Sketch a system. Send traffic. Watch it break.
A whiteboard simulator for learning distributed systems and practicing for system design interviews: drag load balancers, caches, queues and databases onto the canvas, wire them up, push the load until something gives, and then work out how to fix it.
7-day free trial, card required · then US$14.99/month plus applicable tax · cancel anytime
Best on a laptop or desktop
Prepare for the system design round at
Immense help for senior+ level interview preparation.
New to it? The 2-minute tutorial walks you through all four steps.
Drag users, load balancers, app servers, caches, databases, queues and more onto the whiteboard, and wire them together.
Press Play and watch traffic flow along the arrows as pipes: the wider, the busier, banded by reads, writes and retries, with errors flowing back in red.
Servers saturate and turn red, queues fill, errors pile up and p95 latency climbs, while the meter shows what it all costs per hour.
Scale out, add a cache, put a queue in front, shard the database or tune timeouts and retries. Then run it again.
Every component runs a small, honest model of the real thing, so the trade-offs show up on their own.
Round robin vs least connections, and the requests lost before a crashed server is noticed.
LRU, LFU, FIFO and TTLs over realistic item popularity; invalidate or write through on writes; CDNs at the edge.
Extra read capacity, paid for with replication lag and stale reads.
Spread writes across databases by key, and see what one hot key does to a shard.
Absorb write spikes, watch the backlog grow and drain, and keep processing delay in check.
A token bucket that turns overload into cheap, fast 429s instead of a pile-up.
How retries help you ride out a crash, and how they turn into a retry storm.
Locks with leases, geo search and sorted sets, all on one single-threaded Redis.
A request that timed out can still land, and its retry charges the customer twice.
More app servers means more connections, until the database starts refusing them.
Queue order and max queue wait, so servers stop working for users who already left.
A server that is up but ten times slower passes every health check. Route around it anyway.
Each challenge starts with a diagram that fails at least one of its goals (success rate, latency, cost…). Fix it, press Play and earn up to three stars.
One idea at a time, from your first server to a full stack.
Well-known design prompts, scaled down. Find and fix the real bottleneck.
The obvious fix breaks a different goal. Read the requirements first.
Predict before you play: each challenge asks a question or two, like "which node fails first?", and tells you after the run whether you were right. Incident mode freezes the architecture, so you fix the problem with configuration alone.
US$14.99 / month
plus applicable tax
Full access to the simulator, the tutorial and all 28 challenges.
Your exact total, in your local currency where available, is shown at checkout.
One free trial per person. Checkout is on Stripe's secure page, and your subscription is sold through Link, Stripe's merchant of record service. Terms · Refund policy · Privacy
US$14.99/month plus applicable tax, after a 7-day free trial. You need a card to start the trial. Cancel before it ends and you pay nothing.
Yes. You sign in with your email and no password: we email you a sign-in link.
In the simulator, Menu → Manage subscription, or "Cancel subscription" at the bottom of any page. It takes a couple of clicks, with no emails or calls. You keep access until the end of the trial or the month you've paid for. You can also manage or cancel it at link.com, where Link keeps your receipts.
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Automatically, as you work. Use Menu → Download as .json to keep a copy or move your diagrams to another computer. You can download your diagrams even after a subscription ends.
This page does. The simulator is a drag-and-wire whiteboard built for a laptop or desktop (tablets work too); on a phone the toolbar and library don't fit, and it will tell you so before you continue.
It's a simplified discrete-event simulation. Threads, queues, connection pools, caches holding real items, replication lag, retries and timeouts are modeled, and network latency is a fixed delay per arrow; bandwidth, packet loss, disks and plenty of real-world detail are not. Treat the numbers as illustrative and the behavior as the lesson. Help in the simulator explains what each component models.
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Ready to break something?
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