Problem 4.14 — File Storage System (LLD)

The metadata/blob split: the blob is content-addressed, and versioning is just pointing metadata at a new blob key.

public interface BlobStore {                  // PORT — testable with an in-memory fake
  String put(byte[] bytes) throws IOException;   // returns a content-addressed key
  byte[] get(String key) throws IOException;
  void delete(String key) throws IOException;
}

public final class Sha256BlobStore implements BlobStore {
  private final Path root;                       // swap for an S3Adapter without touching callers
  public Sha256BlobStore(Path root) { this.root = root; }

  public String put(byte[] bytes) throws IOException {
    String key = sha256Hex(bytes);                // never trust a user-supplied filename as the key
    Path dest = root.resolve(key.substring(0, 2)).resolve(key);   // fan out into subdirectories
    if (!Files.exists(dest)) { Files.createDirectories(dest.getParent()); Files.write(dest, bytes); }
    return key;                                   // identical content → identical key → free dedupe
  }
  public byte[] get(String key) throws IOException { return Files.readAllBytes(pathFor(key)); }
  public void delete(String key) throws IOException { Files.deleteIfExists(pathFor(key)); }
  private Path pathFor(String key) { return root.resolve(key.substring(0, 2)).resolve(key); }
  private String sha256Hex(byte[] b) { /* MessageDigest.getInstance("SHA-256") */ return "…"; }
}

// Metadata: a small row that POINTS AT a blob key. This is what makes versioning free.
public record FileMeta(String id, String displayName, String ownerId,
                        String currentBlobKey, long size, Instant createdAt) {
  public FileMeta withNewVersion(String newBlobKey, long newSize) {
    return new FileMeta(id, displayName, ownerId, newBlobKey, newSize, Instant.now());
    // the OLD blob is untouched — soft "undo" is just re-pointing currentBlobKey at the old key
  }
}

public final class FileService {
  private final BlobStore blobs;
  private final FileMetaRepository metas;

  public FileMeta upload(String ownerId, String displayName, byte[] content) throws IOException {
    String key = blobs.put(content);
    FileMeta meta = new FileMeta(UUID.randomUUID().toString(), displayName, ownerId, key, content.length, Instant.now());
    return metas.save(meta);
  }
}