> ## Documentation Index
> Fetch the complete documentation index at: https://langchain-zh.cn/llms.txt
> Use this file to discover all available pages before exploring further.

# LangGraph 运行时

[`Pregel`](https://reference.langchain.com/javascript/langchain-langgraph/index/Pregel) 实现了 LangGraph 的运行时，负责管理 LangGraph 应用程序的执行。

编译一个 [StateGraph](https://reference.langchain.com/javascript/langchain-langgraph/index/StateGraph) 或创建一个 [entrypoint](https://reference.langchain.com/javascript/langchain-langgraph/index/entrypoint) 会产生一个 [`Pregel`](https://reference.langchain.com/javascript/langchain-langgraph/index/Pregel) 实例，该实例可以通过输入进行调用。

本指南从高层次解释了运行时，并提供了直接使用 Pregel 实现应用程序的说明。

> **注意：** [`Pregel`](https://reference.langchain.com/javascript/langchain-langgraph/index/Pregel) 运行时以 [Google 的 Pregel 算法](https://research.google/pubs/pub37252/) 命名，该算法描述了一种使用图进行大规模并行计算的高效方法。

## 概述

在 LangGraph 中，Pregel 将 [**执行器**](https://en.wikipedia.org/wiki/Actor_model) 和 **通道** 组合到一个应用程序中。**执行器** 从通道读取数据并向通道写入数据。Pregel 按照 **Pregel 算法**/**批量同步并行** 模型将应用程序的执行组织为多个步骤。

每个步骤包含三个阶段：

* **规划**：确定在此步骤中要执行哪些 **执行器**。例如，在第一步中，选择订阅特殊 **输入** 通道的 **执行器**；在后续步骤中，选择订阅上一步中更新的通道的 **执行器**。
* **执行**：并行执行所有选定的 **执行器**，直到全部完成、某个失败或达到超时。在此阶段，通道更新对执行器不可见，直到下一步。
* **更新**：使用此步骤中 **执行器** 写入的值更新通道。

重复此过程，直到没有 **执行器** 被选中执行，或达到最大步骤数。

## 执行器

一个 **执行器** 是一个 `PregelNode`。它订阅通道，从中读取数据，并向其写入数据。可以将其视为 Pregel 算法中的 **执行器**。`PregelNodes` 实现了 LangChain 的 Runnable 接口。

## 通道

通道用于在执行器（PregelNodes）之间进行通信。每个通道都有一个值类型、一个更新类型和一个更新函数——该函数接收一系列更新并修改存储的值。通道可用于将数据从一个链发送到另一个链，或在未来的步骤中将数据从一个链发送到自身。LangGraph 提供了许多内置通道：

* [`LastValue`](https://reference.langchain.com/javascript/classes/_langchain_langgraph.channels.LastValue.html)：默认通道，存储发送到通道的最后一个值，适用于输入和输出值，或用于将数据从一个步骤发送到下一个步骤。
* [`Topic`](https://reference.langchain.com/javascript/langchain-langgraph/channels/Topic)：一个可配置的发布-订阅主题，适用于在 **执行器** 之间发送多个值，或用于累积输出。可以配置为去重值或在多个步骤中累积值。
* [`BinaryOperatorAggregate`](https://reference.langchain.com/javascript/langchain-langgraph/channels/BinaryOperatorAggregate)：存储一个持久值，通过对当前值和发送到通道的每个更新应用二元运算符来更新，适用于在多个步骤中计算聚合；例如，`total = BinaryOperatorAggregate(int, operator.add)`

## 示例

虽然大多数用户将通过 [StateGraph](https://reference.langchain.com/javascript/langchain-langgraph/index/StateGraph) API 或 [entrypoint](https://reference.langchain.com/javascript/langchain-langgraph/index/entrypoint) 装饰器与 Pregel 交互，但也可以直接与 Pregel 交互。

