# MQTT Shared Contract ## Purpose This document defines the shared contract baseline for all semantic MQTT buses in this repository. It exists to keep adapters, historian ingestion, and future buses aligned on the same transport and payload rules. Bus-specific documents such as `home_bus.md` and `energy_bus.md` define their own topic grammar, but they inherit the rules from this shared contract. The operational namespace under `/sys/...` is specified in more detail by `sys_bus.md`. --- ## Namespace Model Two top-level namespaces are reserved: - semantic bus namespace: `//...` - operational namespace: `/sys/...` Examples: - `vad/home/bedroom/temperature/bedroom-sensor/value` - `vad/energy/load/living-room-tv/active_power/value` - `vad/sys/adapter/z2m-main/error` Rules: - `` MUST be stable and lowercase kebab-case - `` MUST be a reserved bus identifier such as `home`, `energy`, `network`, `compute`, `vehicle` - `sys` is reserved for operational topics and is not a semantic bus; see `sys_bus.md` - topic path versioning is intentionally not used in v1 to keep wildcard subscriptions simple Versioning rule: - breaking contract changes MUST be expressed via document version and `schema_ref` - existing topic paths MUST remain stable for v1 consumers --- ## Shared Streams All semantic buses use the same stream taxonomy: - `value`: live hot-path semantic sample, whether it represents a measurement, durable state, or transition notification - `last`: retained last-known timestamped sample used for cold start and freshness evaluation - `set`: command/request topic - `meta`: retained metadata for the sibling topic family - `availability`: online/offline or degraded health signal Rules: - `set` MUST NOT be retained - `last` SHOULD be retained - `meta` SHOULD be retained - `availability` SHOULD be retained and SHOULD use LWT when supported - buses MUST NOT invent ad-hoc stream names for v1 - adapters SHOULD publish live semantic data on `value`, not split it across separate `state` and `event` streams - adapters SHOULD deduplicate hot `value` publications when consecutive samples are semantically identical - adapters SHOULD update retained `last` whenever the latest observed sample timestamp changes - legacy `state` and `event` topics SHOULD be treated as compatibility-only during migration and SHOULD NOT be introduced by new adapters If a future need appears for diagnostics, replay, dead-letter handling, or adapter metrics, it MUST be modeled under `/sys/...`, not by extending semantic bus streams. --- ## Topic Naming Rules Common naming rules: - identifiers representing physical or logical entities SHOULD use kebab-case - capability and metric names SHOULD use snake_case - topic segments MUST be ASCII lowercase - spaces MUST NOT appear in canonical topics - vendor-native identifiers MUST NOT leak into semantic topics unless they are the chosen canonical identifier Identity rules: - canonical IDs MUST be stable across adapter rewrites - replacing a physical sensor SHOULD NOT force a canonical ID change if the logical endpoint remains the same - vendor IDs SHOULD be carried in `meta.source_ref`, not in the semantic topic path --- ## Lightweight Bus Requirement The semantic MQTT bus is a high-efficiency event bus. It is NOT: - a debugging interface - a transport for rich adapter envelopes - a place to repeat internal mapping state on every sample Normative rule: - semantic bus publications MUST remain minimal and MQTT-ready - the canonical publication boundary is the MQTT topic plus payload, with QoS and retain determined by stream policy - adapters MAY build richer internal normalization objects, but those objects MUST be discarded before publish - adapter-specific fields such as mapping tables, vendor payload snapshots, or internal processing context MUST NOT travel on semantic bus hot paths Canonical publication form: - topic: `//...` - payload: scalar or small JSON envelope - qos: `0` or `1` - retain: according to stream policy Discouraged adapter-side publish shape: ```json { "topic": "vad/home/bedroom/temperature/bedroom-sensor/value", "payload": 23.6, "homeBus": { "location": "bedroom" }, "z2mPayload": { "temperature": 23.6, "battery": 91 }, "mapping": { "source_field": "temperature" } } ``` Those structures may exist inside normalization logic, but MUST be stripped before the MQTT publish boundary. Reason: - the architecture prioritizes low CPU usage, low memory footprint, predictable Node-RED execution, and compatibility with constrained IoT accessories - large message objects increase memory pressure - large nested objects increase garbage collection cost in Node-RED - constrained accessories, SBCs, and thin VMs benefit from structurally simple bus traffic --- ## Payload Profiles Two payload profiles are supported across