HotShard
For system design interviews

57 system design simulations,
running in your browser.

Each one is a working model of a real system — a database index, a cache, a group of servers agreeing on one value. You press Next, it takes one step, and the screen shows what changed.

Open the simulationsSee plans

Everything is free until 1 September 2026. No sign-up needed to run one.

57 simulations6 subject areasFree until 1 September
the LSM tree simulation, paused while it writes memory to disk
The LSM tree simulation, paused mid-flush
01 · How it works

You move it forward, one step at a time.

Nothing plays on its own. You stop on any step and look at it for as long as you want.

  • Press Next. One step happens.
  • The screen shows what the system holds right now — the keys in memory, the bytes on disk, the messages between machines.
  • A short line says what just happened, in plain words.
  • Go back, change the settings, run it again.
Open the simulations
The Raft simulation, paused after a leader is elected and an entry is committed
02 · What’s covered

57 simulations, in 6 subject areas.

Start anywhere. If you want an order to follow, the learning path puts them in one.

Networking basics

8

Client ↔ Server · DNS Resolution · TCP vs UDP · HTTP & APIs · and 4 more

Storage engines

11

Skip List · B+ Tree · Bitcask · LSM Tree · and 7 more

Caching

2

LRU / LFU Cache · Caching Strategies

Indexes & routing

14

Trie Autocomplete · Hash Tables & Resizing · Binary Heap · The LRU Cache Structure · and 10 more

Distributed systems

16

Consistent Hashing · Sharding & Partitioning · Kademlia DHT · Clock Synchronization · and 12 more

Probabilistic structures

6

Bloom Filter · HyperLogLog · Count-Min Sketch · Cuckoo Filter · and 2 more

See all 57 simulations →
03 · Plans

Everything is free until 1 September 2026.

The whole site, for the launch month. These are the plans that start on 1 September.

Free
₹0
forever, no card
  • The foundation simulations, from a request reaching a server to a database writing to disk
  • Two full interview questions — a whole interview you work through on a design board
  • The learning path, the glossary, and 10 review cards a day
Quarterly
₹999 · ₹333 a month
every 3 months, cancel any time
  • All 57 simulations
  • All 12 interview questions, every scenario
  • Unlimited review cards
  • Long enough to cover a full round of interview prep
Monthly
₹399
per month, cancel any time
  • All 57 simulations
  • All 12 interview questions, every scenario
  • Unlimited review cards
  • Enough for a last week of prep

Nothing is locked right now. The split above is what starts on 1 September.

See what each plan includes →

Open a simulation and press Next.

All 57 of them are free until 1 September 2026.

Open the simulationsSee plans