WEEK 04 · 2026-08-19 · WED

Google Docs：operation log、concurrent edits 与实时协同 HLD

把 Google Docs 从“同步文本”拆成 operation log、revision/context、transform/rebase、presence/cursor soft state。

今日固定时间表

08:30 wake 只保留起床；学习从 08:50 开始。三块窗口不互相挤占。

08:30 wake	起床、早餐、准备白板；不安排预读或算法。
08:50—09:10	08:50—10:55 system design 闭卷 HLD — 先定义 Document、Operation、Revision、Session、Presence。
09:10—09:45	08:50—10:55 system design 精确原文 — 读 Google Docs requirements/API/HLD 与 Collaborative Edits Breakdown。
09:45—10:25	08:50—10:55 system design 机制化白板 — snapshot + op log；submit op 带 baseRevision；服务端 sequencer 分配 new revision；客户端 transform/rebase。
10:25—10:55	08:50—10:55 system design DDIA 决策卡 — 多主冲突、本地优先实时协作和 CRDT/OT 的设计影响。
14:30—16:15	NeetCode Tag · Sliding Window 三道题在下方独立算法表执行。
20:30—21:15	20:30—21:15 Google Docs 英语 recall：operation log、base revision、concurrent edits、presence。

算法模块

连续单一 NeetCode Sliding Window tag；每天 3 个 NEW slot，每题 30 分钟作答 + 5 分钟证据，正好 105 分钟。每天 timed review = 0，满足不超过 1。

#	Problem / mode	Pattern	Invariant	Bug risk	Time	Space
7	Max Consecutive Ones II NEW · 30m solve + 5m evidence	至多一个零的可变窗口	窗口内 zeroCount <= 1。	第三个零后只缩一次而非缩到合法。	O(n)	O(1)
8	Longest Substring with At Most Two Distinct Characters NEW · 30m solve + 5m evidence	至多 K 类字符窗口	count 中正频次字符数 <= 2。	频次归零后未删除 key。	O(n)	O(1) alphabet
9	Longest Substring with At Most K Distinct Characters NEW · 30m solve + 5m evidence	至多 K 类字符窗口	收缩结束后 distinct <= k。	k=0；删除左字符时漏减 distinct。	O(n)	O(k)

闭卷起手

20 分钟闭卷画 loadDocument、submitOperation、subscribeOps、updateCursor；写出 LWW 为什么丢用户意图。

Hello Interview 精确目录

day 页面使用 canonical data-toc-id/H4 anchors；周级 overview 不放小节链接。

项目	精确 canonical section	Disposition	今天怎么用
Google Docs	Understanding the Problem	required reading	collaborative document surface
Google Docs	Functional Requirements	required reading	documents, concurrent edits, realtime changes, presence
Google Docs	Non-Functional Requirements	required reading	latency, availability, consistency boundary
Google Docs	Set Up	required reading	scope and simplifications
Google Docs	Planning the Approach	required reading	entities/API/HLD sequence
Google Docs	Defining the Core Entities	required reading	Document, Operation, Revision, Session, Presence
Google Docs	Defining the API	required reading	load document, submit op, subscribe, cursor
Google Docs	High-Level Design	required reading	document service + op log + websocket broadcast
Google Docs	1) Users should be able to create new documents.	required reading	document metadata and initial revision
Google Docs	2) Multiple users should be able to edit the same document concurrently.	required reading	concurrent operation semantics
Google Docs	3) Users should be able to view each other's changes in real-time.	required reading	operation broadcast and replay
Google Docs	4) Users should be able to see the cursor position and presence of other users.	required reading	soft presence, not durable truth
Google Docs	Collaborative Edits Breakdown	deep dive	H4 breakdown of edit path

DDIA 精确决策卡

只读能改变今天设计决定的小节。

项目问题	精确 DDIA subsection	改变的系统决定
Google Docs 多人并发编辑为什么不是简单 last-write-wins？	Ch6 · 处理写入冲突	文档编辑必须保留用户意图，不能用 LWW 丢弃并发操作；需要 OT/CRDT 或服务端 sequencer。
协同编辑的浏览器副本为什么像多主复制？	Ch6 · 实时协作、离线优先和本地优先应用	本地先应用，异步同步到其他客户端；必须显式处理冲突、回放和撤销。

本日项目 artifacts

脚本、音频、PDF 与 mock evidence 直接从对应日打开。

Google Docs 中文完整讲稿 Google Docs 中文完整音频 Google Docs Senior+/Staff English recall Google Docs printable recall PDF

今日产出、修复与英语 recall

课程角色	CORE · requirements/API/data model/HLD
必须交付	Google Docs HLD：Document snapshot、Operation log、baseRevision、ack/rebase、presence/cursor state。
修复规则	如果用 last write wins 覆盖整篇文档，补 OT/CRDT 或服务端 sequencer 如何保留用户意图。
English recall	English recall: “Collaborative editing stores intent-preserving operations, not just the latest document blob.”

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.