Implementing Parallel Task Execution in LangGraph
Never wait sequentially for independent tasks. In this lesson, we master parallel execution in LangGraph: dispatching dynamic fan-out tasks with Send(), synchronizing fan-in branches, and resolving concurrent state reducers.
1. Core Mechanics of Parallel Execution
Select a concurrency patternParallel Execution Inspector
# 1. DYNAMIC FAN-OUT WITH LANGGRAPH Send()
from langgraph.constants import Send
from langgraph.graph import StateGraph, START, END
def fan_out_topics(state: ResearchState):
"""Spawns parallel worker executions for every discovered topic."""
topics = state["topics_to_investigate"]
# Returns a list of Send objects — LangGraph runs them concurrently!
return [Send("research_worker", {"topic": t}) for t in topics]
builder = StateGraph(ResearchState)
builder.add_node("planner", planner_node)
builder.add_node("research_worker", worker_node)
builder.add_node("synthesizer", synthesis_node)
builder.add_conditional_edges("planner", fan_out_topics, ["research_worker"])
builder.add_edge("research_worker", "synthesizer")
builder.add_edge("synthesizer", END)2. Interactive Parallel Execution Benchmark
Sequential vs Parallel Live SimulatorClick Sequential or Parallel above to run the benchmark
Common Engineering Traps
If two parallel nodes return {"results": [...]} without an additive reducer, the last node to finish overwrites the first node's data. Always annotate parallel channels with Annotated[list, operator.add].
Fanning out 50 Send() tasks simultaneously will trip external API rate limits (HTTP 429) instantly. Use an asyncio.Semaphore(5) inside the worker to cap maximum concurrent in-flight requests.
Key Architectural Takeaways
- 1.Send() for Dynamic Fan-Out: Spawn dynamic numbers of parallel workers determined at runtime by the model.
- 2.Pregel Automatic Synchronization: LangGraph handles fan-in synchronization automatically, ensuring the downstream node waits for all parallel branches.
- 3.Massive Latency Reduction: Compress execution time from the sum of all tasks to the maximum duration of the single slowest task.
Comparing Sequential and Parallel Plan Execution
When does parallel execution help, and when does it hurt? Learn how to calculate dependency graphs and choose the optimal execution strategy for cost, latency, and tokens.