langgraph 的核心对象

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class Pregel(
    PregelProtocol[StateT, ContextT, InputT, OutputT],
    Generic[StateT, ContextT, InputT, OutputT],
):
    nodes: dict[str, PregelNode]

    channels: dict[str, BaseChannel | ManagedValueSpec]

    stream_mode: StreamMode = "values"
    """Mode to stream output, defaults to 'values'."""

    stream_eager: bool = False
    """Whether to force emitting stream events eagerly, automatically turned on
    for stream_mode "messages" and "custom"."""

    output_channels: str | Sequence[str]

    stream_channels: str | Sequence[str] | None = None
    """Channels to stream, defaults to all channels not in reserved channels"""

    interrupt_after_nodes: All | Sequence[str]

    interrupt_before_nodes: All | Sequence[str]

    input_channels: str | Sequence[str]

    step_timeout: float | None = None
    """Maximum time to wait for a step to complete, in seconds. Defaults to None."""

    debug: bool
    """Whether to print debug information during execution. Defaults to False."""

    checkpointer: Checkpointer = None
    """Checkpointer used to save and load graph state. Defaults to None."""

    store: BaseStore | None = None
    """Memory store to use for SharedValues. Defaults to None."""

    cache: BaseCache | None = None
    """Cache to use for storing node results. Defaults to None."""

    retry_policy: Sequence[RetryPolicy] = ()
    """Retry policies to use when running tasks. Empty set disables retries."""

    cache_policy: CachePolicy | None = None
    """Cache policy to use for all nodes. Can be overridden by individual nodes.
    Defaults to None."""

    context_schema: type[ContextT] | None = None
    """Specifies the schema for the context object that will be passed to the workflow."""

    config: RunnableConfig | None = None

    name: str = "LangGraph"

    trigger_to_nodes: Mapping[str, Sequence[str]]

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我正在学习 Langgraph的源码，我注意到 Pregel 的类中包含了如下对象:

1. PregelProtocol
2. PregelNode
3. BaseChannel
4. ManagedValueSpec
5. Checkpointer
6. BaseStore
7. BaseCache

请给我从语义上解释一下这些类的作用

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class PregelNode:
    """A node in a Pregel graph. This won't be invoked as a runnable by the graph
    itself, but instead acts as a container for the components necessary to make
    a PregelExecutableTask for a node."""

    channels: str | list[str]
    """The channels that will be passed as input to `bound`.
    If a str, the node will be invoked with its value if it isn't empty.
    If a list, the node will be invoked with a dict of those channels' values."""

    triggers: list[str]
    """If any of these channels is written to, this node will be triggered in
    the next step."""

    mapper: Callable[[Any], Any] | None
    """A function to transform the input before passing it to `bound`."""

    writers: list[Runnable]
    """A list of writers that will be executed after `bound`, responsible for
    taking the output of `bound` and writing it to the appropriate channels."""

    bound: Runnable[Any, Any]
    """The main logic of the node. This will be invoked with the input from 
    `channels`."""

    retry_policy: Sequence[RetryPolicy] | None
    """The retry policies to use when invoking the node."""

    cache_policy: CachePolicy | None
    """The cache policy to use when invoking the node."""

    tags: Sequence[str] | None
    """Tags to attach to the node for tracing."""

    metadata: Mapping[str, Any] | None
    """Metadata to attach to the node for tracing."""

    subgraphs: Sequence[PregelProtocol]
    """Subgraphs used by the node."""

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# 提问
我注意到 langgraph.channels 定义了如下 BaseChannel 的子类，请以表格的形式说明每个子类的语义
any_value.AnyValue
base.BaseChannel
binop.BinaryOperatorAggregate
ephemeral_value.EphemeralValue
last_value.LastValue
last_value.LastValueAfterFinish
named_barrier_value.NamedBarrierValue
named_barrier_value.NamedBarrierValueAfterFinish
topic.Topic
untracked_value.UntrackedValue

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class ManagedValue(ABC, Generic[V]):
    @staticmethod
    @abstractmethod
    def get(scratchpad: PregelScratchpad) -> V: ...


ManagedValueSpec = type[ManagedValue]

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@dataclasses.dataclass(**_DC_KWARGS)
class PregelScratchpad:
    step: int
    stop: int
    # call
    call_counter: Callable[[], int]
    # interrupt
    interrupt_counter: Callable[[], int]
    get_null_resume: Callable[[bool], Any]
    resume: list[Any]
    # subgraph
    subgraph_counter: Callable[[], int]

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我正在阅读 langgraph的源码，注意到 langgraph定义了如下对象

@dataclasses.dataclass(**_DC_KWARGS)
class PregelScratchpad:
    step: int
    stop: int
    # call
    call_counter: Callable[[], int]
    # interrupt
    interrupt_counter: Callable[[], int]
    get_null_resume: Callable[[bool], Any]
    resume: list[Any]
    # subgraph
    subgraph_counter: Callable[[], int]

PregelScratchpad 的定义如上，请先解释他的语义，在以表格的形式说明他的属性

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Checkpointer = Union[None, bool, BaseCheckpointSaver]
"""Type of the checkpointer to use for a subgraph.
- True enables persistent checkpointing for this subgraph.
- False disables checkpointing, even if the parent graph has a checkpointer.
- None inherits checkpointer from the parent graph."""

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class BaseCheckpointSaver(Generic[V]):

    # 序列化器
    serde: SerializerProtocol = JsonPlusSerializer()