Design (LLD) AWS S3 Service - Machine Coding

Features Required:

1. User Authentication:

   • Implement a simple user authentication system where users can register and log in.

   • Each user has a unique identifier and credentials (username and password).

   • Users can only access their own buckets and objects unless granted permission.

2. Bucket Management:

   • Create Bucket: Users can create new buckets to store their objects.

   • List Buckets: Users can list all buckets they own.

   • Delete Bucket: Users can delete a bucket if it's empty.

3. Object Management:

   • Upload Object: Users can upload objects (files) to a bucket.

   • Download Object: Users can download objects from a bucket.

   • List Objects: Users can list all objects within a bucket.

   • Delete Object: Users can delete objects from a bucket.

   • Versioning Support: Maintain different versions of an object.

4. Access Control:

   • Implement permissions to control access to buckets and objects.

   • Permissions include read and write access.

   • Users can grant or revoke permissions to other users.

5. Notifications:

   • Notify users when certain events occur (e.g., object uploaded or deleted).

   • Users can subscribe or unsubscribe to notifications for specific events.

6. Versioning:

   • Keep track of object versions.

   • Users can retrieve previous versions of an object.

Design Patterns Involved:

1. Singleton Pattern:

   • Used For: Ensuring only one instance of the StorageService exists.

   • Reason: Centralizes the management of buckets and objects.

2. Factory Pattern:

   • Used For: Creating instances of Bucket and S3Object.

   • Reason: Encapsulates object creation logic and promotes loose coupling.

3. Strategy Pattern:

   • Used For: Defining different storage strategies (e.g., in-memory storage).

   • Reason: Allows switching between different storage mechanisms without changing the code.

4. Decorator Pattern:

   • Used For: Adding additional responsibilities like versioning and access control to objects.

   • Reason: Enhances objects dynamically without modifying their structure.

5. Observer Pattern:

   • Used For: Implementing the notification feature.

   • Reason: Allows objects to be notified of events without tight coupling.

6. Model-View-Controller (MVC) Pattern:

   • Used For: Separating the application's concerns.

   • Reason: Enhances scalability and maintainability.

Algorithms Involved:

1. Hashing:

   • Used for generating unique identifiers for users, buckets, and objects.

2. Search Algorithms:

   • Used for efficiently searching and listing buckets and objects.

3. Version Control Algorithms:

   • Used in the VersionedObjectDecorator to manage different versions of an object.

Diagram

Code (Java)

1. Singleton Pattern: StorageService

// StorageService.java
public class StorageService {
    private static StorageService instance;
    private StorageStrategy storageStrategy;
    private NotificationService notificationService;

    private StorageService(StorageStrategy storageStrategy) {
        this.storageStrategy = storageStrategy;
        this.notificationService = new NotificationService("upload", "delete");
    }

    public static synchronized StorageService getInstance(StorageStrategy storageStrategy) {
        if (instance == null) {
            instance = new StorageService(storageStrategy);
        }
        return instance;
    }

    // Subscribe and unsubscribe methods for notifications
    public void subscribe(String eventType, EventListener listener) {
        notificationService.subscribe(eventType, listener);
    }

    public void unsubscribe(String eventType, EventListener listener) {
        notificationService.unsubscribe(eventType, listener);
    }

    // Bucket and object management methods...
    // Other Methods are Present in our paid course...
}

2. Factory Pattern: BucketFactory and ObjectFactory

// BucketFactory.java
public class BucketFactory {
    public static Bucket createBucket(String name, User owner) {
        return new Bucket(name, owner);
    }
}

// ObjectFactory.java
public class ObjectFactory {
    public static S3Object createObject(String key, byte[] data) {
        return new S3Object(key, data);
    }
}

3. Strategy Pattern: StorageStrategy and InMemoryStorageStrategy

// StorageStrategy.java
public interface StorageStrategy {
    void saveBucket(Bucket bucket);
    Bucket getBucket(String bucketName);
    void deleteBucket(String bucketName);
    void saveObject(String bucketName, IS3Object object);
    IS3Object getObject(String bucketName, String objectKey);
    void deleteObject(String bucketName, String objectKey);
    List<Bucket> listBuckets(User owner);
    List<IS3Object> listObjects(String bucketName);
}

// InMemoryStorageStrategy.java
public class InMemoryStorageStrategy implements StorageStrategy {
    private Map<String, Bucket> buckets = new HashMap<>();

    // Implement all methods defined in StorageStrategy
    // Other Methods are Present in our paid course...
}

4. Decorator Pattern: S3ObjectDecorator, VersionedObjectDecorator, and AccessControlledObjectDecorator

// IS3Object.java
public interface IS3Object {
    String getKey();
    byte[] getData();
}

