┌──────────┐     ┌──────────────┐     ┌──────────────┐
│  Client  │────▶│   API        │────▶│   Message    │
│  (App)   │◀────│   Gateway    │◀────│   Service    │
└──────────┘     └──────┬───────┘     └──────┬───────┘
                        │                     │
                 ┌──────┴───────┐     ┌──────┴───────┐
                 │  WebSocket   │     │  Message     │
                 │  Manager     │     │  Queue       │
                 └──────┬───────┘     └──────┬───────┘
                        │                     │
                 ┌──────┴───────┐     ┌──────┴───────┐
                 │  Presence    │     │  Database    │
                 │  Service     │     │  (Cassandra) │
                 └──────────────┘     └──────────────┘

sql
-- Messages table (partitioned by conversation)
CREATE TABLE messages (
    conversation_id UUID,
    message_id TIMEUUID,
    sender_id UUID,
    content TEXT,
    content_type TEXT,  -- 'text', 'image', 'video'
    media_url TEXT,
    status TEXT,  -- 'sent', 'delivered', 'read'
    created_at TIMESTAMP,
    PRIMARY KEY (conversation_id, message_id)
) WITH CLUSTERING ORDER BY (message_id DESC);

Message Status Flow:
  ✓  SENT      → Stored in server database
  ✓✓ DELIVERED → Received by recipient's device
  ✓✓ READ      → Recipient opened the chat (blue ticks)

SET user:{userId}:online true EX 30  // Expires in 30 seconds
// Client sends heartbeat every 25 seconds to renew

100B messages / 86,400 seconds = ~1.15M messages/second

Per message: ~1KB average
Storage: 100B × 1KB = 100TB/day = 36.5PB/year

Solution: 
  - Cassandra cluster with 500+ nodes
  - Partition by conversation_id
  - TTL on messages older than 30 days (archive to cold storage)

Signal Protocol (used by WhatsApp):

1. Key Exchange (X3DH):
   - Each user generates identity key pair
   - Pre-keys shared via server
   
2. Message Encryption:
   - Double Ratchet Algorithm
   - New key for every message
   - Forward secrecy: compromised key can't decrypt past messages

3. Server sees:
   ❌ Message content
   ✅ Sender, recipient, timestamp
   ✅ Encrypted blob