Lesson 1: Design a Chat Application#

Goal: Design a real-time chat service like WhatsApp or Slack that supports 1-on-1 and Group messaging.

Requirements#

Functional#

• Send/Receive messages in real-time.
• See user status (Online/Offline).
• Message history (persistent storage).

Non-Functional#

• Low Latency: Messages must appear instantly.
• Consistency: Messages must be delivered in order.
• Availability: High uptime.

Core Design#

1. Communication Protocol#

HTTP is request/response (pull). For chat, we need push.

• WebSockets: Keeps a persistent connection open between client and server.

2. Message Flow#

• User A sends message to Chat Server.
• Chat Server finds which server User B is connected to (using a Session Store like Redis).
• Chat Server pushes message to User B.

3. Storage#

• Chat History: Write-heavy. Cassandra or HBase (Wide-column stores) are good for time-series data.
• User Status: Key-Value store (Redis) with TTL.

🛠️ Sruja Perspective: Modeling Real-Time Flows#

We can use Sruja to model the WebSocket connections and the async message processing.

architecture "Chat System" {
    requirement R1 functional "Real-time messaging"
    requirement R2 functional "Message history"
    requirement R3 latency "Instant delivery"
    requirement R4 consistency "Ordered delivery"

    system ChatApp "WhatsApp Clone" {
        container ChatServer "Chat Server" {
            technology "Node.js (Socket.io)"
            description "Handles WebSocket connections"
            scale {
                min 10
                max 100
                metric "connections > 10k"
            }
        }
        
        datastore SessionStore "Session Store" {
            technology "Redis"
            description "Maps UserID -> WebSocketServerID"
        }
        
        datastore MessageDB "Message History" {
            technology "Cassandra"
            description "Stores chat logs"
        }
        
        queue MessageQueue "Message Queue" {
            technology "Kafka"
            description "Buffers messages for group chat fan-out"
        }

        ChatServer -> SessionStore "Reads/Writes"
        ChatServer -> MessageDB "Persists messages"
        ChatServer -> MessageQueue "Async processing"
    }

    person UserA "Alice"
    person UserB "Bob"

    // Scenario of 1-on-1 chat
    scenario SendMessage "Send Message Flow" {
        UserA -> ChatServer "Send 'Hello'"
        ChatServer -> MessageDB "Persist message"
        ChatServer -> SessionStore "Lookup Bob's connection"
        SessionStore -> ChatServer "Bob is on Server-2"
        ChatServer -> UserB "Push 'Hello'"
    }
}