面试题库

14 天用 Agent 搭一条 AI 短剧生产线

D4 从分镜到镜头：图生视频、异步任务轮询与失败重试

• 让你实现一个异步生成任务的客户端，你会考虑哪些失败情况？You are asked to implement a client for an asynchronous generation task. Which failure cases would you cover?

  国内高频海外高频进阶#async-task#error-handling

  分析过程 · 先想清楚再作答

  1. 这题的区分度不在代码，在你能列出多少种失败。只答「加个 try catch 和重试」的人，通常没在生产上跑过这类接口。
  2. 先把任务的形状说清楚，失败点才有地方挂：提交拿标识、轮询查状态、取件换地址、下载落盘，四步是四类不同的失败。
  3. 然后逐步列：提交阶段有限流、鉴权、参数无效、内容审核；轮询阶段有查询接口自己限流、状态一直不前进、任务返回失败终态；取件阶段有标识存在但取不到地址；下载阶段有地址过期、下到一半断流、写盘失败。
  4. 接着说横跨全程的两类：超时与进程重启。超时的关键在于它不是失败而是「不知道成没成」，必须先按幂等键查一遍再决定要不要重提；进程重启意味着内存里的任务标识没了，所以标识必须先落盘再发请求，否则你会有一批花了钱却找不回来的任务。
  5. 最后给一句能体现工程判断的话：这四步里只有下载是可以无脑重试的，其余每一步的重试都可能产生一次新的计费。
  6. 可以预期的追问：厂商提供回调了还需要轮询吗？需要。回调会因为服务重启、网络抖动、地址不可达而丢失，生产上的标准做法是回调为主、低频轮询兜底扫描长时间没有终态的任务。

  How to reason about it · think before answering

  1. The differentiator here is coverage, not code. Answering 'wrap it in try/catch and retry' usually means you have never run this kind of API in production.
  2. Describe the shape first so the failures have somewhere to hang: submit and get an id, poll for status, retrieve a URL, download to disk — four steps, four families of failure.
  3. Then enumerate: at submit, rate limiting, auth failure, invalid parameters, content moderation; at poll, the query endpoint rate limiting you, a status that never advances, or a terminal failure; at retrieve, a valid id that yields no URL; at download, an expired link, a stream cut halfway, a disk write error.
  4. Then the two that span the whole flow: timeout and process restart. A timeout is not a failure, it is 'I don't know' — you must look up the idempotency key before resubmitting. A restart means in-memory task ids are gone, so the id has to be persisted before or immediately after the request, or you will have paid-for tasks you can never reclaim.
  5. Close with a line that shows judgment: of the four steps, only the download is safely retryable on its own; a retry at any other step can create a new billable job.
  6. Expect the follow-up: if the vendor offers callbacks, do you still poll? Yes. Callbacks get lost to restarts, network blips and unreachable endpoints, so the standard is callback-first with a low-frequency sweep for tasks stuck without a terminal state.

  答题要点

  • 按四步拆失败：提交（限流、鉴权、参数无效、内容审核）、轮询（查询限流、状态停滞、终态失败）、取件（拿不到地址）、下载（地址过期、断流、写盘失败）
  • 超时不是失败而是状态未知，重试前必须先按幂等键查一遍已有产物，否则会为同一个任务付两次钱
  • 任务标识要及时落盘，进程重启后才能把在途任务认回来
  • 四步里只有下载可以无脑重试，其余每一步的重试都可能产生新的计费
  • 有回调也要保留低频兜底轮询，回调会丢

  Key points

  • Break failures down by the four steps: submit (rate limit, auth, invalid params, moderation), poll (query rate limit, stalled status, terminal failure), retrieve (no URL), download (expired link, cut stream, disk error)
  • A timeout means unknown, not failed: look up the idempotency key for an existing artifact before resubmitting, or you pay twice
  • Persist the task id promptly so in-flight tasks survive a process restart
  • Only the download is safely retryable on its own; retries at the other steps can create new billable jobs
  • Keep a low-frequency polling sweep even when callbacks exist, because callbacks get lost
• 轮询间隔怎么定？为什么不能一直用固定间隔死等？How do you choose a polling interval, and why is a fixed interval a bad default?

  国内高频海外高频基础#async-task#backoff

  分析过程 · 先想清楚再作答

  1. 这是一道送分题，但它有三个层次，只答出第一层拿不到高分。面试官想看的是你有没有真的写过这个循环。
  2. 第一层是成本：一个排队五分钟的任务，用一秒的固定间隔就是三百次无效请求。查询接口自己也有速率限制，你很可能自己把自己打到限流，然后在日志里看到「生成失败：限流」，还以为是生成接口的问题。
  3. 第二层是退避要封顶：只乘不封顶的话，退到后面已经是几分钟查一次，任务早就好了你还在睡。上限的选法是「用户能忍受的额外等待」，一般十几到二十秒。
  4. 第三层最容易写错，也是这题真正的区分点：超时判断必须放在睡觉之前，判据是「睡下去会不会越过截止时间」。先睡再判会让你在预算之外多睡整整一轮，退避到二十秒时就是白等二十秒。
  5. 另外提一条顺序：先判终态再判超时。任务恰好在最后一次查询里成功却被当成超时扔掉，等于付了钱还丢了产物。
  6. 可以预期的追问：起步间隔怎么定？按这类任务的典型耗时定，比典型耗时的十分之一略大即可；再往细说就是首次查询可以稍微延后一点，因为刚提交的任务几乎不可能立刻完成。

  How to reason about it · think before answering

  1. This looks like a giveaway, but it has three layers and only the first one is obvious. They want to know whether you have actually written this loop.
  2. Layer one is cost: a task queued for five minutes polled every second is three hundred wasted requests. The query endpoint has its own rate limit, so you can throttle yourself and then misread 'rate limited' in the logs as a generation problem.
  3. Layer two is capping the backoff: multiply without a cap and you end up polling every few minutes, sleeping long after the task finished. Pick the cap from what extra wait a user tolerates — usually in the ten-to-twenty-second range.
  4. Layer three is where people actually get it wrong: the timeout check belongs before the sleep, and the test is whether sleeping would cross the deadline. Sleeping first means overshooting the budget by a full interval, which at a twenty-second backoff is twenty wasted seconds.
  5. Mention ordering too: check terminal states before the timeout. Discarding a task that just succeeded on the final poll means paying for an artifact you then throw away.
  6. Expect the follow-up: how do you pick the initial interval? From the typical duration of this class of task, a bit above a tenth of it; and the very first poll can be delayed slightly, since a just-submitted task is almost never done.

  答题要点

  • 固定间隔要么太密造成大量无效请求并把自己打到限流，要么太疏让完成后的等待过长
  • 用指数退避：从接近典型耗时十分之一的间隔起步，每轮乘一个系数
  • 退避必须封顶，上限按用户能忍受的额外等待来定
  • 超时判断放在 sleep 之前，判据是「睡完会不会越过截止时间」，否则会在预算之外多睡一轮
  • 先判终态再判超时，避免把最后一次查询里刚成功的任务误杀

  Key points

  • A fixed interval is either too tight, wasting requests and throttling yourself, or too loose, adding dead time after completion
  • Use exponential backoff starting near a tenth of the task's typical duration
  • Cap the backoff, choosing the cap from the extra wait a user will tolerate
  • Check the timeout before sleeping, testing whether the sleep would cross the deadline, or you overshoot the budget by a full interval
  • Check terminal states before the timeout so a task that just succeeded is not discarded