System Design - Chat System

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Design Chat System

Requirement clarification

functional requirement

  • one-to-one chatting message(sent/delivered/read)
  • user(online/offline/last seen time)
  • picture, video sharing in the message

non-functional requirement

extend requirement

  • group chat
  • push notification for offline user
  • end-to-end encryption(Whatsapp)

Scale of the system - data size

  • 500 million DAU(daily active users)
  • each user sends 40 messages per day
  • message count per day = 500M * 40 = 20 billion
  • each message take 100bytes

Traffic estimates

Storage estimates

  • per day = 20 billion * 100 bytes = 2 TB/day
  • storage = 2 TB * 265 days * 5 years = 3.6 PB

Bandwidth estimates

  • bandwidth = 2 TB / (24 hours * 3600 sec) = 25 MB/s

Memory cache estimates

API interface

  • sendMessage(User from, User to, String message, Boolean userActivity)
  • isUserOnline(String userID)

Database data-model

  • User

    • String userID
    • TimeStamp time

  • Message

    • String messageID
    • String content

High-level design

Chat System

Chat System

Detail design

  • use websocket to timely nofify

    • heartbeat to keep alive

  • authentication

  • group table

  • MQ for keep message order(userID hash)

Bottlenecks(follow up)

  • loadbalance
  • consistent hashing to solve single point failure; vertual host to solve hash slide
  • idempotence, every message has one ID
  • groupID hash or userID hash for database sharding
  • use UDP to send when big event; client pull message
  • multi-layer message block, to avoid deserialize/serialize all package