以下是几个不同的示例，让您了解 Pregel API。

<Tabs>
  <Tab title="单节点">
    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    import { EphemeralValue } from "@langchain/langgraph/channels";
    import { Pregel, NodeBuilder } from "@langchain/langgraph/pregel";

    const node1 = new NodeBuilder()
      .subscribeOnly("a")
      .do((x: string) => x + x)
      .writeTo("b");

    const app = new Pregel({
      nodes: { node1 },
      channels: {
        a: new EphemeralValue<string>(),
        b: new EphemeralValue<string>(),
      },
      inputChannels: ["a"],
      outputChannels: ["b"],
    });

    await app.invoke({ a: "foo" });
    ```

    ```console theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    { b: 'foofoo' }
    ```
  </Tab>

  <Tab title="多节点">
    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    import { LastValue, EphemeralValue } from "@langchain/langgraph/channels";
    import { Pregel, NodeBuilder } from "@langchain/langgraph/pregel";

    const node1 = new NodeBuilder()
      .subscribeOnly("a")
      .do((x: string) => x + x)
      .writeTo("b");

    const node2 = new NodeBuilder()
      .subscribeOnly("b")
      .do((x: string) => x + x)
      .writeTo("c");

    const app = new Pregel({
      nodes: { node1, node2 },
      channels: {
        a: new EphemeralValue<string>(),
        b: new LastValue<string>(),
        c: new EphemeralValue<string>(),
      },
      inputChannels: ["a"],
      outputChannels: ["b", "c"],
    });

    await app.invoke({ a: "foo" });
    ```

    ```console theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    { b: 'foofoo', c: 'foofoofoofoo' }
    ```
  </Tab>

  <Tab title="主题">
    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    import { EphemeralValue, Topic } from "@langchain/langgraph/channels";
    import { Pregel, NodeBuilder } from "@langchain/langgraph/pregel";

    const node1 = new NodeBuilder()
      .subscribeOnly("a")
      .do((x: string) => x + x)
      .writeTo("b", "c");

    const node2 = new NodeBuilder()
      .subscribeTo("b")
      .do((x: { b: string }) => x.b + x.b)
      .writeTo("c");

    const app = new Pregel({
      nodes: { node1, node2 },
      channels: {
        a: new EphemeralValue<string>(),
        b: new EphemeralValue<string>(),
        c: new Topic<string>({ accumulate: true }),
      },
      inputChannels: ["a"],
      outputChannels: ["c"],
    });

    await app.invoke({ a: "foo" });
    ```

    ```console theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    { c: ['foofoo', 'foofoofoofoo'] }
    ```
  </Tab>

  <Tab title="二元运算符聚合">
    此示例演示如何使用 [`BinaryOperatorAggregate`](https://reference.langchain.com/javascript/langchain-langgraph/channels/BinaryOperatorAggregate) 通道实现一个归约器。

    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    import { EphemeralValue, BinaryOperatorAggregate } from "@langchain/langgraph/channels";
    import { Pregel, NodeBuilder } from "@langchain/langgraph/pregel";

    const node1 = new NodeBuilder()
      .subscribeOnly("a")
      .do((x: string) => x + x)
      .writeTo("b", "c");

    const node2 = new NodeBuilder()
      .subscribeOnly("b")
      .do((x: string) => x + x)
      .writeTo("c");

    const reducer = (current: string, update: string) => {
      if (current) {
        return current + " | " + update;
      } else {
        return update;
      }
    };

    const app = new Pregel({
      nodes: { node1, node2 },
      channels: {
        a: new EphemeralValue<string>(),
        b: new EphemeralValue<string>(),
        c: new BinaryOperatorAggregate<string>({ operator: reducer }),
      },
      inputChannels: ["a"],
      outputChannels: ["c"],
    });

    await app.invoke({ a: "foo" });
    ```
  </Tab>

  <Tab title="循环">
    此示例演示如何通过让一个链写入它订阅的通道来在图中引入循环。执行将持续进行，直到向通道写入一个 `null` 值。

    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    import { EphemeralValue } from "@langchain/langgraph/channels";
    import { Pregel, NodeBuilder, ChannelWriteEntry } from "@langchain/langgraph/pregel";

    const exampleNode = new NodeBuilder()
      .subscribeOnly("value")
      .do((x: string) => x.length < 10 ? x + x : null)
      .writeTo(new ChannelWriteEntry("value", { skipNone: true }));

    const app = new Pregel({
      nodes: { exampleNode },
      channels: {
        value: new EphemeralValue<string>(),
      },
      inputChannels: ["value"],
      outputChannels: ["value"],
    });

    await app.invoke({ value: "a" });
    ```

    ```console theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    { value: 'aaaaaaaaaaaaaaaa' }
    ```
  </Tab>
</Tabs>

