Staff (L6) · System design

Staff Backend Engineer

Staff-level backend loops push past the boxes-and-arrows diagram. Expect to quantify bounds: how much over-admission, how much lag, which invariant holds during a partition, and what you sacrifice to keep it.

How the loop runs

Roughly 37 minutes across three rounds.

1. 1

   High-level design

   15 min

   Invariants, ownership of state, API and data model

2. 2

   Chaos injection

   12 min

   Hot keys, partitions, retries after timeouts, clock skew

3. 3

   Cost & edge cases

   10 min

   Reconciliation, migrations, operational cost

Where candidates lose points

• Naming Redis or Kafka without saying what invariant they protect
• No idempotency story for client retries
• Hand-waving partition behavior
• Not quantifying the error bound of local batching