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Ideas you will meet

Distributed Systems

Consensus, time, failure and the fundamentals of scale-out.

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 minVideo catalogue (Netflix-style)Titles, profiles, playback and recommendations: modelling content, per-profile state and a swappable recommendation strategy.Read hereClose

  The problem

  Subscribers have accounts with several profiles. They browse a catalogue of movies and series, play titles, resume where they stopped, and see recommendations. This lesson models the domain. The streaming infrastructure (transcoding, CDN) is in the video-streaming lesson of the classic designs course.

  Entities

  100%

  Figure SDA-LLD-16-1 · Video catalogue classes · © forcsguy.com

  A playable unit is a movie or an episode; the player only needs an asset id and a duration.

  Playback and resume

  100%

  Figure SDA-LLD-16-2 · Playback state · © forcsguy.com

  class Player {
    state: "STOPPED" | "PLAYING" | "PAUSED" | "FINISHED" = "STOPPED";
    constructor(private profile: Profile, private progress: ProgressStore) {}
  
    play(unit: Playable) {
      if (!canPlay(this.profile, unit)) throw new Error("not allowed");   // rating and plan rules, one place
      this.state = "PLAYING";
      this.seek(this.progress.get(this.profile.id, unit.id) ?? 0);
    }
    tick(unit: Playable, pos: number) {
      this.progress.save(this.profile.id, unit.id, pos);                  // every few seconds, not every frame
      if (pos >= unit.duration * 0.95) this.state = "FINISHED";
    }
  }
  

  Recommendations as a strategy

  interface Recommender { forProfile(p: Profile, n: number): Title[] }
  
  const popularInGenre: Recommender = {
    forProfile: (p, n) => catalog.byGenres(topGenresOf(p)).filter(t => !watched(p, t) && allowed(p, t)).sort(byPopularity).slice(0, n),
  };
  

  Start with a simple strategy (popular in the genres you watch, not yet seen, allowed for the profile). Replace it later with collaborative or learned models behind the same interface.

  Design notes

  • One rule for access: canPlay and allowed combine plan limits, region licensing, maturity rating and device limits, and are used everywhere.
  • Concurrent streams: the plan limits simultaneous streams; track active sessions and refuse extra ones with a clear message.
  • Progress writes are frequent and approximate; buffer and save periodically, and let the latest write win.
  • Catalogue changes (titles added or removed by region and date) are data, not code.
  • Extensions: downloads, parental controls, ratings, search, and "because you watched" rows.

  Real-world reading

  Related reading

  • ByteByteGo: System Design Video SeriesAlex Xu · ByteByteGoYouTube
  • Performance Optimization on Image, Video, Page LoadGoogle engineers · GoogleCase Studies
  • Netflix: What Happens When You Press Play?High Scalability engineers · High ScalabilityBlogs
  • SVE: Distributed Video Processing at Facebook Scale (paper)Huang et al. · Facebook ScalePapers

  Show 6 moreShow fewer

  • CS Video CoursesDeveloper-Y · GitHubGitHub
  • Deterministic Aperture: A Distributed, Load Balancing Algorithm at TwitterTwitter engineers · TwitterCase Studies
  • The Google File System (paper)Ghemawat, Gobioff and Leung · GooglePapers
  • Aroma: Using ML for Code Recommendation at FacebookFacebook engineers · FacebookCase Studies
  • Wealthfront Engineering BlogWealthfront engineers · WealthfrontBlogs
  • Xero BlogXero engineers · XeroBlogs

  Open this lesson with its quiz and exercise