Design Pattern to be used in Google Calendar(LLD)

Designing Google Calendar involves many complex components and features. Given its wide range of functionality, various design patterns can be applied to different aspects of the system.

Below are some key design patterns that could be used in building a system like Google Calendar:

1. Factory Method Pattern

Use case: For creating different types of events, reminders, and schedules.

Explanation: When you need to create different kinds of objects (e.g., different types of events such as one-time events, recurring events, and reminders), you can use a factory method to abstract the object creation process. This keeps the object creation logic decoupled from the rest of the application.

Example:

• Different types of events (meeting, personal event, public event) could be created by respective factory methods.

• Similarly, reminders for events (email reminder, push notification) could also be created via factories.

public abstract class Event {
    abstract void createEvent();
}

public class MeetingEvent extends Event {
    @Override
    void createEvent() {
        // Create a meeting event
    }
}

public class PersonalEvent extends Event {
    @Override
    void createEvent() {
        // Create a personal event
    }
}

public class EventFactory {
    public static Event createEvent(String type) {
        if (type.equals("meeting")) {
            return new MeetingEvent();
        } else if (type.equals("personal")) {
            return new PersonalEvent();
        }
        return null;
    }
}

2. Observer Pattern

Use case: To notify users about changes such as event updates, reminders, and invitations.

Explanation: The Observer pattern is useful when you want to notify multiple users about changes, such as an event update or a reminder notification. In Google Calendar, this could be used for sending notifications to all participants of a calendar event when the event's time or details change.

Example:

• Observers can be the users who have subscribed to notifications about a particular event or calendar.

• When an event changes, all subscribed users are notified

public interface Observer {
    void update(String message);
}

public class User implements Observer {
    private String userName;

    public User(String userName) {
        this.userName = userName;
    }

    @Override
    public void update(String message) {
        System.out.println(userName + " received update: " + message);
    }
}

public class Event {
    private List<Observer> observers = new ArrayList<>();

    public void addObserver(Observer observer) {
        observers.add(observer);
    }

    public void removeObserver(Observer observer) {
        observers.remove(observer);
    }

    public void notifyObservers(String message) {
        for (Observer observer : observers) {
            observer.update(message);
        }
    }

    public void setEventDetails(String newDetails) {
        // Update event details
        notifyObservers("Event updated: " + newDetails);
    }
}

3. Command Pattern

Use case: For managing user actions such as creating, editing, or deleting events.

Explanation: The Command pattern is useful when you want to encapsulate all the information required to perform an action, such as creating or updating an event. This allows users to undo/redo actions, keep a history of commands, or queue actions.

Example:

• Every action a user performs (e.g., create event, delete event) can be encapsulated in a command object.

• A CommandInvoker can execute the actions and allow users to undo or redo them

public interface Command {
    void execute();
}

public class CreateEventCommand implements Command {
    private Event event;

    public CreateEventCommand(Event event) {
        this.event = event;
    }

    @Override
    public void execute() {
        // Logic to create event
        System.out.println("Event created: " + event.getTitle());
    }
}

public class CommandInvoker {
    private Command command;

    public CommandInvoker(Command command) {
        this.command = command;
    }

    public void invoke() {
        command.execute();
    }
}

4. Singleton Pattern

Use case: For managing a centralized calendar system (e.g., a central CalendarManager for the entire platform).

Explanation: The Singleton pattern ensures that a class has only one instance and provides a global point of access to it. In the context of Google Calendar, you may want a CalendarManager class to manage all events, reminders, and users, ensuring that only one instance of this manager exists throughout the system.

Example:

• A CalendarManager could handle all operations related to calendar events, ensuring consistency in how events are handled across the system.

public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {
        // Private constructor to prevent instantiation
    }

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

In the Singleton pattern, synchronization is often needed when the singleton instance is created in a multithreaded environment. However, if you use synchronization incorrectly, it can lead to performance issues, so let's walk through how to use synchronization efficiently with the Singleton pattern.

