Week 8 · Wed Sep 16, 2026 · Sep 14-20, 2026

Day 3: Job Scheduler CORE HLD

A scheduler HLD that separates job definitions, schedules, execution instances, leases, attempts, and user-visible status.

What is durable state: the job definition, the scheduled occurrence, or the execution attempt?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: What is durable state: the job definition, the scheduled occurrence, or the execution attempt?
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	Blank-page the scheduler: create job, update recurring schedule, store next fire time, scan due buckets, enqueue executable work, run worker, write attempt/status, expose monitor API. Then read Hello through HLD and update entity names.
10:05—10:45	Deep dive + DDIA decision	Name the state transitions: scheduled -> due -> leased -> running -> succeeded/failed/retry/dead. Store idempotency key, lease token, attempt number, not_before, and poison reason. User status reads should not depend on worker memory.
10:45—10:55	Spoken close	Explain why recurring schedules create execution instances instead of mutating one long-running job row.
14:30—16:15	NeetCode Heap/Priority Queue	Complete contiguous slots W8-H07—W8-H09; 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 3

CORE scope and requirements gate

Functional RequirementsDay 3

CORE schedule, monitor, cancel contract

Non-Functional RequirementsDay 3

CORE precision, scale, durability constraints

The Set UpDay 3

CORE setup and interviewer framing

Planning the ApproachDay 3

CORE planning spine

Defining the Core EntitiesDay 3

Job, schedule, execution, attempt, lease

The APIDay 3

CORE API

Data FlowDay 3

CORE write, schedule, dispatch, execute, observe flow

High-Level DesignDay 3

CORE architecture

1) Users should be able to schedule jobs to be executed immediately, at a future date, or on a recurring scheduleDay 3

CORE scheduling flow

2) Users should be able monitor the status of their jobs.Day 3

CORE observability surface

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	The workflow definition is separate from each execution instance.
Ch5	消息代理	The dispatch layer decouples time discovery from worker execution.
Ch12	消息传递系统	Execution messages must support acknowledgement and redelivery semantics.

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-H07	Find Median From Data Stream	two heaps	All left items <= all right items, and sizes differ by at most one.	O(log n) · O(n)
W8-H08	Merge K Sorted Lists	heap of list heads	Only the next head of each list belongs in the frontier.	O(n log k) · O(k)
W8-H09	Design Twitter	heap merge of recency streams	News feed is a bounded k-way merge over followed users.	O(f log f) · O(f)

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.