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Concepts: one system design idea in under 10 minutes
Each concept builds one picture in short steps. A drawing grows as you read, and every two or three steps a quick check asks you to predict what happens next. Get it wrong and you see exactly why. The numbers come from real runs, not from memory.
- DatabasesHow a Database Index Finds a Row: the B-Tree, Step by StepA million rows, one id. Without an index, how many rows does the database read?10 min · 14 steps · 6 checks
- Distributed dataSharding With Consistent Hashing: Why Adding a Server Moves So Few KeysAdd a fifth server to four. Guess how many keys hash mod N makes you move.10 min · 14 steps · 6 checks
- MessagingKafka Partitions and Consumer Groups: Who Reads What, and WhySix partitions, eight consumers in one group. How many sit idle?10 min · 14 steps · 7 checks
How a concept works
- Read one short step, then press Continue or Enter. The drawing changes with each step.
- Every two or three steps, answer a check. One click locks it, and every answer shows its own reason.
- There is no fail state. Right or wrong, you carry on.
- The last step is the way an interviewer would ask it, with a model answer.
- Your place is saved in this browser, so you can come back to it.