WhatsApp System Design (High-Level)

1️⃣ What Problem Does WhatsApp Solve?

WhatsApp enables:

• Real-time messaging

• 1:1 and group chats

• Low latency delivery

• Offline message sync

• Strong privacy (End-to-End Encryption)

The system must work reliably for billions of users, across:

• Mobile networks

• Intermittent connectivity

• Different devices and regions

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2️⃣ Core Requirements

Functional

• Send and receive messages

• Support 1:1 and group chats

• Message ordering

• Delivery & read receipts

• Presence (online/offline) and typing indicators

• Media sharing (images, videos, documents)

Non-Functional

• Very low latency

• High availability

• Massive scalability

• Message durability

• Privacy and security

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3️⃣ High-Level Architecture

Mobile Client
    |
    v
WebSocket Gateway
    |
    v
Message Service
    |
    +--> Message Store (durable)
    |
    +--> Fan-out to Online Users
    |
    +--> Push Notifications (offline users)

Supporting systems:

• Redis → presence & typing

• Object Storage (S3/GCS) → media

• APNs / FCM → mobile push notifications

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4️⃣ Message Flow (Send → Receive)

Step 1: Send

• User sends a message

• App sends message to server via WebSocket

• Server stores message first (durability)

Step 2: Deliver

• If recipient is online → push instantly

• If recipient is offline → send push notification

Step 3: Sync

• When offline user opens app:

  • Client fetches missing messages

  • Uses last seen message id or sequence

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5️⃣ Message Storage & Ordering

How messages are stored

• Messages are stored once per chat (conversation)

• Each message gets a sequence number

• Sequence numbers guarantee ordered delivery

Conversation A:
  seq 1 → "Hi"
  seq 2 → "Hello"
  seq 3 → "How are you?"

Why not global ordering?

• Global ordering does not scale

• Ordering is required only inside a chat

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6️⃣ Group Chats (Important)

How group messages work

• Message stored once in group chat

• Server sends to:

  • All online members immediately

  • Offline members via pull on reconnect

Messages are not synced continuously to all phones.

This avoids:

• Battery drain

• Network waste

• Write amplification

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7️⃣ Fan-out Explained (Simply)

Fan-out = deliver one message to many users

In WhatsApp:

• Small groups → push to all online members

• Large groups → push online, pull for offline

WhatsApp uses a hybrid fan-out model.

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8️⃣ Offline Messaging

• Messages always stored on server

• Offline users receive:

  • Push notification (wake-up)

• On reconnect:

  • Client fetches messages after last seen id

GET /history?after=last_message_id

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9️⃣ Presence & Typing Indicators

These are ephemeral states:

• Stored in Redis

• Have short TTL (seconds)

Examples:

• Online / Offline

• “Typing…”

They are not persisted in message storage.

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🔟 Media (Images, Videos, Files)

Media is handled separately from messages:

1. Client uploads media to object storage using a pre-signed URL

2. Message contains only media reference

3. Media is downloaded directly from storage/CDN

This keeps messaging fast and scalable.

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1️⃣1️⃣ End-to-End Encryption (E2E)

WhatsApp uses E2E encryption:

• Messages encrypted on sender’s device

• Decrypted only on recipient’s device

• Server stores encrypted content only

Trade-offs:

• Server cannot read messages

• Searching messages is harder

• Abuse detection is limited

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1️⃣2️⃣ Reliability & Safety

• Messages are acknowledged after persistence

• Retries handled by client using message IDs

• Duplicate messages avoided using idempotency

• Abuse handled via metadata & rate limiting

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1️⃣3️⃣ Why This Design Scales

• WebSockets for real-time delivery

• Store once, deliver many

• Pull-based offline sync

• Separation of:

  • messages

  • presence

  • media

• Eventual consistency is acceptable

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1️⃣4️⃣ Interview Talking Points (Key)

• Store message before delivery

• Per-chat ordering, not global

• Push for online, pull for offline

• Fan-out trade-offs in group chats

• Redis for ephemeral state

• Object storage for media

• E2E encryption trade-offs

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1️⃣5️⃣ One-Line Summary (Interview Gold)

WhatsApp stores each message once, pushes to online users in real time, and lets offline users fetch messages when they reconnect, all while preserving per-chat ordering and strong privacy.