HLD: Design Instagram - Photo Sharing at Scale

public class PhotoUploadService {
    public UploadResponse upload(MultipartFile file, String userId) {
        String photoId = generateId();
        
        String s3Key = userId + "/" + photoId;
        s3Client.putObject(BUCKET, s3Key, file.getInputStream());
        
        List<String> thumbnails = imageProcessor.generateThumbnails(
            file, Arrays.asList(150, 320, 640, 1080)
        );
        
        Post post = new Post(photoId, userId, s3Key, thumbnails);
        postRepository.save(post);
        
        fanoutService.publishToFollowers(userId, post);
        
        return new UploadResponse(post.getId(), getCdnUrl(s3Key));
    }
}

public class FeedService {
    private static final int CELEBRITY_THRESHOLD = 10000;
    
    public List<Post> getFeed(String userId, int limit) {
        List<Post> feed = new ArrayList<>();
        
        feed.addAll(feedCache.get(userId));
        
        List<String> celebrities = followService.getCelebritiesFollowed(userId);
        for (String celeb : celebrities) {
            feed.addAll(postService.getRecentPosts(celeb, 10));
        }
        
        feed.sort(Comparator.comparing(Post::getCreatedAt).reversed());
        
        return feed.stream().limit(limit).collect(Collectors.toList());
    }
    
    public void fanout(String userId, Post post) {
        int followerCount = followService.getFollowerCount(userId);
        
        if (followerCount < CELEBRITY_THRESHOLD) {
            List<String> followers = followService.getFollowers(userId);
            for (String followerId : followers) {
                feedCache.prepend(followerId, post);
            }
        }
    }
}

CREATE TABLE users (
    user_id BIGINT PRIMARY KEY,
    username VARCHAR(50) UNIQUE,
    email VARCHAR(100),
    profile_pic_url TEXT,
    bio TEXT,
    created_at TIMESTAMP
);

CREATE TABLE posts (
    post_id UUID,
    user_id BIGINT,
    media_url TEXT,
    caption TEXT,
    location TEXT,
    created_at TIMESTAMP,
    PRIMARY KEY (user_id, created_at)
) WITH CLUSTERING ORDER BY (created_at DESC);

CREATE TABLE follows (
    follower_id BIGINT,
    followee_id BIGINT,
    created_at TIMESTAMP,
    PRIMARY KEY (follower_id, followee_id)
);