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Hashrocket Engineering Blog
Articles by Hashrocket about Ruby on Rails, Elixir, React and mobile development.
By Hashrocket engineers · Hashrocket · Added 2026-10-12
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About 37 min left. Your work is saved as you go.
What you are about to study
Hashrocket wrote this to explain how they thought about a problem. You should know the basics: caching, queues and replication.
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 minHotel managementRooms, bookings and availability: modelling date ranges, preventing double booking and pricing by room type and season.Read here
The problem
Guests search for a room type and dates, book, check in, check out and pay. Staff manage rooms. The system must never double-book a room for overlapping dates.
Entities
100%Figure SDA-LLD-04-1 · Hotel classes · © forcsguy.com
Booking.statusmoves through reserved, checked in, checked out, cancelled.Availability is a range problem
A room is free for
[in, out)if no active booking overlaps it. Two ranges overlap whenstartA < endB && startB < endA.const overlaps = (a: Range, b: Range) => a.start < b.end && b.start < a.end; function freeRooms(rooms: Room[], bookings: Booking[], want: Range, type: string) { return rooms.filter((r) => r.type === type && !bookings.some((b) => b.room === r && b.status !== "CANCELLED" && overlaps(b, want))); }Booking safely
100%Figure SDA-LLD-04-2 · Booking flow · © forcsguy.com
The check and the insert must be one atomic step. In a database that means a transaction with a lock on the room row (or an exclusion constraint on the date range). In memory it means a lock per room.
Pricing
Keep price calculation behind
RatePlan: base price per room type, multiplied by season, weekday and demand, with member discounts. ABookingstores the price it was sold at, so later rate changes do not alter old bookings.Cancellation, check-in and check-out
- Cancel: apply a cancellation policy (strategy) to compute the refund from how close the stay is.
- Check-in: assign a specific room if the booking was for a type, mark checked in, create a folio for extra charges.
- Check-out: add extras, settle the payment, mark the room "needs cleaning" and release it for new bookings after housekeeping.
Extensions
Group bookings, room upgrades, overbooking with a controlled allowance, and housekeeping as a small state machine (dirty, cleaning, ready).
Real-world reading
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Go deeper on this
A path through videos, articles, other case studies and code on the same ideas as Hashrocket Engineering Blog.
Warm upGet the vocabulary first.
Books · Free from the author
Concurrent and Distributed Systems (Cambridge lecture notes)
Eight lectures on time, broadcast, replication, consensus and transactions, with notes, slides and video. Free from the author under CC BY-SA.
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Computer Networks: A Systems Approach
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WatchHow others solve the same problem, on video.
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ReadEngineers explain it in their own words.
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Scalable Web Architecture and Distributed Systems
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Read the codeSee how it looks when it is real.
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