real-time presence status

systemdesign.one

• leverage heartbeat signal through server-send event (SSE) to check status of client
• heartbeat single is connectionless and lightweight for improving performance -> use UDP

high-level design

data storage

• store last active time of client
• distributed key-value storage supports high read/write operations (no SQL: ACID is overkill/noSQL: slower reads) -> use redis (with sets)
• redis can use keyspace notification (monitor redis changes in real time) to notify connected clients when presence status changes

pub/sub server

• servers broadcast presence status to clients through pub/sub
• redis’ message bus should be confirgured in ephemeral storage mode
• limitations
  • no guarantee at least one-time message delivery
  • degraded latency if consumers use pull-based model

deep dive design

live presence

when user is online

• subscribers establish SSE connection to real-time platform and subscribe to any change of connection status
• real-time platform sends heartbeat signal to presence service to detect status of client, presence service publishes event to notify when status changes

when user goes offline

• web browser/app triggers unload event to change presence status when closing app -> delayed trigger can be configured to detect absence of heartbeat signal and queries whether DB record of user has expired

handle jittery connections

• offline and timeout events can be treated as offline, longer time interval set for timeout to prevent fluctuations status -> handle jittery connection

pub/sub flow