Week 8 · Fri Sep 18, 2026 · Sep 14-20, 2026

Day 5: Price Tracking Service TRANSFER

A blind-first transfer artifact with deltas, invalid assumptions, new bottleneck, follow-up, and direct links to every Price Tracking heading.

Which crawler/scheduler assumptions break when the product is price history and alerts?Frontier · leases · retries · dedupe · recovery3 Heap/Priority Queue slots

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: Which crawler/scheduler assumptions break when the product is price history and alerts?
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	Start with a 25-minute blind attempt. Do not read first. Reuse only Week 8 mechanisms that actually transfer: crawl frontier, scheduler leases, retry/backoff, dedupe, and event logs. Then read all Price Tracking headings and mark deltas.
10:05—10:45	Deep dive + DDIA decision	The new bottleneck is not crawling pages; it is trustworthy change detection and threshold fanout. Extension reports are untrusted hints. The canonical price sample needs source validation, product identity, timestamp, old/new comparison, and a deduped PriceChanged event.
10:45—10:55	Spoken close	Write one follow-up question that exposes shallow transfer: if Amazon blocks direct scraping and extensions are noisy, what source becomes authoritative and how do you protect alert correctness?
14:30—16:15	NeetCode Heap/Priority Queue	Complete contiguous slots W8-H13—W8-H15; 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.

Understanding the ProblemDay 5

TRANSFER blind scope check

Functional RequirementsDay 5

TRANSFER requirements

Non-Functional RequirementsDay 5

TRANSFER scale and latency constraints

The Set UpDay 5

TRANSFER setup

Planning the ApproachDay 5

TRANSFER planning

Defining the Core EntitiesDay 5

Product, price sample, subscription, alert

The APIDay 5

TRANSFER API

Data FlowDay 5

TRANSFER scrape/sample/notify flow

High-Level DesignDay 5

TRANSFER architecture

1) Users should be able to view price history for Amazon products (via website or Chrome extension)Day 5

TRANSFER price history surface

2) Users should be able to subscribe to price drop notifications with thresholds (via website or Chrome extension)Day 5

TRANSFER notification contract

Potential Deep DivesDay 5

TRANSFER deep-dive index

1) How do we efficiently discover and track 500 million Amazon products?Day 5

TRANSFER discovery scale

2) How do we handle potentially malicious price updates from Chrome extension users?Day 5

TRANSFER trust boundary

3) How do we efficiently process price changes and notify subscribed users?Day 5

TRANSFER change detection and fanout

4) How do we serve fast price history queries for chart generation?Day 5

TRANSFER query serving

Final DesignDay 5

TRANSFER final synthesis

What is Expected at Each Level?Day 6

Transfer rubric calibration

Mid-levelDay 6

Rubric contrast

SeniorDay 6

Senior+ bar

Staff+Day 6

Staff transfer bar

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
Ch12	变更数据捕获	Price changes should be detected once and emitted as events, not rediscovered by every notification worker.
Ch12	日志压缩	Latest price state and historical samples serve different query contracts.
Ch12	幂等性	Repeated price samples and notification retries require idempotent change IDs.
Ch12	失败后重建状态	Notification state and materialized price views must be rebuildable from the price event log.

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-H13	Car Pooling	time-ordered deltas	Capacity is a prefix-sum invariant over stop events.	O(n log n) · O(n)
W8-H14	Hand of Straights	min-heap/count map	Always close the smallest unfinished card first.	O(n log n) · O(n)
W8-H15	Find K Closest Elements	binary window / heap contrast	Tie-breaking must prefer the smaller value.	O(log(n-k)+k) · O(1)

Evidence To Save

One compact artifact is enough if it proves mechanism depth.

Invariant

State safety

Name the state that survives restart and the write that makes retry safe.

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.

Local study materials
Detailed lecture notes, audio, recall scripts, PDFs, Staff Q&A, and mock packs are archived locally and are intentionally not published on this site.