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如何取消执行

前提条件

本指南假定您熟悉以下概念：

在构建长时间运行的链或 LangGraph 代理时，您可能希望在某些情况下中断执行，例如用户离开您的应用程序或提交新查询。

LangChain 表达式语言 (LCEL) 通过运行时 signal 选项支持中止正在进行的可运行对象。

兼容性

内置的信号支持需要 @langchain/core>=0.2.20。有关升级指南，请参见此处。

注意： 个别集成（如聊天模型或检索器）可能在中止执行的实现上存在缺失或差异。本指南中描述的信号支持将适用于链的各个步骤之间。

要查看其工作原理，可以构建一个如下的链，该链执行检索增强生成。它通过首先使用 Tavily 在网络上搜索，然后将结果传递给聊天模型以生成最终答案来回答问题：

Pick your chat model:

Install dependencies

tip

See this section for general instructions on installing integration packages.

yarn add @langchain/groq

Add environment variables

GROQ_API_KEY=your-api-key

Instantiate the model

import { ChatGroq } from "@langchain/groq";

const model = new ChatGroq({
  model: "llama-3.3-70b-versatile",
  temperature: 0
});
import { TavilySearchAPIRetriever } from "@langchain/community/retrievers/tavily_search_api";
import type { Document } from "@langchain/core/documents";
import { StringOutputParser } from "@langchain/core/output_parsers";
import { ChatPromptTemplate } from "@langchain/core/prompts";
import {
  RunnablePassthrough,
  RunnableSequence,
} from "@langchain/core/runnables";

const formatDocsAsString = (docs: Document[]) => {
  return docs.map((doc) => doc.pageContent).join("\n\n");
};

const retriever = new TavilySearchAPIRetriever({
  k: 3,
});

const prompt = ChatPromptTemplate.fromTemplate(`
Use the following context to answer questions to the best of your ability:

<context>
{context}
</context>

Question: {question}`);

const chain = RunnableSequence.from([
  {
    context: retriever.pipe(formatDocsAsString),
    question: new RunnablePassthrough(),
  },
  prompt,
  llm,
  new StringOutputParser(),
]);

如果以正常方式调用它，可以看到它返回的是最新的信息：

await chain.invoke("what is the current weather in SF?");
Based on the provided context, the current weather in San Francisco is:

Temperature: 17.6°C (63.7°F)
Condition: Sunny
Wind: 14.4 km/h (8.9 mph) from WSW direction
Humidity: 74%
Cloud cover: 15%

The information indicates it's a sunny day with mild temperatures and light winds. The data appears to be from August 2, 2024, at 17:00 local time.

现在，让我们尽早中断它。初始化一个 AbortController，并将其 signal 属性传入链式执行中。为了说明取消操作会尽可能快地发生，我们设置一个 100 毫秒的超时：

const controller = new AbortController();

const startTimer = console.time("timer1");

setTimeout(() => controller.abort(), 100);

try {
  await chain.invoke("what is the current weather in SF?", {
    signal: controller.signal,
  });
} catch (e) {
  console.log(e);
}

console.timeEnd("timer1");
Error: Aborted
    at EventTarget.<anonymous> (/Users/jacoblee/langchain/langchainjs/langchain-core/dist/utils/signal.cjs:19:24)
    at [nodejs.internal.kHybridDispatch] (node:internal/event_target:825:20)
    at EventTarget.dispatchEvent (node:internal/event_target:760:26)
    at abortSignal (node:internal/abort_controller:370:10)
    at AbortController.abort (node:internal/abort_controller:392:5)
    at Timeout._onTimeout (evalmachine.<anonymous>:7:29)
    at listOnTimeout (node:internal/timers:573:17)
    at process.processTimers (node:internal/timers:514:7)
timer1: 103.204ms

你可以看到执行在超过 100 毫秒后结束。查看this LangSmith 跟踪，你会发现模型从未被调用。

流式传输​

在流式传输时，你也可以传递一个 signal。这使你能够更精确地控制在 for await...of 循环中使用 break 语句来取消当前运行，该操作仅在最终输出已经开始流式传输后才会触发。以下示例使用了 break 语句——请注意取消操作发生前经过的时间：

const startTimer2 = console.time("timer2");

const stream = await chain.stream("what is the current weather in SF?");

for await (const chunk of stream) {
  console.log("chunk", chunk);
  break;
}

console.timeEnd("timer2");
chunk
timer2: 3.990s

现在将其与使用信号进行比较。请注意，你需要将流包装在 try/catch 块中：

const controllerForStream = new AbortController();

const startTimer3 = console.time("timer3");

setTimeout(() => controllerForStream.abort(), 100);

try {
  const streamWithSignal = await chain.stream(
    "what is the current weather in SF?",
    {
      signal: controllerForStream.signal,
    }
  );
  for await (const chunk of streamWithSignal) {
    console.log(chunk);
    break;
  }
} catch (e) {
  console.log(e);
}

console.timeEnd("timer3");
Error: Aborted
    at EventTarget.<anonymous> (/Users/jacoblee/langchain/langchainjs/langchain-core/dist/utils/signal.cjs:19:24)
    at [nodejs.internal.kHybridDispatch] (node:internal/event_target:825:20)
    at EventTarget.dispatchEvent (node:internal/event_target:760:26)
    at abortSignal (node:internal/abort_controller:370:10)
    at AbortController.abort (node:internal/abort_controller:392:5)
    at Timeout._onTimeout (evalmachine.<anonymous>:7:38)
    at listOnTimeout (node:internal/timers:573:17)
    at process.processTimers (node:internal/timers:514:7)
timer3: 100.684ms

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