Concurrency / Realtime• Patterns: Command, Observer, Strategy

Google Docs

Hard

Problem Summary

Design a real-time Collaborative Rich-Text Document Editor supporting concurrent multi-user editing, Operational Transformation (OT) conflict resolution, and version history snapshots.

Functional Scope

Concurrent document modifications by multiple remote collaborators.
Operational Transformation (OT) or CRDT strategy to guarantee eventual consistency.
Command pattern encapsulating document mutations (Insert, Delete, Format).
Observer pattern broadcasting transformed operations to connected WebSocket peers.
Monotonically increasing document revisions and snapshot recovery.

Entity-Relationship (ER) Schema

Document [1] --> [*] Operation
Document [1] --> [1] ConflictResolverStrategy
Document [1] --> [*] CollaboratorObserver

Design Approach

Apply the Command pattern for editing operations and Strategy for Operational Transformation (OT) transformation matrices, ensuring all collaborators converge on identical document contents.

Core Classes & Models

Document (DocId, revision, text buffer)Operation (Type: INSERT, DELETE, position, text content, authorId, clientRevision)ConflictResolverStrategy (OperationalTransformationStrategy, CRDTStrategy)CollaboratorObserver (Broadcasts transformed operations to connected peers)HistoryManager (Snapshot creation and revision rollback)

Code Blueprint

89 lines

/**
 * Low-Level Design: Google Docs Collaborative Text Editor
 * Design Patterns Applied:
 *  - Command Pattern: Encapsulating keystrokes, inserts, deletes with Undo / Redo support
 *  - Observer Pattern: Real-time broadcast of document edits to active collaborators
 *  - Strategy Pattern: Operational Transformation (OT) conflict resolution strategies
 */

import java.util.*;

// --- [DESIGN PATTERN: Observer] Real-time collaborator sync ---
interface DocumentObserver {
    void onDocumentModified(int version, String fullText);
}

class CollaboratorView implements DocumentObserver {
    private final String userId;
    public CollaboratorView(String userId) { this.userId = userId; }
    @Override
    public void onDocumentModified(int version, String fullText) {
        System.out.printf("💻 [Collaborator %s] Sync v%d: \"%s\"%n", userId, version, fullText);
    }
}

// --- [DESIGN PATTERN: Command] Text editing command with undo ---
interface EditorCommand {
    void execute();
    void undo();
}

class InsertTextCommand implements EditorCommand {
    private final StringBuilder doc;
    private final int index;
    private final String text;

    public InsertTextCommand(StringBuilder doc, int index, String text) {
        this.doc = doc; this.index = index; this.text = text;
    }

    @Override
    public void execute() {
        doc.insert(index, text);
    }

    @Override
    public void undo() {
        doc.delete(index, index + text.length());
    }
}

class CollaborativeDocument {
    private final StringBuilder content = new StringBuilder();
    private int version = 0;
    private final Stack<EditorCommand> history = new Stack<>();
    private final List<DocumentObserver> observers = new ArrayList<>();

    public void addObserver(DocumentObserver o) { observers.add(o); }

    public synchronized void applyCommand(EditorCommand cmd) {
        cmd.execute();
        history.push(cmd);
        version++;
        for (DocumentObserver o : observers) o.onDocumentModified(version, content.toString());
    }

    public synchronized void undo() {
        if (!history.isEmpty()) {
            EditorCommand cmd = history.pop();
            cmd.undo();
            version++;
            for (DocumentObserver o : observers) o.onDocumentModified(version, content.toString());
        }
    }

    public StringBuilder getContent() { return content; }
}

public class Main {
    public static void main(String[] args) {
        CollaborativeDocument doc = new CollaborativeDocument();
        doc.addObserver(new CollaboratorView("Alice"));
        doc.addObserver(new CollaboratorView("Bob"));

        System.out.println("--- Collaborative Editing ---");
        doc.applyCommand(new InsertTextCommand(doc.getContent(), 0, "Hello "));
        doc.applyCommand(new InsertTextCommand(doc.getContent(), 6, "World!"));
        doc.undo();
    }
}