Problem with Unsynchronized Singleton

If multiple threads attempt to create an instance of a singleton at the same time, they might end up creating more than one instance. This breaks the singleton principle, where only one instance of the class should exist. This problem is typically encountered when the singleton class is lazy-loaded, i.e., the instance is created only when it's needed.

Solution: Thread-Safe Singleton with Synchronization

To make the Singleton thread-safe, you can synchronize the method that creates the instance. Here's how:

public class Singleton {
    private static Singleton instance;

    private Singleton() {
        // Private constructor to prevent instantiation
    }

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

In this version:

• The getInstance() method is synchronized, which means only one thread can execute it at a time.

• The synchronization ensures that only one thread will create the instance, and subsequent calls will simply return the already created instance.

Drawback of This Approach:

• Performance: Every time getInstance() is called, it will acquire a lock, which can result in performance bottlenecks, especially in high-performance or heavily-concurrent systems. This happens even when the instance has already been created, which is unnecessary.

Example 2: Double-Checked Locking Singleton

A better approach to mitigate the performance overhead is to use double-checked locking. In this approach, you only synchronize the code that actually creates the instance, and you check whether the instance is null before and after synchronization.

public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {
        // Private constructor to prevent instantiation
    }

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

Key Points:

1. Double-Checked Locking:

   • The first check (if (instance == null)) is done outside the synchronized block, ensuring that we avoid unnecessary synchronization when the instance is already created.

   • The second check (if (instance == null)) inside the synchronized block ensures that only one thread actually creates the instance when it’s still null.

2. volatile Keyword:

   • The volatile keyword is used for the instance variable to ensure that its value is correctly synchronized across all threads. Without it, the instance might not be properly initialized across different threads, leading to potential issues in a multithreaded environment.

Benefits of Double-Checked Locking:

• Performance: It avoids the performance cost of synchronization after the instance has been created.

• Thread-Safety: Ensures only one instance of the class is created, even in a multithreaded environment.

5. Strategy Pattern

Use case: For implementing different types of reminders or scheduling strategies.

Explanation: The Strategy pattern allows you to define a family of algorithms (e.g., different ways to schedule an event or remind a user) and make them interchangeable. For Google Calendar, you might have different strategies for scheduling events (e.g., recurring events like daily, weekly) or sending reminders (e.g., email, SMS, push notifications).

Example:

• Different reminder strategies can be defined for various notification types.

public interface ReminderStrategy {
    void sendReminder(User user, Event event);
}

public class EmailReminder implements ReminderStrategy {
    @Override
    public void sendReminder(User user, Event event) {
        System.out.println("Sending email reminder to " + user.getName());
    }
}

public class SmsReminder implements ReminderStrategy {
    @Override
    public void sendReminder(User user, Event event) {
        System.out.println("Sending SMS reminder to " + user.getName());
    }
}

public class ReminderContext {
    private ReminderStrategy strategy;

    public ReminderContext(ReminderStrategy strategy) {
        this.strategy = strategy;
    }

    public void executeReminder(User user, Event event) {
        strategy.sendReminder(user, event);
    }
}

6. Decorator Pattern

Use case: To extend the functionality of events or users, such as adding additional features like prioritization or color-coding.

Explanation: The Decorator pattern allows you to dynamically add responsibilities to an object. For example, you could use this to extend the behavior of events or users (e.g., adding priority levels to events, adding labels, or providing color-coding).

Example:

• You could decorate an event to add priorities or labels dynamically.

public interface Event {
    String getDetails();
}

public class BasicEvent implements Event {
    @Override
    public String getDetails() {
        return "Basic Event";
    }
}

public abstract class EventDecorator implements Event {
    protected Event decoratedEvent;

    public EventDecorator(Event event) {
        this.decoratedEvent = event;
    }

    public String getDetails() {
        return decoratedEvent.getDetails();
    }
}

public class PriorityEventDecorator extends EventDecorator {
    public PriorityEventDecorator(Event event) {
        super(event);
    }

    @Override
    public String getDetails() {
        return decoratedEvent.getDetails() + " with Priority";
    }
}

Github Link Google Calendar