LangGraph Parallel Execution

Run independent nodes concurrently for performance.

When to Use

• Independent agents can run together
• Map-reduce over task lists
• Scatter-gather patterns
• Performance optimization

Fan-Out/Fan-In Pattern

python
from langgraph.graph import StateGraph

def fan_out(state):
    """Split work into parallel tasks."""
    state["tasks"] = [{"id": 1}, {"id": 2}, {"id": 3}]
    return state

def worker(state):
    """Process one task."""
    task = state["current_task"]
    result = process(task)
    return {"results": [result]}

def fan_in(state):
    """Combine parallel results."""
    combined = aggregate(state["results"])
    return {"final": combined}

workflow = StateGraph(State)
workflow.add_node("fan_out", fan_out)
workflow.add_node("worker", worker)
workflow.add_node("fan_in", fan_in)

workflow.add_edge("fan_out", "worker")
workflow.add_edge("worker", "fan_in")  # Waits for all workers

Using Send API

python
from langgraph.constants import Send

def router(state):
    """Route to multiple workers in parallel."""
    return [
        Send("worker", {"task": task})
        for task in state["tasks"]
    ]

workflow.add_conditional_edges("router", router)

Parallel Agent Analysis

python
from typing import Annotated
from operator import add

class AnalysisState(TypedDict):
    content: str
    findings: Annotated[list[dict], add]  # Accumulates

async def run_parallel_agents(state: AnalysisState):
    """Run multiple agents in parallel."""
    agents = [security_agent, tech_agent, quality_agent]

    # Run all concurrently
    tasks = [agent.analyze(state["content"]) for agent in agents]
    results = await asyncio.gather(*tasks, return_exceptions=True)

    # Filter successful results
    findings = [r for r in results if not isinstance(r, Exception)]

    return {"findings": findings}

Map-Reduce Pattern

python
def map_node(state):
    """Map: Process each item independently."""
    items = state["items"]
    results = []

    for item in items:
        result = process_item(item)
        results.append(result)

    return {"mapped_results": results}

def reduce_node(state):
    """Reduce: Combine all results."""
    results = state["mapped_results"]

    summary = {
        "total": len(results),
        "passed": sum(1 for r in results if r["passed"]),
        "failed": sum(1 for r in results if not r["passed"])
    }

    return {"summary": summary}

Error Isolation

python
async def parallel_with_isolation(tasks: list):
    """Run parallel tasks, isolate failures."""
    results = await asyncio.gather(*tasks, return_exceptions=True)

    successes = []
    failures = []

    for task, result in zip(tasks, results):
        if isinstance(result, Exception):
            failures.append({"task": task, "error": str(result)})
        else:
            successes.append(result)

    return {"successes": successes, "failures": failures}

Timeout per Branch

python
import asyncio

async def parallel_with_timeout(agents: list, content: str, timeout: int = 30):
    """Run agents with per-agent timeout."""
    async def run_with_timeout(agent):
        try:
            return await asyncio.wait_for(
                agent.analyze(content),
                timeout=timeout
            )
        except asyncio.TimeoutError:
            return {"agent": agent.name, "error": "timeout"}

    tasks = [run_with_timeout(a) for a in agents]
    return await asyncio.gather(*tasks)

Key Decisions

| Decision | Recommendation | |----------|----------------| | Max parallel | 5-10 concurrent (avoid overwhelming APIs) | | Error handling | return_exceptions=True (don't fail all) | | Timeout | 30-60s per branch | | Accumulator | Use Annotated[list, add] for results |

Common Mistakes

• No error isolation (one failure kills all)
• No timeout (one slow branch blocks)
• Sequential where parallel possible
• Forgetting to wait for all branches

Related Skills

• langgraph-state - Accumulating state
• multi-agent-orchestration - Coordination patterns
• langgraph-supervisor - Supervised parallel execution

Capability Details

fanout-pattern

Keywords: fanout, parallel, concurrent, scatter Solves:

• Run agents in parallel
• Implement fan-out pattern
• Distribute work across workers

fanin-pattern

Keywords: fanin, gather, aggregate, collect Solves:

• Aggregate parallel results
• Implement fan-in pattern
• Collect worker outputs

parallel-template

Keywords: template, implementation, parallel, agent Solves:

• Parallel agent fanout template
• Production-ready code
• Copy-paste implementation