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Bus Messages

The Message class is the on-the-wire envelope that OVOS components exchange — utterances coming in, intent matches going out, skills announcing handler outcomes, hosts dispatching to skills. The shape is defined by OVOS-MSG-1 and this module is the reference implementation.

What is in a Message

Every Message is a JSON object with exactly three top-level fields:

Field Type Meaning
type string The topic — ovos.intent.matched, speak, <skill_id>:<intent_name>, …
data object Topic-specific payload; shape fixed by whichever spec defines type
context object Routing keys, the session carrier, and any layer-2 metadata 
from ovos_spec_tools import Message

m = Message("speak", {"utterance": "hello"}, {"source": "skill.id"})

The constructor rejects malformed input: an empty type, a non-string type, or a non-dict data / context raise MalformedMessage. Pass the dicts by reference — they are stored as-is; if you need a detached copy use the derivations below.

Routing keys

Two context keys mark the OVOS/handler-code boundary:

  • source — opaque identifier of the producer;
  • destination — opaque identifier (or list of identifiers) of the intended consumer(s). Absent or empty means broadcast.

The envelope treats both as opaque strings — no parsing, no validation beyond "is it there". How identifiers are minted is a deployment concern. Hivemind, for example, uses source / destination to thread remote peers through the bus without OVOS itself learning about them.

The session carrier

context.session is the carrier for one conversational session — the unit of "this wake-word interaction" or "this HiveMind client connection". OVOS-MSG-1 makes two of its keys normative:

  • session.session_id — string identifier; the value "default" is reserved and means originates from the device itself (used by ovos-audio to keep TTS local).
  • session.lang — BCP-47 tag for the user's preferred output language (distinct from data.lang which describes the payload).

The rest of session's shape is opaque to this Message — full Session structure is the job of the consumer (typically ovos-bus-client's Session class or a future session spec).

Absent session = session_id: "default". A Message with no session in its context is treated the same as one carrying the reserved value. Producers don't have to emit a session for device-local messages.

The three derivations

Each derivation returns a new Message with the right routing/session fields for its role. The runtime class is preserved — subclasses of Message get back instances of their own subclass.

forward(type, data) — relay under a new topic

Keeps context (including source, destination, and session) exactly as it was. The forwarder does not become the new source; the original producer remains named.

ack = m.forward("ovos.utterance.handled", {"id": "u-7"})
# ack.context == m.context  (deep-copied)

reply(type, data, context=None) — send back to the asker

Copies context, then swaps the routing keys so the new Message is addressed back to the original producer:

  • the new destination is the old source;
  • the new source is the old destination (the first entry if it was an array — exact choice is implementation-defined).

Other context keys, including session, pass through unchanged. The optional context argument is overlaid before the swap, matching ovos-bus-client.Message.reply behaviour:

ack = m.reply("speak", {"utterance": "got it"})
# ack.context["source"] == m.context["destination"]
# ack.context["destination"] == m.context["source"]

response(data, context=None) — sugar for .response-suffixed reply

A response is a reply whose topic is the source topic with .response appended. Used by request/response chains so an observer can recognize the answer:

res = Message("ovos.intent.list").response({"intents": ["..."]})
res.msg_type   # 'ovos.intent.list.response'

Serialization

serialize() produces a single UTF-8 JSON object per OVOS-MSG-1 §6:

wire = m.serialize()
recovered = Message.deserialize(wire)
assert recovered == m

Nested objects in data / context that expose a .serialize() method (the duck-typed protocol used by ovos-bus-client.Session and similar carriers) are converted before JSON encoding — Message itself doesn't know about Session or any specific carrier type, but its .serialize() walks containers and calls the method when present.

For a JSON-decoded view without the string intermediate, m.as_dict returns the same envelope as a dictionary:

m.as_dict   # {'type': 'ovos.test', 'data': {...}, 'context': {...}}

deserialize rejects malformed payloads as MalformedMessage: unparsable JSON, non-object root, unknown top-level keys, missing type, wrong value types. Both str and bytes are accepted, and an already-parsed dict short-circuits the JSON step.

A note on correlation

There is no per-message identifier, no in-reply-to chain, no host-managed request/response bookkeeping. Messages on the bus are fully asynchronous. The .response suffix and the preserved session give you enough raw material to do your own correlation if you need it — match an incoming <topic>.response against an outstanding request in the same session_id.

Components that need per-conversation state track it themselves keyed on session.session_id. A Message on the bus is self-contained; any state a later consumer needs is either inside the Message or kept by some component out of band — never recovered by a hidden host-side correlation index.

What this module does not do

By design, no transport (websocket, queue, …), no encryption, no authentication, no delivery guarantees, no ordering guarantees, no session lifecycle, no identifier-assignment policy. Each of those is out of scope for the envelope and belongs in the layer that consumes it — ovos-bus-client for the websocket transport, layer-2 systems (HiveMind) for multi-tenant routing, individual subsystems for their own per-session state.

Subclassing

Message is designed to be subclassed. Both deserialize (a classmethod) and forward / reply / response return instances of the runtime class, so a transport-layer subclass propagates through derivation chains without losing identity:

class TransportMessage(Message):
    def serialize(self):
        body = super().serialize()
        # add framing, encryption, … here
        return body

m = TransportMessage("ovos.test")
forwarded = m.forward("ovos.next")
assert isinstance(forwarded, TransportMessage)

This is exactly how ovos-bus-client.Message is expected to extend the spec primitive once it adopts this class.

Spec section map

Every part of the Message API traces to a specific OVOS-MSG-1 clause. The module's docstrings cite these inline; this table is the quick index.

API surface OVOS-MSG-1 § What it implements
Message(type, data, context) §2, §2.1–§2.3 the three-field envelope; constructor type checks
MalformedMessage (constructor) §2.1–§2.3 reject non-string type, non-dict data/context
msg_type / data / context §2 the wire fields type / data / context
context["source"] / ["destination"] §3, §3.2–§3.4 opaque routing keys, carried not parsed
context["session"] §4 session carrier (inner shape owned by OVOS-SESSION-1)
DEFAULT_SESSION_ID §4.1 the reserved "default" session id
forward(type, data) §5.1 relay, context preserved unchanged (deep-copied)
reply(type, data, context) §5.2 swap routing keys, address back to the asker
response(data, context) §5.3 .response-suffixed reply
(no correlation API) §5.4 messages are fully async; no host-side correlation
serialize() §6 single UTF-8 JSON object, finite numbers only
as_dict §2, §6 JSON-decoded envelope view
deserialize(payload) §6, §7 parse + reject malformed payloads
MalformedMessage (deserialize) §7 the consumer "MUST reject" rules

See also

  • Bus namespaces — SpecMessage, MIGRATION_MAP, and the transparent legacy↔ovos.* bridge that rides on top of these derivations (forward for the handler-lifecycle trio, reply for the stop pong, and so on).
  • Spec traceability — every public symbol in ovos-spec-tools mapped to its authoritative spec section.