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D20 定时 Job 与主动关怀：时区、quiet hours、每日上限、通知 provider 抽象

• 系统主动发给用户的消息，和用户自己触发的消息，在系统设计上有什么不同？How does a system-initiated message differ from a user-triggered one, from a system design point of view?

  国内高频海外高频基础#proactive-messaging#system-design#product-engineering

  分析过程 · 先想清楚再作答

  1. 这题看着像概念题，其实是筛人题。答「都是发消息，只是触发方不同」就落进了最浅的一层——面试官想听的是这个差别会逼你多写哪些代码。
  2. 先给三条结构化的差别：谁在等（用户触发时他正盯着屏幕，主动消息没有人在等）；失败怎么处理（用户触发的失败必须报错给他看，主动消息的失败多数时候应该安静地推迟或放弃）；凭什么发（用户触发是他开了口，主动消息你得自己说出理由）。
  3. 第三条是题眼，要说透：主动消息的默认答案是不发。每一条都要能回答为什么是现在、为什么是这个用户、为什么这条内容值得打断他，三个问题答不上任何一个就不该发。
  4. 然后给出这个差别在系统里的落点：主动消息这一侧必须多出一层准入判断，本课叫三道闸——按用户时区算本地时间、安静时段命中就推迟、每日上限满了就拦下。用户触发那一侧完全不需要这层。
  5. 代价也要算清楚，这是区分「读过文章」和「做过系统」的地方：用户对主动消息的容忍度极低，连着几条无关紧要的推送之后他不会争论内容对不对，直接关掉通知权限——而权限一关，你连真正重要的那条也送不出去了。你消耗的是一个用完就拿不回来的额度。
  6. 可以预期的追问：那定时任务和主动消息是不是一回事？答不是。定时任务解决的是「能按时触发」（中心调度、幂等键锚在计划触发的那一分钟），主动消息解决的是「该不该发」，前者是机制、后者是准入，两层要分开做。

  How to reason about it · think before answering

  1. This looks like a definition question but it is really a filter. Answering both send a message, only the trigger differs stays at the shallowest layer — the interviewer wants to know what extra code the difference forces you to write.
  2. Give three structured differences: who is waiting (a user-triggered reply has someone staring at the screen, a proactive message has nobody waiting); how failure is handled (user-triggered failures must surface as errors, proactive failures should usually be silently deferred or dropped); and what justifies sending (the user asked, versus you having to justify it yourself).
  3. The third is the hinge, so make it explicit: the default answer for a proactive message is do not send. Every one must answer why now, why this user, and why this content is worth interrupting them. Fail any of the three and it should not go out.
  4. Then land the difference in the system: the proactive path needs an admission layer the reactive path does not — compute the user's local time from their timezone, defer if it falls inside quiet hours, drop if the daily cap is used up.
  5. Quantify the cost, which is what separates having read about this from having shipped it: tolerance for proactive messages is very low. After a few irrelevant pushes the user will not argue about the content, they will revoke the notification permission — and once revoked, the genuinely important message cannot reach them either. You are spending a budget that never refills.
  6. Expect: so is a cron job the same thing as a proactive message? No. The scheduler solves firing on time (central scheduling, an idempotency key anchored to the scheduled minute); proactive care solves whether to send at all. One is mechanism, the other is admission, and they belong in separate layers.

  答题要点

  • 三条差别：谁在等、失败怎么处理、凭什么发；第三条是关键
  • 主动消息的默认答案是不发，每条要能回答为什么是现在、为什么是这个用户、为什么值得打断他
  • 落到系统上就是多一层准入判断：时区换算、安静时段、每日上限，用户触发那一侧不需要
  • 代价是一个不可再生的额度：推送惹烦了用户，他关掉权限之后重要消息也送不出去
  • 定时机制（能按时触发）和主动关怀（该不该发）是两层，不要混在一起做

  Key points

  • Three differences: who is waiting, how failure is handled, and what justifies sending — the third is the crux
  • The default answer for a proactive message is no; each one must justify why now, why this user, why worth interrupting
  • In the system this becomes an admission layer — timezone, quiet hours, daily cap — that the reactive path does not need
  • The cost is a non-renewable budget: annoy the user and they revoke notifications, taking the important messages down with them
  • Scheduling (fire on time) and proactive care (should we send) are two separate layers