How the interviewer opens it
“Design a service where drivers find and reserve parking spaces at participating facilities. The service must show availability for a requested time and prevent double booking.”
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
- Facilities and spaces
- Time rules
- Traffic and read:write ratio
- Discovery behavior
- Holds and cancellations
- Payments
- Latency
- Consistency and availability
- Durability and retention
- Scope boundaries
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
- Find nearby facilities with a compatible space for the requested interval.
- Hold a space for checkout.
- Confirm a reservation after payment authorization.
- Retrieve and cancel reservations, including payment reversal.
Usually out of scope
- Control gates or enforce physical occupancy.
- Detect vehicles that overstay.
- Build a payment processor.
- Set prices or support resale.
- Keep reservation writes available during a region failure.
What interviewers ask about
- Space availability across time
- A facility can have spare capacity at every instant without one space remaining free for the entire requested stay.
- Discovery results and reservation attempts
- Location searches combine many interval checks. A correct result can become stale before the driver requests a hold.
- Reservation confirmation and expiry
- A payment response can arrive after a hold expires and another driver reserves the same space.
- Cancellation and payment outcomes
- Cancellation can overlap a charge whose result remains unknown. The service must release the space without losing the payment reversal obligation.
Common mistakes
- Treating a space as globally free or occupied instead of checking the requested interval.
- Using aggregate free capacity when the driver needs one space for the entire stay.
- Trusting discovery results as authority for a reservation.
- Allowing payment success to restore an expired hold.
- Assuming the database automatically removes expired holds from an overlap constraint.
- Treating cancellation as complete while an unresolved charge can still retain the driver's money.
The bar by level
- Senior (L5/E5)
- Explains interval allocation and database enforcement. Bounds discovery work and data age. Resolves authorization, expiry, capture, and cancellation races through durable state transitions.
- Staff (L6+/E6+)
- Explains the limits of fixed space assignment and measures its utilization cost. Defines safe payment outcomes across crashes and delayed responses. Controls hot searches without weakening reservation guarantees. If the interviewer asks the regional failure follow-up, also defines regional write ownership, fencing, and recovery.