E9 · Message bus
Family: Multi-agent
Agents publish events and subscribe to topics through a router; the flow emerges from the events.
Principle
Agents publish events and subscribe to topics through a router. No sequence is written in advance: the flow emerges from the events. It is classic publish-subscribe, applied to agents.
The problem it solves
In a system already driven by events, you need to plug in agents without fixing the order of steps, which depends on what happens.
When to use it
- Event-driven pipelines, where the flow emerges from events rather than from a predefined sequence.
- The agentic system slots into an existing event-driven architecture.
When to avoid it
- Debugging is harder than with a sequential orchestrator, and bad routing fails silently: without fine-grained observability, prefer E1.
- The flow is known in advance: a workflow (B1, B6) is simpler.
Common pitfalls
The anti-patterns that misuse this pattern, best established first. Each sheet describes the symptoms, the public incidents and the remedies.
Loop, stopping and orchestration
- AP42 · Untraceable multi-agent system — The deliverable is wrong, and nobody can tell which agent introduced the error, or at which step.
Synonyms by vendor
- Message bus (Anthropic (2026))
- Publish/subscribe (architecture distribuée classique)
Neighbouring patterns
Sources
- Multi-agent coordination patterns: five approaches and when to use them — Anthropic (C. Phillips), 2026-04-10
Content reviewed on 2026-10-02