Invariant
State safety
Name the state that survives restart and the write that makes retry safe.
Week 8 · Sun Sep 20, 2026 · Sep 14-20, 2026
Day 7: Repair, Synthesis, Scorecard
A final scorecard with artifact proof, heading coverage, DDIA decisions, Heap evidence, and a READY_FOR_REVIEW production record.
Exact Time Blocks
Follow the Week 4 delivery pattern: blank-page first, source reading second, DDIA decision third, spoken close, contiguous algorithm practice, evening recall.
| Time | Block | Required output |
|---|---|---|
| 08:30 | Wake + setup | Open this page, start the board, write the day question: Can every Week 8 mechanism be restarted without hidden state? |
| 08:50—09:00 | Board set | List entities, queues, leases, retry states, dedupe keys, metrics before reading. |
| 09:00—10:05 | Blank-page attempt + Hello reading | Repair the weakest answer from Saturday and regenerate the final Staff paragraph for that system. Then walk every local artifact link. |
| 10:05—10:45 | Deep dive + DDIA decision | Synthesize the week into one mechanism ladder: frontier ownership -> leases -> retries/backoff -> dedupe/idempotence -> change detection -> recovery metrics. |
| 10:45—10:55 | Spoken close | Stop only when the production record says READY_FOR_REVIEW. |
| 14:30—16:15 | NeetCode Heap/Priority Queue | Complete contiguous slots W8-H19—W8-H21; capture invariant and bug risk for each. |
| 20:30—21:15 | Recall / Q&A / mock | Use Staff Q&A, lecture assets, or mock script for the day's system. |
Canonical Heading Anchors
Read only the listed Hello Interview headings for today's assignment. Every link is a direct day-page anchor.
- Use local artifacts and scorecard evidence; no new source scope.
Decision Notes
- CORE means you must answer requirement, HLD, deep dive, failure, recovery, metric, and Staff-level tradeoff.
- TRANSFER means blind attempt first, then delta against source, invalid assumption, new bottleneck, and follow-up pressure question.
- Do not add open-ended AI radar unless it changes a concrete Week 8 design decision; this week it does not.
DDIA Decision Map
Use exact sections only from Ch5, Ch9, and Ch12.
| Chapter | Exact DDIA decision section | Concrete Week 8 use |
|---|---|---|
| Ch5 | 持久化执行与工作流 | Final audit: every workflow has durable state and a resume rule. |
| Ch9 | 隔离僵尸进程和延迟请求 | Final audit: every lease path rejects stale completions. |
| Ch12 | 失败后重建状态 | Final audit: every materialized view can be rebuilt from events/logs. |
Heap/Priority Queue Slots
Three contiguous slots today. Keep one invariant and one failure mode per problem.
| Slot | Problem | Pattern | Invariant / bug risk | Time · Space |
|---|---|---|---|---|
| W8-H19 | Cheapest Flights Within K Stops | bounded-state heap | State includes stops used; city alone is not enough. | O(EK log EK) · O(VK) |
| W8-H20 | Min Cost to Connect Points | Prim heap | The cut grows by the cheapest edge crossing from chosen to unchosen. | O(n^2 log n) · O(n^2) |
| W8-H21 | Take Gifts From the Richest Pile | max-heap update | Each operation changes exactly one largest pile. | O(k log n) · O(n) |
Evidence To Save
One compact artifact is enough if it proves mechanism depth.
Failure
Concrete break
Use a worker crash, zombie lease, redelivery, poison message, stale robots decision, or duplicate price event.
Metric
Operational proof
Attach lag, retry rate, duplicate suppression, stale lease reject count, host politeness violation, or alert freshness.
Detailed lecture notes, audio, recall scripts, PDFs, Staff Q&A, and mock packs are archived locally and are intentionally not published on this site.