Data Infrastructure at LinkedIn

Ideas you will meet

Machine Learning Systems

Recommendations, ranking, feature stores and serving models at scale.

What we already know about this

Our own lessons on these ideas. Read them here before you start, so it makes sense.

• Low-Level Design Practice · 10 minChat applicationUsers, conversations and messages: delivery states, group chats and notifying clients, modelled as classes rather than infrastructure.Read hereClose

  The problem

  Users send text messages one-to-one or in groups. The app shows who is online, message status (sent, delivered, read) and notifies offline users. This lesson is about the classes; the infrastructure view (WebSockets, storage at scale) is in the chat-system lesson of the classic designs course.

  Entities

  100%

  Figure SDA-LLD-14-1 · Chat classes · © forcsguy.com

  A conversation has a type (direct or group) and participants. A message belongs to one conversation and has one receipt per recipient.

  Sending a message

  100%

  Figure SDA-LLD-14-2 · Sending a message · © forcsguy.com

  type Status = "SENT" | "DELIVERED" | "READ";
  class Conversation {
    private subscribers = new Map<string, (m: Message) => void>();
    constructor(readonly id: string, readonly members: string[]) {}
  
    subscribe(userId: string, fn: (m: Message) => void) { this.subscribers.set(userId, fn); return () => this.subscribers.delete(userId); }
  
    send(from: string, text: string): Message {
      if (!this.members.includes(from)) throw new Error("not a member");
      const m = new Message(newId(), this.id, from, text);
      m.receipts = Object.fromEntries(this.members.filter(u => u !== from).map(u => [u, "SENT" as Status]));
      store.save(m);                                         // persist first
      for (const [u, fn] of this.subscribers) if (u !== from) fn(m);   // push to online users
      return m;                                              // offline users get it when they reconnect, or by push notification
    }
  }
  

  Offline users and ordering

  • Offline users receive messages on reconnect by asking for everything after the last message id they have, and may get a push notification.
  • Order within a conversation comes from a server-assigned, increasing message id.
  • Read receipts are updates to the per-recipient receipt, then aggregated for the sender (in a group, "read by 3 of 5").

  Design notes

  • Observer decouples the conversation from how clients are connected.
  • Group size matters: a small group can notify each member directly; very large channels need a different fan-out approach.
  • Presence (online, last seen) is a separate component with its own throttled updates.
  • Privacy: block lists, muted conversations, and "delete for everyone" are policies checked before delivery.
  • Extensions: media messages (store files elsewhere, send a reference), replies, reactions, end-to-end encryption.

  Real-world reading

  Related reading

  • RFC 6455: The WebSocket ProtocolIETFDocumentation
  • Scaling Real-time Infrastructure at Alibaba for Global Shopping Holiday - Xiaowei Jiang, Senior Director at AlibabaAlibaba engineers · AlibabaCase Studies
  • The Architecture Twitter Uses to Deal with 150M Active Users, 300K QPS, a 22 MB/S Firehose, and Send Tweets in Under 5 SecondsHigh Scalability engineers · High ScalabilityCase Studies
  • Scaling Mailbox - From 0 to One Million Users in 6 Weeks and 100 Million Messages Per Day - High Scalability -High Scalability engineers · High ScalabilityCase Studies

  Show 6 moreShow fewer

  • Scaling Live Videos to a Billion Users at Facebook - Sachin Kulkarni, Director of Engineering at FacebookFacebookYouTube
  • MDN: WebSockets APIMDN Contributors · MozillaDocumentation
  • The WhatsApp Architecture Facebook Bought for $19BHigh ScalabilityCase Studies
  • Infrastructure to Run User Forecasts at SpotifySpotify engineers · SpotifyCase Studies
  • Data Infrastructure at LinkedInLinkedIn engineers · LinkedInCase Studies
  • Real-Time Data Warehouse with MemSQL on Cisco UCSCisco engineers · CiscoCase Studies

  Open this lesson with its quiz and exercise