## 高级 API

LangGraph 提供了两个用于创建 Pregel 应用程序的高级 API：[StateGraph（图 API）](/oss/javascript/langgraph/graph-api) 和 [函数式 API](/oss/javascript/langgraph/functional-api)。

<Tabs>
  <Tab title="StateGraph（图 API）">
    [StateGraph（图 API）](https://reference.langchain.com/javascript/langchain-langgraph/index/StateGraph) 是一个更高级别的抽象，简化了 Pregel 应用程序的创建。它允许您定义节点和边的图。当您编译图时，StateGraph API 会自动为您创建 Pregel 应用程序。

    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    import { START, StateGraph } from "@langchain/langgraph";

    interface Essay {
      topic: string;
      content?: string;
      score?: number;
    }

    const writeEssay = (essay: Essay) => {
      return {
        content: `Essay about ${essay.topic}`,
      };
    };

    const scoreEssay = (essay: Essay) => {
      return {
        score: 10
      };
    };

    const builder = new StateGraph<Essay>({
      channels: {
        topic: null,
        content: null,
        score: null,
      }
    })
      .addNode("writeEssay", writeEssay)
      .addNode("scoreEssay", scoreEssay)
      .addEdge(START, "writeEssay")
      .addEdge("writeEssay", "scoreEssay");

    // 编译图。
    // 这将返回一个 Pregel 实例。
    const graph = builder.compile();
    ```

    编译后的 Pregel 实例将与一系列节点和通道相关联。您可以通过打印它们来检查节点和通道。

    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    console.log(graph.nodes);
    ```

    您将看到类似这样的内容：

    ```console theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    {
      __start__: PregelNode { ... },
      writeEssay: PregelNode { ... },
      scoreEssay: PregelNode { ... }
    }
    ```

    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    console.log(graph.channels);
    ```

    您应该看到类似这样的内容

    ```console theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    {
      topic: LastValue { ... },
      content: LastValue { ... },
      score: LastValue { ... },
      __start__: EphemeralValue { ... },
      writeEssay: EphemeralValue { ... },
      scoreEssay: EphemeralValue { ... },
      'branch:__start__:__self__:writeEssay': EphemeralValue { ... },
      'branch:__start__:__self__:scoreEssay': EphemeralValue { ... },
      'branch:writeEssay:__self__:writeEssay': EphemeralValue { ... },
      'branch:writeEssay:__self__:scoreEssay': EphemeralValue { ... },
      'branch:scoreEssay:__self__:writeEssay': EphemeralValue { ... },
      'branch:scoreEssay:__self__:scoreEssay': EphemeralValue { ... },
      'start:writeEssay': EphemeralValue { ... }
    }
    ```
  </Tab>

  <Tab title="函数式 API">
    在 [函数式 API](/oss/javascript/langgraph/functional-api) 中，您可以使用 [`entrypoint`](https://reference.langchain.com/javascript/langchain-langgraph/index/entrypoint) 来创建 Pregel 应用程序。`entrypoint` 装饰器允许您定义一个接收输入并返回输出的函数。

    ```typescript theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    import { MemorySaver } from "@langchain/langgraph";
    import { entrypoint } from "@langchain/langgraph/func";

    interface Essay {
      topic: string;
      content?: string;
      score?: number;
    }

    const checkpointer = new MemorySaver();

    const writeEssay = entrypoint(
      { checkpointer, name: "writeEssay" },
      async (essay: Essay) => {
        return {
          content: `Essay about ${essay.topic}`,
        };
      }
    );

    console.log("节点: ");
    console.log(writeEssay.nodes);
    console.log("通道: ");
    console.log(writeEssay.channels);
    ```

    ```console theme={"theme":{"light":"catppuccin-latte","dark":"catppuccin-mocha"}}
    节点:
    { writeEssay: PregelNode { ... } }
    通道:
    {
      __start__: EphemeralValue { ... },
      __end__: LastValue { ... },
      __previous__: LastValue { ... }
    }
    ```
  </Tab>
</Tabs>

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