How the interviewer opens it
“Design a shipment tracking service that collects updates from carriers. Customers use the service to view each shipment's history and current delivery status.”
The prompt is vague on purpose. The first minutes are for you. Reduce the prompt to the core features and agree on the numbers before you draw.
What to clarify first
- Scale
- Read and write traffic
- Carrier access
- Shipment identity
- Event identity
- Status rules
- Latency and freshness
- Unresolved and quarantined updates
- Consistency, durability, and retention
- Access and exclusions
The interviewer has the answers. In a SysRound round, Alex gives these answers, as a real interviewer does.
Scope of a strong answer
Core requirements
- Register a shipment with its carrier, tracking number, and ship date
- Collect carrier updates through webhooks and polls
- Show history with exact duplicate suppression for reliable event identities and best-effort suppression otherwise
- Show the current status despite delayed events and events that arrive out of order
Usually out of scope
- Delivery time prediction
- Route plans
- Customer notifications
- Cross-region failover
What interviewers ask about
- Repeated carrier updates
- A carrier can repeat one event, revise that event, or reuse a tracking number. Missing event identity prevents perfect duplicate detection.
- Current delivery status
- Arrival order differs from event order. Delivery states can repeat or reverse, and carrier timestamps do not always establish a reliable order.
- Updates after acknowledgment
- Acknowledgment commits the service to retain an update. Durable retention alone does not satisfy the promise to publish it.
- Carrier limits and outages
- Carrier quotas control freshness. One carrier's outage can produce a large backlog while other carriers still send live updates.
Common mistakes
- Treating receipt time as the true event order
- Using a tracking number as a permanent shipment identity
- Promising perfect duplicate suppression when carrier data cannot establish event identity
- Discarding corrections as duplicate events
- Acknowledging updates before a durable write
- Counting unresolved or quarantined updates as successful publication
The bar by level
- Senior (L5/E5)
- Defines event identity, correction rules, and atomic status updates. Calculates poll capacity and explains durable acceptance, concurrent workers, retries, and publication misses.
- Staff (L6+/E6+)
- Defines guarantees when carrier data cannot establish order or identity. Plans replay, policy changes, carrier capacity allocation, and recovery within measured limits. Separates durable acceptance, receipt visibility, and shipment freshness without hiding unresolved updates.