Build LangGraph Payment Router State Machine

from typing import TypedDict, Annotated
from langgraph.graph import StateGraph, END

class PaymentState(TypedDict):
    transaction_id: str
    amount: float
    currency: str
    failed_gateways: list[str]
    current_gateway: str
    status: str
    routing_reason: str

def evaluate_health_node(state: PaymentState):
    # Node logic to check gateway health metrics...
    pass

def route_fallback_node(state: PaymentState):
    # Node logic to pick optimal operational fallback...
    pass

workflow = StateGraph(PaymentState)
workflow.add_node("health_check", evaluate_health_node)
workflow.add_node("route_fallback", route_fallback_node)
workflow.set_entry_point("health_check")
# Define conditional edges and compile graph...