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D8 A Workflow Engine: Turning the Pipeline Into a Resumable Task Graph

• What state must you persist to support resuming a workflow? Is per-node completion status enough?要支持断点续跑，你需要持久化哪些状态？只存每个节点的完成状态够不够？

  Common in ChinaCommon overseasDeep dive#workflow-engine#state-persistence#resume

  How to reason about it · think before answering

  1. The words 'is it enough' hint that it is not. A system storing only completion status knows a node ran, but not which version ran, so it happily skips after you change the code.
  2. Frame it as three questions a resume must answer: which nodes are done, are they the version I want now, and are their artifacts still there? Each maps to something you must persist.
  3. So beyond status you need the fingerprint and the artifact location. The fingerprint answers 'same version?', the location answers 'still there?'. Storing artifacts in a content-addressed directory named by the fingerprint collapses the third question into a file-existence check.
  4. Also separate two layers: the artifact cache is global and shared across runs, providing idempotency; node state is per run, providing resume. Collapse them and a new run id costs you full price again.
  5. Write timing is part of the answer: persist state right after each node completes, not once at the end. Hard kills, power loss and container eviction are not rare during ten-minute video jobs.
  6. Expect the follow-up 'do you delete a failed node's partial artifacts'. No. Keep them, and make the hit condition 'every declared output exists'. Missing one means redo, so partials are never mistaken for success.

  分析过程 · 先想清楚再作答

  1. 题眼在「够不够」三个字，它在暗示你答案是不够。只存完成状态的系统，重跑时只知道「这个节点做过」，却答不出「做的是哪一版」——于是改完代码重跑，它照样跳过。
  2. 拆的角度是：续跑要回答三个问题。哪些节点做完了？它们做的是不是我现在要的那一版？它们的产物还在不在？三个问题分别对应三样要持久化的东西。
  3. 所以除了状态，还要存指纹和产物位置。指纹回答「是不是同一版」，产物位置回答「东西还在不在」。本课的做法是把产物按指纹落进内容寻址的目录，这样第三个问题退化成一次文件存在性检查，连记都不用记。
  4. 还要区分两层：产物缓存是全局的，跨运行共享，它提供的是幂等；节点状态是每次运行一份，它提供的是断点续跑。混成一层的话，换个运行标识就得重花一次钱。
  5. 落盘时机也是这题的一部分：状态必须在每个节点跑完之后立刻写，而不是整个流程结束再写一次。进程被强杀、机器掉电、容器被驱逐，在跑十几分钟的视频任务时并不罕见。
  6. 可预期的追问是「失败节点的残产物要不要删」。答：不删。留着它，下一次跑到这里判断产物齐不齐就直接得到结论；但判定必须是「outputs 里每个文件都在」才算命中，缺一个就重做，否则残产物会被当成成功的。

  Key points

  • Completion status alone is not enough: persist the fingerprint and artifact location to answer 'which version' and 'still present'.
  • Content-addressed artifact directories reduce 'still present' to a file-existence check.
  • Keep two layers: a global cache for idempotency, per-run node state for resume.
  • Persist state immediately after each node, not once at the end of the run.
  • Keep failed nodes' partial artifacts, but only count a hit when every declared output exists.

  答题要点

  • 只存完成状态不够，还要存指纹和产物位置，分别回答「哪一版」和「还在不在」。
  • 产物按指纹落进内容寻址目录后，「还在不在」退化成一次文件存在性检查。
  • 两层分开：缓存全局共享提供幂等，节点状态每次运行一份提供断点续跑。
  • 状态要在每个节点跑完后立刻落盘，不能等整个流程结束再写。
  • 失败节点的残产物保留，但命中判定必须是全部产物齐全才算数。