Google Drive / Dropbox

Problem statement (interviewer prompt)

Design Google Drive / Dropbox: multi-device file sync, sharing (per-file ACL + link sharing), versioning, full-text search, previews, and live collaborative editing. Cover content-defined chunking, block-level dedup, and conflict resolution.

flowchart LR
  C([Client])
  SYNC[Sync Service]
  CHUNK[Chunker]
  S3[(Block Store)]
  META[(Metadata DB)]
  NOTIFY[Notify / Long-poll]
  C --> SYNC
  SYNC --> CHUNK --> S3
  SYNC --> META
  C2[Client B] --> SYNC
  SYNC --> NOTIFY --> C2

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class C,C2 client;
    class SYNC,CHUNK,NOTIFY service;
    class S3,META datastore;

flowchart TB
  subgraph Clients
    DESK([Desktop client])
    MOB([Mobile])
    WEB([Web])
    OFF[Office plugins]
  end

  subgraph Edge
    LB
    GW[API Gateway]
  end

  subgraph Sync[Sync Engine]
    WATCH[Filesystem watcher / FSEvents / inotify]
    DIFF[Local change diff]
    CHUNK[Content-defined chunking<br/>Rabin / FastCDC, ~4 MB]
    HASH[SHA-256 per chunk]
    DEDUP([Block-level dedup<br/>against user + global])
    COMP[Compression]
    ENC([Client-side / At-rest encryption])
    UP[Resumable upload]
    DOWN[Range download]
  end

  subgraph Services
    META[Metadata Service]
    NSP([Namespace Service<br/>per user / shared drives])
    NOTIF[[Notification fan-out]]
    ACL[Permissions / Sharing]
    SEARCH([Search / Tika OCR + ML])
    PREVIEW[Preview / Thumbnail]
    OFFLINE[[Offline sync queue]]
    VER[Versioning / Snapshots]
    TRASH[Trash / Restore]
  end

  subgraph Storage
    BLOB[(Block store<br/>S3 / Colossus / Magic Pocket)]
    META_DB[(Metadata SQL/Spanner)]
    INDEX[(Search index)]
    AUDIT[(Audit log)]
  end

  subgraph Realtime
    LP[Long-poll / SSE / WS]
    PUSH((APNS / FCM))
  end

  subgraph Office[Docs integration]
    DOC[Docs / Sheets / Slides]
    LIVE[Live collab CRDT/OT]
  end

  subgraph Safety
    AV[Virus scan]
    DLP([DLP scanner])
    LEGAL[Legal hold]
  end

  Clients --> LB --> GW --> Sync
  Sync --> BLOB
  Sync --> META
  META --> META_DB
  META --> NOTIF
  NOTIF --> LP --> Clients
  NOTIF --> PUSH
  ACL --- META
  SEARCH --- INDEX
  PREVIEW --- BLOB
  VER --- META
  TRASH --- META
  Office --- META
  Office --- LIVE
  Safety --- BLOB

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class DESK,MOB,WEB,DEDUP,ENC,NSP client;
    class GW edge;
    class OFF,WATCH,DIFF,CHUNK,HASH,COMP,UP,DOWN,META,ACL,PREVIEW,VER,LP,DOC,LIVE,AV,LEGAL service;
    class TRASH,META_DB,INDEX,AUDIT datastore;
    class NOTIF,OFFLINE queue;
    class SEARCH,DLP compute;
    class BLOB storage;
    class PUSH external;

Block-level sync

1. Watcher detects change.
2. Chunker (FastCDC) splits file into ~4 MB content-defined chunks.
3. SHA-256 per chunk → lookup in dedup table.
4. Only NEW chunks are uploaded.
5. Metadata service updates file → [chunk hashes] mapping.
6. Notifier wakes other clients on same file/namespace.

Why content-defined chunking

• Edits in the middle of a file shift bytes; fixed-size chunking re-uploads everything after.
• Rolling-hash boundaries align to content, so only changed regions create new chunks.

Dedup scopes

• Per-user dedup: safe by default.
• Cross-user (global) dedup: large savings but enables side-channel attacks (proof of ownership needed; Dropbox abandoned cross-user dedup years ago).

