langgraph-architecture/COMPONENTS.md

LangGraph Components

Version: 1.0.0
Last Updated: 2026-02-23

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Overview

This document lists and describes the key components in the LangGraph codebase.

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Directory Structure

langgraph/libs/langgraph/langgraph/
├── pregel/           # Core execution engine
├── channels/         # Inter-node communication
├── graph/            # Graph building DSL
├── checkpoint/       # Persistence (in separate lib)
├── managed/          # Managed values
├── _internal/        # Internal utilities
├── utils/           # Helper utilities
├── types.py         # Core types
├── config.py        # Configuration
├── constants.py     # Constants
└── errors.py       # Error definitions

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Core Components

1. Pregel Engine (pregel/)

The heart of LangGraph - execution engine.

File	Lines	Purpose
main.py	~4400	Public API, entry point
_loop.py	~1300	Core PregelLoop class
_algo.py	~1500	Task scheduling, write application
_runner.py	~1000	Async execution
_read.py	~300	PregelNode (node wrapper)
_write.py	~250	Write application
_checkpoint.py	~100	Checkpoint creation
_executor.py	~250	Task execution
_retry.py	~250	Retry logic
_validate.py	~150	Graph validation

2. Channels (channels/)

Inter-node communication.

File	Purpose
base.py	Abstract BaseChannel
last_value.py	LastValue channel
any_value.py	AnyValue channel
topic.py	Topic (pub/sub)
named_barrier_value.py	Barrier synchronization
binop.py	Binary operation
ephemeral_value.py	One-time values
untracked_value.py	Non-checkpointed values

3. Graph Building (graph/)

DSL for building graphs.

File	Purpose
state.py	StateGraph builder (~1800 lines)
_node.py	Node definition
_branch.py	Conditional edges
message.py	Message graph utilities
ui.py	Graph visualization

4. Core Types (types.py)

~600 lines of type definitions.

Key types:

# Durability
Durability = Literal["sync", "async", "exit"]

# Checkpointer
Checkpointer = None | bool | BaseCheckpointSaver

# Streaming
StreamMode = Literal["values", "updates", "checkpoints", "tasks", "debug"]

# Execution
class Send(NamedTuple):
    node: str
    arg: Any

class Interrupt(NamedTuple):
    value: Any
    when: str

class Command(NamedTuple):
    update: dict | None
    resume: dict | None

5. Errors (errors.py)

class GraphRuntimeException(Exception):
    """Base exception."""
    pass

class EmptyInputError(GraphRuntimeException):
    """No input provided."""
    pass

class GraphInterrupt(GraphRuntimeException):
    """Graph interrupted."""
    pass

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Public API

From pregel/__init__.py

# Main classes
class Pregel:
    """Main graph executor."""
    
    def invoke(self, input: Any, config: RunnableConfig) -> Any: ...
    def stream(self, input: Any, config: RunnableConfig) -> Iterator: ...
    async def ainvoke(self, input: Any, config: RunnableConfig) -> Any: ...
    async def astream(self, input: Any, config: RunnableConfig) -> AsyncIterator: ...
    def get_state(self, config: RunnableConfig) -> StateSnapshot | None: ...
    def get_state_history(self, config: RunnableConfig) -> Iterator[StateSnapshot]: ...
    def update_state(self, config: RunnableConfig, values: dict) -> StateSnapshot: ...

# Graph builders
class StateGraph:
    """Build a stateful graph."""
    
    def add_node(self, name: str, action: Callable) -> Self: ...
    def add_edge(self, start: str, end: str) -> Self: ...
    def add_conditional_edges(self, source: str, path: Callable) -> Self: ...
    def compile(self) -> Pregel: ...

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Configuration

RunnableConfig

From config.py:

class RunnableConfig:
    """Configuration for graph execution."""
    
    configurable: dict = {}
    tags: list[str] = []
    metadata: dict = {}
    recursion_limit: int = 25
    max_concurrency: int = None

Config Keys

From constants.py:

CONFIG_KEY_THREAD_ID = "thread_id"
CONFIG_KEY_CHECKPOINT_ID = "checkpoint_id"
CONFIG_KEY_CHECKPOINTER = "checkpointer"
CONFIG_KEY_CHECKPOINT_MAP = "checkpoint_map"
CONFIG_KEY_DURABILITY = "durability"
CONFIG_KEY_RESUMING = "resuming"
CONFIG_KEY_RESUME_MAP = "resume_map"

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Dependencies

External

• langchain-core — Core utilities
• langchain — LangChain integration
• pydantic — Type validation
• xxhash — Fast hashing
• typing_extensions — Type extensions

Internal

• langgraph.checkpoint.* — Checkpoint backends
• langgraph.store — Long-term storage

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Key Files Summary

Component	Main File	Key Classes
Execution	pregel/main.py	Pregel
Loop	pregel/_loop.py	PregelLoop
Channels	channels/base.py	BaseChannel
Graph	graph/state.py	StateGraph
Types	types.py	Send, Interrupt, Command
Config	config.py	RunnableConfig

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Generated from source code analysis