Interview prompt
Problem context
A stateful service has 256 shards, uneven growth, and several hot partitions. Adding nodes requires moving terabytes while requests continue. Pausing writes violates the availability target, and previous manual moves caused stale writers and divergent data. Design automatic shard movement.
Skills being evaluated
state transferfencingonline migrationcontrol-plane correctness
The full reasoning guide is part of Pro
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Clarify the decision
Establish scale assumptions
Functional and non-functional requirements
High-level architecture
Data model and flow
Consistency and transaction boundaries
Failure modes and recovery
Security and privacy
Observability and SLOs
Capacity and cost
Alternatives and trade-offs
Evolution and migration
What Staff and Principal candidates should emphasize