// S3Object.java
public class S3Object implements IS3Object {
    private String key;
    private byte[] data;

    // Constructor, getters, and setters
}

// S3ObjectDecorator.java
public abstract class S3ObjectDecorator implements IS3Object {
    protected IS3Object s3Object;

    public S3ObjectDecorator(IS3Object s3Object) {
        this.s3Object = s3Object;
    }

    // Implement IS3Object methods
}

// VersionedObjectDecorator.java
public class VersionedObjectDecorator extends S3ObjectDecorator {
    private Map<Integer, byte[]> versions = new HashMap<>();
    private int currentVersion = 0;

    public VersionedObjectDecorator(IS3Object s3Object) {
        super(s3Object);
        saveVersion();
    }

    public void saveVersion() {
        currentVersion++;
        versions.put(currentVersion, s3Object.getData());
    }

    public byte[] getVersionData(int version) {
        return versions.get(version);
    }

    public int getCurrentVersion() {
        return currentVersion;
    }

    @Override
    public byte[] getData() {
        return versions.get(currentVersion);
    }
}

// AccessControlledObjectDecorator.java
public class AccessControlledObjectDecorator extends S3ObjectDecorator {
    private Set<String> readPermissions = new HashSet<>();
    private Set<String> writePermissions = new HashSet<>();

    public AccessControlledObjectDecorator(IS3Object s3Object) {
        super(s3Object);
    }

    // Methods to grant and revoke permissions
    // Other Methods are Present in our paid course...
}

5. Observer Pattern: EventListener and NotificationService

// EventListener.java
public interface EventListener {
    void update(String eventType, String message);
}

// NotificationService.java
public class NotificationService {
    private Map<String, List<EventListener>> listeners = new HashMap<>();

    public NotificationService(String... eventTypes) {
        for (String eventType : eventTypes) {
            listeners.put(eventType, new ArrayList<>());
        }
    }

    // Methods to subscribe, unsubscribe, and notify
    // Other Methods are Present in our paid course...
}

6. MVC Pattern: Controllers and Models

// User.java
public class User implements EventListener {
    private String userId;
    private String name;
    private String password;

    // Constructor, getters, setters, and update method
}

// Bucket.java
public class Bucket {
    private String name;
    private User owner;
    private Map<String, IS3Object> objects = new HashMap<>();

    // Constructor, methods to manage objects
}

// UserController.java
public class UserController {
    private Map<String, User> users = new HashMap<>();

    public User createUser(String userId, String name, String password) {
        User user = new User(userId, name, password);
        users.put(userId, user);
        return user;
    }

    public User login(String userId, String password) {
        // Authentication logic
        // Other Methods are Present in our paid course...
    }
}

// BucketController.java
public class BucketController {
    private StorageService storageService;

    public BucketController(StorageService storageService) {
        this.storageService = storageService;
    }

    // Methods to create, delete, and list buckets
}

// ObjectController.java
public class ObjectController {
    private StorageService storageService;

    public ObjectController(StorageService storageService) {
        this.storageService = storageService;
    }

    // Methods to upload, download, delete, and list objects
    // Other Methods are Present in our paid course...
}

7. Main Class

// Main.java
public class Main {
    public static void main(String[] args) {
        // Initialize storage service with in-memory strategy
        StorageStrategy storageStrategy = new InMemoryStorageStrategy();
        StorageService storageService = StorageService.getInstance(storageStrategy);

        // Initialize controllers
        UserController userController = new UserController();
        BucketController bucketController = new BucketController(storageService);
        ObjectController objectController = new ObjectController(storageService);

        // User registration and login
        User user = userController.createUser("user1", "Alice", "password1");
        User loggedInUser = userController.login("user1", "password1");

        // Subscribe to notifications
        storageService.subscribe("upload", loggedInUser);
        storageService.subscribe("delete", loggedInUser);

        // Bucket operations
        bucketController.createBucket("my-bucket");

        // Object operations
        byte[] data = "Hello, S3!".getBytes();
        objectController.uploadObject("my-bucket", "my-object", data);

        // List objects
        List<IS3Object> objects = objectController.listObjects("my-bucket");
        for (IS3Object obj : objects) {
            System.out.println("Object Key: " + obj.getKey());
        }

        // Download object
        IS3Object downloadedObject = objectController.downloadObject("my-bucket", "my-object");
        System.out.println("Downloaded Data: " + new String(downloadedObject.getData()));

        // Delete object and bucket
        objectController.deleteObject("my-bucket", "my-object");
        bucketController.deleteBucket("my-bucket");
    }
}

Explanation of the Code:

• Singleton Pattern (StorageService): Ensures a single instance manages all storage operations, providing a global point of access.