Sharing & permissions

• Permission models: per-file ACL, link-based (capability tokens), domain-restricted.
• Permission inheritance and explicit overrides.

Versioning

• Keep historical chunk lists per file version.
• Versions never delete chunks until all refs gone (refcount or GC).

Real-time notification

• Long-poll or WebSocket per client; namespace = (user_id, root_dir).
• Mobile fallback to push.

Capacity

• Dropbox: ~700 M users, EB scale storage (Magic Pocket).
• Drive: tightly integrated with Google's Colossus.

Glossary & fundamentals

Concepts referenced in this design. Each row links to its canonical page; the tag column shows whether it is a high-level (HLD) or low-level (LLD) concept.

Tag	Concept	What it is	Page
HLD	API gateway / BFF	single ingress, auth, rate limit, routing	api-gateway
HLD	CRDTs	commutative replicated data types	crdts
HLD	Realtime protocols	WS / SSE / polling / gRPC streaming	realtime-protocols
HLD	Search internals	inverted index, BM25, embeddings, ANN	search-internals
LLD	REST API design	verbs, statuses, pagination, errors	rest-api-design
LLD	Async models	futures / async-await / coroutines / actors	async-models
LLD	OOP pillars	encapsulation, abstraction, inheritance, polymorphism	oop-pillars

Quick reference

Functional

• Multi-device file sync (incremental).
• Sharing (per-file ACL, link sharing).
• Versioning + restore.
• Preview, search.
• Office integration / live collab.

Non-functional

• Eventual consistency tolerated (seconds).
• Durability: 11×9s (S3-class).
• Conflict resolution: rename-on-conflict by default.

Capacity

• Avg user: 10 GB stored, hundreds of files.
• Dedup ratio: 25-50% within user, lower if encrypted client-side.
• 1B users × 10 GB = 10 EB; with dedup → ~5 EB physical.

Schema

• files(id, owner, parent_id, name, mime, version, size, chunks[])
• chunks(hash PK, refcount, blob_url) - global or per-user table
• acl(file_id, principal_id, role)
• events(user_id, ts, type, file_id) for change feed

Trade-offs

• Block dedup is the killer feature; CPU cost on client.
• Long-poll vs WebSocket vs push: combine; reliability over elegance.
• Conflict resolution: rename-on-conflict is simple; OT/CRDT for live collab on docs.
• Server-side encryption vs client-side: client-side breaks search and preview.
• Magic Pocket / Colossus vs S3: own infra at scale saves big money.

Refs

• Dropbox engineering blog: Magic Pocket, "Why we built our own…", FSEvents chains.
• Google Drive APIs + Docs collab papers.
• Casado: "Towards Internet-scale conflict-free replicated data types".
• ByteByteGo "Design Google Drive", Alex Xu Vol 2.

FAQ

How does Google Drive sync files across devices in near real time?

Clients hold a long poll to a notification service that pushes deltas the moment metadata changes. The client then pulls only the changed chunks from the block store.

How does Google Drive handle large file uploads?

Uploads are split into chunks via content defined chunking. Each chunk is uploaded once, hashed, and resumable, so a flaky network just re-tries the missing chunks instead of the whole file.

How does Google Drive implement per file permissions?

Every file has an ACL with explicit grants plus link sharing roles. Permission checks are cached aggressively because they are read on nearly every request.

How does Google Drive resolve conflicts when two users edit offline?

For binary files, Drive keeps both versions, marking one as a conflict copy. For Docs, Sheets, and Slides, real time collaborative editing uses operational transforms to merge concurrent edits.

How does Google Drive deduplicate storage?

Block level deduplication stores each unique content hash once. If many users share the same attachment or template, only one physical copy exists in the block store.

Further reading

Curated, high-credibility sources for going deeper on this topic.

• ✍️ Blog - Dropbox - Inside the Magic Pocket
• ✍️ Blog - Dropbox - Streaming File Synchronization (atomic delta)
• 📑 Docs - Google Drive API documentation

Video walkthrough

Design Dropbox/Google Drive : via Hello Interview