跳至主要内容

如何取消执行

先决条件

本指南假设您熟悉以下概念

在构建运行时间更长的链或 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) 支持通过运行时 信号 选项中止正在进行的 Runnable。

兼容性

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

注意：像聊天模型或检索器这样的单个集成可能缺少或具有不同的中止执行实现。在本指南中描述的信号支持将在链的步骤之间应用。

要查看其工作原理，请构建如下链，它执行 检索增强生成。它通过首先使用 Tavily 搜索网络，然后将结果传递给聊天模型以生成最终答案来回答问题

选择您的聊天模型

安装依赖项

提示

查看 本节以获取有关安装集成软件包的常规说明.

yarn add @langchain/openai

添加环境变量

OPENAI_API_KEY=your-api-key

实例化模型

import { ChatOpenAI } from "@langchain/openai";

const model = new ChatOpenAI({
  model: "gpt-4o-mini",
  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(),
]);

如果您正常调用它，您会看到它返回最新信息

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 属性传递到链执行中。为了说明取消尽快发生的，将超时设置为 100ms

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

您会看到执行仅在 100ms 后结束。查看 此 LangSmith 跟踪，您会发现模型从未被调用。

流式传输​

您也可以在流式传输时传递 signal。这使您可以更好地控制在 for await... of 循环中使用 break 语句来取消当前运行，这只会触发最终输出已开始流式传输后。以下示例使用 break 语句 - 请注意取消发生前的时间

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，您需要将流包装在 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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