all buses. ### Profile A: Scalar Payload This is the default profile for hot paths. Examples: - `23.6` - `41` - `true` - `on` Profile A requirements: - the payload MUST be a scalar: number, boolean, or short string enum - units and metadata MUST be published on retained `meta` - if exact source observation time matters, Profile A MUST NOT be used unless broker receive time is acceptable - Profile A SHOULD be the default for high-rate telemetry and hot `value` streams - Profile A is the preferred format for live `value` streams consumed by lightweight clients Profile A historian rule: - historian workers SHOULD use ingestion time as `observed_at` unless an equivalent timestamp is provided out of band ### Profile B: Envelope JSON This profile is used when timestamp, quality, or extra annotations must travel with each sample. Canonical shape: ```json { "value": 23.6, "unit": "C", "observed_at": "2026-03-08T10:15:12Z", "quality": "good" } ``` Optional fields: - `published_at`: adapter publish time - `source_seq`: source-side monotonic counter or sequence id - `annotations`: free-form object for low-rate streams Profile B requirements: - `value` is REQUIRED - `observed_at` SHOULD be included when the source provides a timestamp - `quality` SHOULD be included if the adapter had to estimate or degrade data - `unit` MAY be omitted for unitless, boolean, or enum values Use Profile B when: - source timestamp must be preserved - historian ordering must follow source time, not broker receive time - per-sample quality matters - the stream is low-rate enough that JSON overhead is acceptable - a retained `last` sample is used for startup decisions and consumers must evaluate whether it is still usable Profile B restriction: - adapters SHOULD avoid envelope JSON on high-rate streams unless there is no acceptable scalar alternative - Profile B MUST remain a small canonical envelope and MUST NOT be extended into a general-purpose transport for adapter internals - repeated metadata belongs on retained `meta`, not inside every `value` sample --- ## Meta Contract Each retained `meta` topic describes the sibling `value` and `last` stream family. Minimum recommended shape: ```json { "schema_ref": "mqbus.home.v1", "payload_profile": "scalar", "data_type": "number", "unit": "C", "adapter_id": "z2m-main", "source": "zigbee2mqtt", "source_ref": "0x00158d0008aa1111", "source_topic": "zigbee2mqtt/bedroom_sensor", "precision": 0.1, "historian": { "enabled": true, "mode": "sample" } } ``` Recommended fields: - `schema_ref`: stable schema identifier such as `mqbus.home.v1` - `payload_profile`: `scalar` or `envelope` - `data_type`: `number`, `boolean`, `string`, or `json` - `unit`: canonical engineering unit when applicable - `adapter_id`: canonical adapter instance id - `source`: source system such as `zigbee2mqtt`, `modbus`, `snmp` - `source_ref`: vendor or physical device identifier - `source_topic`: original inbound topic or equivalent source path - `precision`: numeric precision hint - `display_name`: human-readable label - `tags`: optional list for analytics and discovery - `historian`: ingestion policy object Historian metadata contract: - `historian.enabled`: boolean - `historian.mode`: one of `sample`, `state`, `event`, `ignore` - `historian.retention_class`: optional storage class such as `short`, `default`, `long` - `historian.sample_period_hint_s`: optional expected cadence Rules: - `meta` SHOULD be published before live `value` traffic for new streams - `meta` updates MUST remain backward-compatible for existing consumers during v1 - consumers MUST tolerate missing `meta` and continue with degraded defaults - repeated descriptive metadata MUST be published on retained `meta`, not repeated on each hot-path `value` publication Example: - `vad/home/bedroom/temperature/bedroom-sensor/meta` -> `{"unit":"C","precision":0.1,"adapter_id":"z2m-main"}` - `vad/home/bedroom/temperature/bedroom-sensor/value` -> `23.6` --- ## Time Semantics The following timestamps are distinct: - `observed_at`: when the source system observed or measured the value - `published_at`: when the adapter normalized and published the message - `ingested_at`: when the downstream worker processed the message Rules: - if the source provides a trustworthy timestamp, adapters SHOULD preserve it as `observed_at` - if the source does not provide a timestamp, adapters MAY omit `observed_at` - if `observed_at` is omitted, historian workers SHOULD use `ingested_at` - adapters MUST NOT fabricate source time and mark it as fully trustworthy - if an adapter estimates time, it SHOULD use Profile B with `quality=estimated` This rule is the key tradeoff between low-overhead scalar payloads and strict time fidelity. --- ## Quality Model The following quality values are recommended: - `good`: trusted value from source - `estimated`: value or timestamp estimated by adapter - `degraded`: source known to be unstable or partially invalid - `stale`: source not updated within expected cadence - `invalid`: malformed or failed validation Rules: - omit `quality` only when `good` is implied - `invalid` payloads SHOULD NOT be emitted on semantic bus topics - invalid or unmappable messages SHOULD be routed to operational error topics under `sys` --- ## Delivery Policy Shared defaults: - `value`: QoS 1, retain false unless a bus-specific contract explicitly requires otherwise - `last`: QoS 1, retain true - `set`: QoS 1, retain false - `meta`: QoS 1, retain true - `availability`: QoS 1, retain true Additional rules: - `value` is the live stream and SHOULD remain lightweight - `last` is the cold-start bootstrap mechanism for latest known measurements on the semantic bus - `last` SHOULD use Profile B and include `observed_at` - consumers MUST treat a retained `last` sample as the latest known observation, not as proof of freshness - if freshness matters, consumers SHOULD evaluate `observed_at`, `availability`, and expected cadence from `meta` - adapters MAY deduplicate `value` publications, but `last` SHOULD be updated whenever the latest observed sample timestamp changes - command acknowledgements SHOULD be emitted on normalized `value`, not by retaining `set` - late joiners MUST be able to reconstruct stream meaning and last known sample from retained `meta`, retained `last`, and retained `availability` - consumers that require deterministic retained bootstrap SHOULD use a dedicated MQTT client session rather than sharing a session with unrelated subscribers on the same broker config --- ## Command Envelope Guidance Simple `set` commands may use scalar payloads: - `on` - `off` - `21.5` If correlation or richer semantics are required, a JSON envelope is allowed: ```json { "value": "on", "request_id": "01HRN8KZQ2D7P0S4M6B4CJ3M8Y", "requested_at": "2026-03-08T10:20:00Z" } ``` Rules: - command topics MUST remain bus-specific and capability-specific - acknowledgements SHOULD be published separately on normalized `value` - adapters SHOULD avoid command-side business logic --- ## Operational Namespace Operational topics are for adapter health, replay control, and malformed input handling. Detailed operational namespace rules are defined in `sys_bus.md`. Recommended topics: - `/sys/adapter//availability` - `/sys/adapter//stats` - `/sys/adapter//error` - `/sys/adapter//dlq` Recommended uses: - `availability`: retained adapter liveness - `stats`: low-rate counters such as published points or dropped messages - `error`: structured adapter errors that deserve operator attention - `dlq`: dead-letter payloads for messages that could not be normalized This keeps operational concerns separate from semantic buses and avoids polluting historian input. Debugging, replay diagnostics, and adapter-internal observability MUST be published under `/sys/...`, not embedded into semantic bus payloads. --- ## Retained Message Lifecycle Retained topics are part of the contract and require explicit lifecycle handling. Rules: - `meta`, `last`, and `availability` SHOULD be retained when they represent current truth - when a retained topic must be deleted, publish a zero-byte retained message to the same topic - adapters SHOULD clear retained topics when an entity is decommissioned or renamed - consumers MUST tolerate retained data arriving before or after live traffic --- ## Historian Ingestion Defaults Historian workers SHOULD apply the following defaults: - ingest `value` streams by default - interpret `meta.historian.mode` as the semantic category of the `value` stream, for example `sample`, `state`, or `event` - ignore `last`, `set`, `meta`, and `availability` as time-series samples Current PostgreSQL historian compatibility: - numeric and boolean samples are directly compatible with `tdb_ingestion/mqtt_ingestion_api.md` - string enum states are valid on the semantic bus, but SHOULD stay out of historian until an explicit encoding policy exists - counter-style cumulative metrics such as `energy_total`, `*_bytes_total`, and `*_packets_total` are valid bus metrics, but the current measurement API does not define their storage semantics; see `tdb_ingestion/counter_ingestion_api.md` - if enum state ingestion is needed, the worker MUST map it to an agreed numeric or boolean representation before calling PostgreSQL Default field mapping: - `value` from payload or envelope - `observed_at` from envelope if present, otherwise ingestion time - `unit` from envelope if present, otherwise cached retained `meta.unit` - `quality` from envelope if present, otherwise `good` This allows historian workers to stay generic while bus contracts remain strict.