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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 · 11 minMovie ticket bookingSeats, shows and temporary holds: how to let many people pick seats at once without selling the same seat twice.Read hereClose

  The problem

  A theatre chain runs screens that show movies at set times. Customers browse shows, pick seats, pay and receive a ticket. Many customers pick seats for the same popular show at once. A seat must never be sold twice.

  Entities

  100%

  Figure SDA-LLD-05-1 · Movie booking classes · © forcsguy.com

  The key idea: ShowSeat is a seat in one show. A physical seat exists for all shows; it is available or taken per show. Status values: AVAILABLE, HELD, BOOKED.

  The seat lifecycle

  100%

  Figure SDA-LLD-05-2 · Show seat status · © forcsguy.com

  class ShowSeat {
    status: "AVAILABLE" | "HELD" | "BOOKED" = "AVAILABLE";
    heldBy?: string; heldUntil = 0;
  
    // must be atomic: a lock per seat in memory, a conditional UPDATE in a database
    tryHold(user: string, ms = 10 * 60_000): boolean {
      if (this.status === "HELD" && Date.now() > this.heldUntil) this.status = "AVAILABLE";
      if (this.status !== "AVAILABLE") return false;
      this.status = "HELD"; this.heldBy = user; this.heldUntil = Date.now() + ms;
      return true;
    }
    confirm(user: string): boolean {
      if (this.status !== "HELD" || this.heldBy !== user) return false;
      this.status = "BOOKED"; return true;
    }
  }
  

  In a database this is one statement: UPDATE show_seat SET status='HELD', held_by=?, held_until=? WHERE id=? AND (status='AVAILABLE' OR held_until < now()) and the affected-row count tells you whether you won.

  Booking several seats

  100%

  Figure SDA-LLD-05-3 · Booking flow · © forcsguy.com

  If any seat in the group cannot be held, release the ones you did hold and tell the customer. This avoids a half-booked group.

  Design notes

  • Pricing by seat category, time and day is a strategy or a price table on ShowSeat.
  • Expiry can be a background job or a check at read time (as above). A job keeps the displayed seat map accurate.
  • Hot shows: at huge scale put the seat state in a fast store with atomic operations, and queue requests; this overlaps with the flash-sale design in the real-world systems course.
  • Idempotent payment: a retried payment callback must not issue a second ticket.
  • Extensions: seat recommendations, food add-ons, cancellations with refund policy, and notifications on confirm.

  Real-world reading

  Related reading

  • Rewriting the heart of our sync engineDropboxCase Studies
  • Megastore: Providing Scalable, Highly Available Storage (paper)Baker et al. · GooglePapers
  • Scaling Dropbox - Kevin Modzelewski, Back-end Engineer at DropboxDropboxYouTube
  • Hussein Nasser: Backend Engineering ShowHussein Nasser · Hussein Nasser (YouTube)YouTube

  Show 6 moreShow fewer

  • Uber Engineering — Where the Digital World Meets the Physical OneUber engineers · UberBlogs
  • Handles Millions of Digital Transactions Safely Everyday at PaytmPaytm engineers · PaytmCase Studies
  • Distributed systems theory for the distributed systems engineerHenry Robinson · The Paper TrailBlogs
  • The Pragmatic EngineerGergely Orosz · SubstackPodcasts & Newsletters
  • Soundcloud Engineering BlogSoundcloud engineers · SoundcloudBlogs
  • Pluralsight Engineering BlogPluralsight engineers · PluralsightBlogs

  Open this lesson with its quiz and exercise