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30 天从前端工程师到 Agent 工程师

D29 弱项补强 + 编码热身：rate limiter、LRU、并发控制、流式 JSON 解析

• 为什么流式场景下不能直接用 JSON.parse？你会怎么做增量解析？Why can't you just call JSON.parse in a streaming response, and how would you parse incrementally?

  国内高频海外高频深入#streaming#json-parsing

  分析过程 · 先想清楚再作答

  1. 这题的区分度几乎全在你开口的第一句话。听到「流式 JSON 解析」就动手写状态机的人，会花二十多分钟写一个大概率有 bug 的东西；先反问一句「是一行一个完整 JSON，还是一个大对象被切成很多片」的人，已经赢了一半。
  2. 先回答为什么不能直接解析：网络分包不认语法边界，一次读取拿到的很可能是半个 JSON。直接扔给解析器只会抛异常，而且这个异常没有任何可恢复的信息。
  3. 然后做那个关键区分。情况 a 是 SSE：每条事件是一行以 data 开头的文本，行内是完整 JSON，LLM 场景 99% 是这一种，解法是行缓冲加逐行解析，二十行代码——把切分出来的最后一段（可能是半行）留在缓冲区里，等下一次读到更多数据再拼。情况 b 是单个大对象跨分片到达，才需要真正的增量解析。
  4. 情况 b 的核心是三个状态变量：括号深度（深度归零说明一个完整对象结束）、是否在字符串内部（字符串里的括号不能计入深度）、前一个字符是不是反斜杠（转义中的引号不切换字符串状态）。三者缺一不可，少一个遇到含括号的文本就算错深度。
  5. 还有一条几乎没人主动说、但一说就加分的坑：分片是按字节切的，一个汉字在 UTF-8 里占三个字节，边界可能落在中间。必须用流式解码器（TextDecoder 的 stream 选项、Python 的增量解码器、Java 的 InputStreamReader），否则会拿到一个永远补不回来的乱码字符。这是「半行缓冲」在字节层的同款问题。
  6. 可以预期的追问：那结尾那个终止标记怎么办？答案是它不是 JSON，必须在解析前先判断并直接返回，拿它去解析必然抛异常——这是这道题里最常见的一行 bug。

  How to reason about it · think before answering

  1. Almost all the signal is in your first sentence. Whoever starts writing a state machine will spend twenty-plus minutes on something probably buggy; whoever first asks 'is it one complete JSON per line, or one big object split across chunks?' has already won half the question.
  2. Answer the why first: network chunking ignores syntax boundaries, so a single read often holds half a JSON document. Handing that to a parser only throws, and the exception carries nothing you can recover from.
  3. Then draw the distinction. Case (a) is SSE: each event is one line prefixed with data, holding one complete JSON object. This covers 99% of LLM work, and the fix is line buffering plus per-line parsing — keep the trailing fragment in the buffer and stitch it onto the next chunk.
  4. Case (b) — one large object arriving in pieces — is the only case needing real incremental parsing, and it rests on three state variables: bracket depth (back to zero means a complete object), whether you are inside a string (brackets in text must not count), and whether the previous character was a backslash (an escaped quote must not toggle string state). Drop any one and text containing brackets breaks the depth count.
  5. One trap almost nobody volunteers: chunks are split on bytes, and a CJK character takes three bytes in UTF-8, so a boundary can land mid-character. Use a streaming decoder — TextDecoder with the stream option, an incremental decoder in Python, InputStreamReader in Java — or you get a replacement character you can never recover. It is the half-line problem one layer down.
  6. Expect the follow-up: what about the terminator line? It is not JSON, so check for it and return before parsing. Feeding it to the parser is the single most common one-line bug in this question.

  答题要点

  • 网络分包不认语法边界，一次读取可能拿到半个 JSON，直接解析必然抛异常且不可恢复
  • 先问清是哪一种：一行一个完整 JSON（SSE，占 LLM 场景的绝大多数）还是一个大对象跨分片到达
  • 前者只需行缓冲加逐行解析：把最后一段可能的半行留在缓冲区，等下一次读到更多数据再拼
  • 后者才需要状态机，核心是括号深度、是否在字符串内部、前一个字符是否为转义反斜杠三个状态
  • 字节层还有一个同款坑：UTF-8 多字节字符可能被分片切开，必须用流式解码器；结尾的终止标记不是 JSON，解析前要先判断

  Key points

  • Network chunking ignores syntax boundaries, so a read can hold half a document; parsing it throws an unrecoverable error
  • Ask which case it is first: one complete JSON per line (SSE, the overwhelming majority of LLM work) or one large object split across chunks
  • The first case only needs line buffering plus per-line parsing, keeping the trailing partial line for the next chunk
  • Only the second case needs a state machine, tracking bracket depth, inside-string, and escaped-previous-character
  • One layer down, a multi-byte UTF-8 character can be split across chunks, so use a streaming decoder; and the terminator line is not JSON, so check for it before parsing