code editing cannot rely on Observer pattern alone.

💡 Requires:

OT (Operational Transform) or CRDT
Version vectors
Operation sequencing

Solution

will be covered in our premium course.

Current

shareCode(String code)
codeEditor.addCode(code)

Fatal Issue

Entire document replaced
No diff
No merge
No conflict detection

📌 Correct Mental Model

Collaborative editing shares operations, not text blobs.

Solution

will be covered in our premium course.

5️⃣ No Concurrency Control on Code Editing

Missing Entirely

Concurrent edits
Cursor positions
Selection ranges
Intent preservation

📌 Expected

Two users typing at the same time must converge deterministically.

Solution

will be covered in our premium course.

6️⃣ Undo/Redo Is Incorrectly Modeled

Current

state.undo()

Why This Breaks

Undo is user-local
Remote operations interfere
No per-user operation stack

📌 Interview Insight

Undo/redo in collaborative editors is per-user, operation-based, not global.

Solution

will be covered in our premium course.

7️⃣ CodeSession State Pattern Is Incomplete

Current States

Editing

Missing States

Initializing
Syncing
Reconnecting
Read-only
Closed
Network-partitioned

📌 Expected

State machine must model network realities, not just UI modes.

Solution

will be covered in our premium course.

8️⃣ No Session Ownership or Leadership Model

Missing

Who is authoritative?
Driver vs Navigator enforcement
Write permissions
Role transitions

📌 Interview Red Flag

Roles must be enforced at operation level, not UI level.

Solution

will be covered in our premium course.

9️⃣ Video / Audio Strategy Pattern Is Too Shallow

Current

CommunicationStrategy.startCommunication()

Missing

ICE negotiation
Signaling server
NAT traversal
Reconnection logic
Media state sync

📌 Reality

Video is peer-to-peer with centralized signaling, not a simple strategy.

Solution

will be covered in our premium course.

🔟 No Network Failure Handling

Missing

Packet loss
Out-of-order messages
Client reconnect
State rehydration

📌 Interview Expectation

Real-time systems must assume clients disappear at any time.

Solution

will be covered in our premium course.

1️⃣1️⃣ No Version Control Semantics

Current

“Recording and managing versions”

Missing

Snapshot vs delta
Commit graph
Branching model
Merge strategy

📌 Expected

Versioning must be append-only and immutable, not overwritten.

Solution

will be covered in our premium course.

1️⃣2️⃣ No Persistence Model

Missing

Session recovery
Crash recovery
History replay
Late join sync

📌 Real-world

Collaboration platforms must survive server restarts.

Solution

will be covered in our premium course.

Problem

Chat floods block code updates
No prioritization
No QoS separation

📌 Expected

Control plane ≠ data plane.

Solution

will be covered in our premium course.

1️⃣4️⃣ Security Model Is Absent

Missing

AuthZ per operation
Session tokens
Invitation validation
Replay protection

📌 Interview Insight

“Who can send what to which session?”

Solution

will be covered in our premium course.

1️⃣5️⃣ Composite Pattern Is Underutilized

Current

CodeSessionComponent

Missing

Lifecycle propagation
Permission inheritance
Failure isolation

📌 Expected

Composite must manage behavior, not just structure.

Solution

will be covered in our premium course.

1️⃣6️⃣ No Backpressure or Rate Limiting

Missing

Typing bursts
Paste floods
Chat spam

📌 Failure Mode

One fast typist can DoS the session.

Solution

will be covered in our premium course.

1️⃣7️⃣ No Ordering Guarantees Across Commands

Current

CommandExecutor.execute()

Problem

Commands may arrive out of order
No sequencing
No idempotency

📌 Expected

Every operation must carry sequence number + session version.

Solution

will be covered in our premium course.

1️⃣8️⃣ No Multi-Session Isolation

Missing

Resource limits per session
User quotas
CPU / memory caps

📌 Real-World

One large session must not starve others.

Solution

will be covered in our premium course.

Issues in Above Design

1️⃣ Singleton SessionManager Is a Scalability Bottleneck

Current

Problems

Solution

2️⃣ UserSession Is Not Thread-Safe

Current

Issues

Solution

3️⃣ Observer Pattern Is Insufficient for Real-Time Editing

Current

Why This Fails

Solution

4️⃣ Code Sharing = Overwriting State (Data Loss)

Current

Fatal Issue

Solution

5️⃣ No Concurrency Control on Code Editing

Missing Entirely

Solution

6️⃣ Undo/Redo Is Incorrectly Modeled

Current

Why This Breaks

Solution

7️⃣ CodeSession State Pattern Is Incomplete

Current States

Missing States

Solution

8️⃣ No Session Ownership or Leadership Model

Missing

Solution

9️⃣ Video / Audio Strategy Pattern Is Too Shallow

Current

Missing

Solution

🔟 No Network Failure Handling

Missing

Solution

1️⃣1️⃣ No Version Control Semantics

Current

Missing

Solution

1️⃣2️⃣ No Persistence Model

Missing

Solution

1️⃣3️⃣ Chat + Code Share Use Same Observer Channel (Coupling)

Problem

Solution

1️⃣4️⃣ Security Model Is Absent

Missing

Solution

1️⃣5️⃣ Composite Pattern Is Underutilized

Current

Missing

Solution

1️⃣6️⃣ No Backpressure or Rate Limiting

Missing

Solution

1️⃣7️⃣ No Ordering Guarantees Across Commands

Current

Problem

Solution

1️⃣8️⃣ No Multi-Session Isolation

Missing

Solution

1️⃣ Singleton `SessionManager` Is a Scalability Bottleneck

Design (LLD) Pair Programming Platform like HackerRank CodePair - Machine Coding