System Design Capstone Checkpoint

Calculate workload and recovery, diagnose retry boundaries, bound read fan-in, and assess design evidence. Select one option per multiple-choice question.

Get every question correct to pass this checkpoint. You can review feedback, revise your answers, and submit again.

1. A feed serves 12 million viewers making 18 requests/day each. What is its average request rate per second over an 86,400-second day? Enter one integer.

2. A 6-million-follower author posts while the fanout service has 40,000 spare inserts/s after ordinary traffic. Which conclusion follows? Select one answer.

Select one answer.

Write fanout finishes within 5 seconds Read-time merging costs nothing The extra work needs about 150 seconds to drain, so a 5-second freshness target needs a different fanout policy or capacity Mean follower count proves the burst is harmless

3. A client loses the response after a post transaction commits. Which mechanism makes retrying creation safe? Select one answer.

Select one answer.

The feed cache has a long TTL A stable client request ID and its result are recorded atomically with the post, and mismatched reuse is rejected The client generates a new request ID for every retry A worker acknowledges the queue message before writing

4. A worker commits a feed reference, crashes before acknowledging its chunk, and then receives that chunk again. Which record rule prevents duplicate entries? Select one answer.

Select one answer.

Assign a fresh post ID on each delivery Never retry failed chunks Count each delivery as a new post Upsert a reference under unique (viewer_id,post_id)

5. Workers stop for 90 seconds while 40,000 reference writes/s arrive. After restart they serve 70,000/s, and arrivals remain 40,000/s. How many seconds after restart are needed to drain the extra backlog? Enter one integer.

6. A feed page returns 30 posts and merges one ordinary source plus at most 12 exceptional-author sources. Each source returns its newest 30 entries. What is the maximum candidate count per read? Select one answer.

Select one answer.

390 30 360 12

7. A ride offer to driver A times out. The service reserves the ride for B, then receives A’s late acceptance. Which design evidence addresses the conflict? Select one answer.

Select one answer.

A map showing nearby drivers only A queue that guarantees there will never be retries Atomic ride/driver reservation states with version and expiry checks that reject stale acceptance A faster location refresh interval alone

8. A design document lists products and adds each stage’s measured p99 to claim its total meets 200 ms. What evidence is still required? Select one answer.

Select one answer.

Only a more detailed architecture drawing End-to-end latency under the stated peak and cache conditions, plus fault drills against explicit recovery targets A larger average-throughput number without latency data A statement that all queues are exactly once

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