RFC 0017: host.queueBus capability

Summary

Adds an optional host.queueBus capability covering publish + inbound-consume + ack/nack/dead-letter, plus stream subscribe/publish. Required by core.openwop.messaging. Sibling to existing host.messaging (which is outbound-egress-only) — host.queueBus covers full message-queue semantics including delivery acknowledgement.

Motivation

n8n RabbitMQ/Kafka/SQS/NATS/MQTT/Pub-Sub nodes all expose the same pattern: a trigger fires on inbound, an action publishes outbound, plus ack/nack on the trigger side. openwop currently has no inbound-queue surface at all. host.messaging only handles outbound chat egress.

Proposal

§A Capability schema

{
  "queueBus": {
    "type": "object",
    "properties": {
      "supported": { "type": "boolean" },
      "backends": { "type": "array", "items": { "type": "string", "enum": ["rabbitmq","kafka","sqs","sns","pubsub","mqtt","nats","redis-streams","in-memory"] } },
      "deadLetterSupported": { "type": "boolean" },
      "stream": {
        "type": "object",
        "properties": {
          "supported": { "type": "boolean" },
          "fromBeginning": { "type": "boolean" }
        }
      }
    },
    "additionalProperties": false
  }
}

§B Host-contract MUSTs

1. Cross-tenant message isolation. A consumer for tenant A MUST NOT receive messages published by tenant B, even on the same logical topic. 2. Ack semantics: ack removes the message from the queue; nack returns it for redelivery; deadLetter routes it to the configured dead-letter queue. 3. Trigger-mode delivery: when a workflow registers core.messaging.consume as a trigger, the host MUST deliver one workflow run per message.

§C SECURITY invariant

- id: queue-cross-tenant-isolation
  tier: protocol
  summary: "host.queueBus MUST partition messages by tenant. Cross-tenant consumption MUST NOT occur."
  conformance_test: "queue-cross-tenant-isolation.test.ts"

§D Conformance

• queue-publish-consume-roundtrip.test.ts.
• queue-cross-tenant-isolation.test.ts.
• queue-ack-nack-dlq.test.ts.
• stream-subscribe-from-beginning.test.ts (gated on stream.fromBeginning).

Compatibility

Additive. New capability. host.messaging (existing, outbound-egress-only) remains unchanged.

Implementation notes (non-normative)

• Schema diff in §A lands in schemas/capabilities.schema.json on Active promotion, not at Draft.
• SECURITY invariant queue-cross-tenant-isolation lands in SECURITY/invariants.yaml alongside the matching scenario.
• Reference impl candidate: in-memory pub/sub bus per tenant in examples/hosts/in-memory/.

Acceptance criteria

• [x] Capability block.
• [x] Prose section. (Landed at commit c5831fe.)
• [x] Invariant + 3 mandatory scenarios + 1 gated.
• [x] In-memory reference impl.
• [x] CHANGELOG entry.

References

• n8n RabbitMQ/Kafka/SQS/NATS/MQTT triggers (prior art).
• Existing host.messaging (sibling, outbound-only).
• core.openwop.messaging pack.