Episode 10. System design plus behavioral. Days 10 through 12 of your sprint.

Scenario one, design: "Marketplace buy, sell, rent at Roblox scale." This is the 2022 Principal onsite prompt. Open with the shard key.

Say: "I shard by the query pattern and isolate by the tenant. Then I defend the cross-shard transaction story and the failover story."

Say the review line: "My trade-offs are explicit: consistency where money moves, availability where feeds read, and measurements for both."

Scenario two, behavioral: every STAR story needs a metric. Situation in two sentences, Task with I owned, Action longest, Result as X measured by Y by doing Z.

Say the intro spine once, ninety seconds, San Mateo close: correctness plus scale for millions of players is the career problem.

Lab days ten to twelve: whiteboard the marketplace flow, drill six STAR stories timed, then run a full mock loop and keep the gap list to five.

Drill: deliver the ninety-second intro spine twice, timed, no notes.
Say: shard by the query pattern, isolate by the tenant, fence every failover.
Say the STAR mechanic: I owned, Action longest, metric with by-doing-Z.
Done-check: six stories each with a metric, gap list capped at five with owners.
OSS framing: issue, repro, patch, review. Narrate one upstream-shaped story.

Close: Say: "Mechanism, Measure, Move. That is the whole interview." Good luck. Now say it again.