{
  "$id": "realtime-message-envelope.json",
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Realtime Message Envelope",
  "description": "A canonical envelope describing a single message flowing across a realtime channel — the framing that providers wrap around user payloads. This envelope generalizes the message shapes used by Ably, Pusher, PubNub, Socket.IO, MQTT 5 (with user properties), graphql-ws, and CloudEvents-over-WebSocket.",
  "type": "object",
  "required": [
    "id",
    "channel",
    "data"
  ],
  "properties": {
    "id": {
      "type": "string",
      "description": "A unique identifier for the message — typically a ULID, UUID, or provider-assigned message ID. Used for deduplication and resume-from-checkpoint."
    },
    "channel": {
      "type": "string",
      "description": "Channel name to which the message was published."
    },
    "event": {
      "type": "string",
      "description": "Application-defined event name or message type (e.g., 'message.created', 'order.updated', 'user.joined'). Maps to Ably 'name', Pusher 'event', Socket.IO event names, and CloudEvents 'type'."
    },
    "data": {
      "description": "The application payload. Often JSON, but may be a base64-encoded binary buffer, plain text, or a primitive value.",
      "oneOf": [
        {
          "type": "object",
          "additionalProperties": true
        },
        {
          "type": "string"
        },
        {
          "type": "number"
        },
        {
          "type": "boolean"
        },
        {
          "type": "array",
          "items": {}
        },
        {
          "type": "null"
        }
      ]
    },
    "encoding": {
      "type": "string",
      "description": "Encoding of the data field when not JSON.",
      "enum": [
        "json",
        "utf-8",
        "base64",
        "protobuf",
        "msgpack",
        "cbor",
        "binary"
      ]
    },
    "contentType": {
      "type": "string",
      "description": "IANA media type of the payload (e.g., 'application/json', 'application/vnd.example.event+json')."
    },
    "publishedAt": {
      "type": "string",
      "format": "date-time",
      "description": "Time the message was accepted by the realtime provider."
    },
    "receivedAt": {
      "type": "string",
      "format": "date-time",
      "description": "Time the message was delivered to the subscribing client (set client-side)."
    },
    "clientId": {
      "type": "string",
      "description": "Identifier of the publishing client connection or user. Maps to Ably clientId, MQTT ClientID, Socket.IO socket.id."
    },
    "connectionId": {
      "type": "string",
      "description": "Provider-assigned identifier for the transport connection that originated the message."
    },
    "qualityOfService": {
      "type": "string",
      "description": "Delivery guarantee applied to this message.",
      "enum": [
        "at-most-once",
        "at-least-once",
        "exactly-once"
      ]
    },
    "sequence": {
      "type": "integer",
      "description": "Per-channel monotonically increasing sequence number for ordering and gap detection.",
      "minimum": 0
    },
    "correlationId": {
      "type": "string",
      "description": "Correlation identifier linking this message to a request, trace, or higher-level workflow."
    },
    "replyTo": {
      "type": "string",
      "description": "Channel name on which the publisher expects a response (request/response pattern over pub/sub)."
    },
    "expiresAt": {
      "type": "string",
      "format": "date-time",
      "description": "Time at which the message is considered stale and may be discarded by the broker or consumer."
    },
    "headers": {
      "type": "object",
      "description": "Provider-level metadata or user properties (modeled on MQTT 5 User Properties, AMQP application properties, and HTTP headers).",
      "additionalProperties": {
        "type": "string"
      }
    },
    "trace": {
      "type": "object",
      "description": "Distributed tracing context propagated with the message.",
      "properties": {
        "traceparent": {
          "type": "string",
          "description": "W3C traceparent header value."
        },
        "tracestate": {
          "type": "string",
          "description": "W3C tracestate header value."
        }
      },
      "additionalProperties": false
    }
  },
  "additionalProperties": false
}