• Factory Pattern (BucketFactory, ObjectFactory): Encapsulates the creation of Bucket and S3Object instances, making the code more modular and easier to maintain.

• Strategy Pattern (StorageStrategy): Allows for different storage implementations (e.g., in-memory, file system). Currently, InMemoryStorageStrategy is implemented for simplicity.

• Decorator Pattern: Enhances S3Object with additional features:

  • VersionedObjectDecorator: Adds versioning capability.

  • AccessControlledObjectDecorator: Adds access control features.

• Observer Pattern (NotificationService, EventListener): Implements the notification system, allowing users to receive updates on events like uploads and deletions.

• MVC Pattern: Separates the application into:

  • Models: Represent the data structures (User, Bucket, S3Object).

  • Views: Not explicitly implemented but could be added for a UI.

  • Controllers: Handle the logic and interact with the models (UserController, BucketController, ObjectController).

Note: This implementation is single-threaded and focuses on the core features and design patterns requested. In a production environment, considerations for concurrency, security, data persistence, and error handling would be necessary.

Complete Code Present in Premium Course

Issues in the Above Design and Multi-threading Considerations

1. Thread Safety and Concurrency Issues

Problem:

• Shared Data Structures without Synchronization:

  • The use of non-thread-safe collections like HashMap, ArrayList, and HashSet without synchronization mechanisms can lead to race conditions.

  • Multiple threads accessing and modifying these collections concurrently may corrupt the data or cause unpredictable behavior.

• Singleton Pattern Limitations:

  • While the getInstance method of StorageService is synchronized to ensure a single instance, the instance methods themselves are not thread-safe.

  • Concurrent access to shared resources within StorageService can cause conflicts.

• Lack of Atomicity in Operations:

  • Operations such as checking if a bucket is empty before deleting it are not atomic. Between the check and the delete, another thread could modify the bucket's contents.

Solution:

Covered in Premium Course

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2. Data Consistency and Integrity

Problem:

• Inconsistent State:

  • Without proper synchronization, data structures can become inconsistent, leading to incorrect application behavior.

• Versioning Conflicts:

  • The versioning system in VersionedObjectDecorator uses a simple integer counter without atomic operations, which can cause duplicate or skipped version numbers in concurrent environments.

Solution:

Covered in Premium Course

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3. Lack of Persistence and Scalability

Problem:

• In-Memory Storage Limitations:

  • Data is lost when the application stops or restarts.

  • Not suitable for large-scale or distributed systems.

• Scalability Constraints:

  • Single instance of StorageService can become a bottleneck.

  • The design does not support distributed environments or load balancing.

Solution:

Covered in Premium Course

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4. Security Concerns

Problem:

• Password Management:

  • Passwords are stored in plain text, posing a security risk.

• Access Control Weaknesses:

  • The access control system is rudimentary and does not enforce permissions consistently.

  • Lack of authentication tokens or session management.

Solution:

Covered in Premium Course

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5. Error Handling and Logging

Problem:

• Insufficient Error Handling:

  • The code uses System.out.println for error messages, which is not suitable for error management.

• Lack of Logging Framework:

  • No structured logging makes it difficult to trace issues.

Solution:

Covered in Premium Course

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6. Inefficient Use of Design Patterns

Problem:

• Overcomplicating the Design:

  • Some design patterns might be unnecessary or could be simplified.

  • For example, the use of both Factory and Decorator patterns for simple object creation may add unnecessary complexity.

Solution:

Covered in Premium Course

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7. Notification System Limitations

Problem:

• Thread Safety:

  • The NotificationService is not thread-safe, leading to potential issues with event delivery.

• Potential Memory Leaks:

  • Subscribers are not weak references; if they are not unsubscribed, they may prevent garbage collection.

Solution:

Covered in Premium Course

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8. User Experience and Feedback

Problem:

• Poor Feedback Mechanisms:

  • The application relies on console output, which is not user-friendly.

Solution:

Covered in Premium Course

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Will the Above Code Work in a Multi-threaded System?

No, the current code will not work correctly in a multi-threaded system without modifications to address thread safety and concurrency issues.

Detailed Explanation:

The code lacks synchronization mechanisms required for safe execution in a multi-threaded environment. Without addressing the concurrency issues, running this code with multiple threads will likely result in:

• Race Conditions:

  • Multiple threads may read and write shared data simultaneously, leading to inconsistent or corrupted data.

• Data Corruption:

  • Shared collections like HashMap are not thread-safe, so concurrent modifications can corrupt the internal state.

• Unpredictable Behavior:

  • Operations may fail or produce incorrect results due to interleaved thread execution.

Recommendations for Multi-threaded Operation

To make the code suitable for multi-threaded environments, the following steps should be taken:

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