Day 52: Blackboard vs Message Bus Orchestration

A shared Google Doc “blackboard” feels collaborative until you cannot prove who wrote which step or why the Implementer ignored the Critic. For production agents, prefer a message bus (SQS / EventBridge) with typed envelopes — and reserve blackboards for read-mostly scratch that is still versioned.

⚡ TL;DR: Use EventBridge/SQS for agent handoffs with run_id, role, schema_version, and payload hash. Keep a small append-only artifact store for plans/diffs. Prefer bus over shared mutable docs when you need auditability.

When a blackboard is enough

Blackboards work for short, single-tenant research loops where humans watch live. They fail when multiple writers mutate the same plan section without optimistic locking.

# blackboard/scratch.py — OK for demos, not for prod apply
class Scratch:
    def __init__(self):
        self.notes: list[dict] = []
    def append(self, role: str, text: str):
        self.notes.append({"role": role, "text": text})  # ❌ no hash, no schema

Message bus envelopes

{
  "run_id": "run_7f3a",
  "role": "planner",
  "type": "PlanReady",
  "schema_version": 3,
  "payload_s3": "s3://agent-artifacts/run_7f3a/plan.json",
  "payload_sha256": "ab12…",
  "ts": "2026-09-11T15:00:00Z"
}

# bus/emit.py
import boto3, hashlib, json
eb = boto3.client("events")
s3 = boto3.client("s3")

def emit_plan(run_id: str, plan: dict, bucket: str):
    body = json.dumps(plan, sort_keys=True).encode()
    key = f"{run_id}/plan.json"
    s3.put_object(Bucket=bucket, Key=key, Body=body, ContentType="application/json")
    eb.put_events(Entries=[{
        "Source": "agents.planner",
        "DetailType": "PlanReady",
        "Detail": json.dumps({
            "run_id": run_id,
            "payload_s3": f"s3://{bucket}/{key}",
            "payload_sha256": hashlib.sha256(body).hexdigest(),
            "schema_version": 3,
        }),
        "EventBusName": "agent-orchestration",
    }])

✅ Consumers verify SHA before trusting the plan — bus messages stay small and auditable.

SQS vs EventBridge

// consumer/idempotency.ts
const seen = new Set<string>();
export function once(key: string, fn: () => Promise<void>) {
  if (seen.has(key)) return; // ✅ at-least-once bus, exactly-once side effect
  seen.add(key);
  return fn();
}

Closing checklist

• [ ] Typed envelopes with schema_version
• [ ] Artifacts in object storage with content hash
• [ ] DLQ + alarm on poison messages
• [ ] Idempotency keys on apply tools
• [ ] Prefer bus when multiple roles write sequentially

Series navigation

Day 51: Multi-Agent Roles: Planner, Implementer, Critic · Day 53: Avoiding Agent Ping-Pong

Last updated September 11, 2026