System Design Core Checkpoint

Trace fresh cases, diagnose a failed invariant, then revisit the linked explanation and implementation.

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

1. A cache serves 900 of 1,000 reads. If misses each make one database call and hits make none, how many database reads occur? Answer with the integer only.

2. A product price changes, but clients keep reading an old cached value. Which missing decision is most relevant?

Select one answer.

The HTTP port number The shard filename extension Whether the service uses recursion An explicit freshness and invalidation policy

3. Every write uses the current date alone as a partition key. What is the likely hot-partition risk?

Select one answer.

Only old partitions receive traffic All current writes concentrate on one partition Dates guarantee uniform distribution Partition keys affect reads but never writes

4. An at-least-once consumer receives event IDs [a,b,a,c,b]. With durable deduplication by event ID, how many distinct effects should it apply? Answer with the integer only.

5. A search index lags the transactional store by 20 seconds. A user expects a just-created record immediately in search. What must the design address?

Select one answer.

The database password length Whether index entries are compressed Only alphabetical sorting The index freshness contract or a read-after-write fallback

6. Adding five secondary indexes speeds selected reads. What write-side cost should be measured?

Select one answer.

No storage overhead is possible Maintaining additional index entries and associated I/O Writes become free Replication becomes unnecessary

7. A transaction acknowledges on a leader, then a user reads from a lagging follower. What can violate read-your-writes?

Select one answer.

The response is JSON The follower may not yet contain the acknowledged write The read uses the same user ID The leader stores a timestamp

8. A worker commits a processed-event marker, crashes before applying its effect, then skips the replay. Which repair addresses the lost effect?

Select one answer.

Commit the marker and database effect atomically, then acknowledge Acknowledge before receiving the event Keep the marker but never retry Rely only on a longer cache TTL

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