Requirements Clarification

Functional Requirements

1. One-to-one text messaging with delivery/read receipts
2. Group messaging (up to 1024 members)
3. Media sharing (images, videos, documents up to 2GB)
4. Online/offline presence indicators
5. Message history and synchronization across devices
6. End-to-end encryption (E2EE)

Non-Functional Requirements

1. Availability: 99.99% uptime
2. Latency: < 100ms for message delivery (P99)
3. Durability: Messages must not be lost once sent
4. Consistency: Causal ordering within conversations
5. Scale: 2B registered users, 500M daily active users

Back-of-the-Envelope Estimation

High-Level Architecture

Core Components Deep Dive

1. Connection Manager

Maintains persistent WebSocket connections with clients:

class ConnectionManager:
    def __init__(self):
        self.user_connections = {}  # user_id -> {device_id: server_id}
        self.server_connections = {}  # server_id -> {connection_id: user_id}
    
    def register(self, user_id, device_id, server_id):
        self.user_connections[user_id][device_id] = server_id
        self.server_connections[server_id][connection_id] = user_id
    
    def get_servers(self, user_id):
        return set(self.user_connections.get(user_id, {}).values())

2. Message Router

Routes messages between senders and receivers:

3. Message Flow (1:1 Chat)

4. End-to-End Encryption

5. Group Messaging

6. Presence System

Data Model

Scaling Strategies

Message Storage Partitioning

Messages are partitioned by chat_id using consistent hashing:

Push vs Pull for Delivery

Practice Exercises

1. Design: How would you implement message synchronization across multiple devices for the same user? Consider ordering and conflict resolution.

2. Scale: WhatsApp has 50M concurrent users. Estimate the number of WebSocket connections needed and the memory overhead per connection.

3. Reliability: Design a mechanism to ensure messages are never lost, even if the sender's device crashes after sending but before receiving acknowledgment.

4. Optimization: How would you reduce bandwidth usage for users on slow networks? Propose a compression strategy for text and media.

What to Learn Next