notes✳
FREE LAB · NO ACCOUNT NEEDED

The system design
playground.

Four building blocks every backend interview touches, running live. Change the traffic, break things, and watch what moves.

LOAD BALANCER / 4 SERVERSLive simulation
Clients14 req/sLoadbalancerround-robinServer 10 in progress or queuedServer 20 in progress or queuedServer 30 in progress or queuedServer 40 in progress or queued
S1
S2
S3
S4
Served
0
Failed
0
p95 latency
0.00 s
Capacity
32 req/s

Try it: make Server 2 slow. Round robin keeps sending it a quarter of the traffic, so its queue fills and p95 latency climbs. Switch to least connections and watch the queue drain. Then kill a server: requests fail until the next health check removes it from the pool.

GO DEEPER

The building blocks lesson explains each simulator with interview questions. Paid lessons add real PostgreSQL labs and incident workflows. See what is included.