📝 System Design Interview Classics · Sequence
Chat App Message Delivery (WhatsApp-style)
How a one-to-one chat message is delivered in a WhatsApp-style system, with sent, delivered and read ticks, and push notifications when the receiver is offline.
Drawing diagram…
What this diagram shows
- Messages go over persistent WebSocket connections
- Offline users get a push notification and messages wait in storage
- Ticks come from delivery and read acknowledgements
Prompt used
Chat message sequence: Alice's app sends a message over its WebSocket to chat server 1, which stores it in Cassandra and acknowledges (one tick). The presence service says which server Bob is on. If Bob is online, chat server 2 delivers it and Bob's app acknowledges (two ticks). If Bob is offline, a push notification is sent and the message is delivered when he reconnects. When Bob opens the chat a read receipt goes back (blue ticks).
Mermaid code
sequenceDiagram
actor A as Alice
participant CS1 as Chat Server 1
participant DB as Message Store
participant PR as Presence Service
participant CS2 as Chat Server 2
participant PN as Push Service
actor B as Bob
A->>CS1: Send message over WebSocket
CS1->>DB: Store message
CS1-->>A: Sent - one tick
CS1->>PR: Where is Bob?
alt Bob online
PR-->>CS1: On Chat Server 2
CS1->>CS2: Forward message
CS2->>B: Deliver
B-->>CS2: Delivered ack
CS2-->>A: Delivered - two ticks
else Bob offline
PR-->>CS1: Offline
CS1->>PN: Send push notification
PN-->>B: New message alert
end
B->>CS2: Opened chat
CS2-->>A: Read - blue ticksRelated templates
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