{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://kiosk.tech/spec/schemas/event.schema.json",
  "title": "Kiosk event",
  "description": "One message the operator pushes on the event stream at <endpoint>/events. The root is CLOSED: an event carries the five members declared here. The topic's `payload_schema` in the catalog gives the shape of `data`. `$defs` holds the two other messages of a subscription: `subscribed` and `unsubscribed`.",
  "type": "object",
  "required": ["id", "topic", "subject", "occurred_at", "data"],
  "additionalProperties": false,
  "properties": {
    "id": {
      "type": "integer",
      "minimum": 1,
      "maximum": 9007199254740991,
      "description": "A per-origin monotonic integer, at most 2^53 - 1, which a subscriber presents as `since`. It names one event in ONE identity's stream, not the change behind it. An event can come more than once: a subscriber MUST ignore an id it has already seen."
    },
    "topic": {
      "type": "string",
      "pattern": "^[a-z][a-z0-9_]*$",
      "description": "The topic this event belongs to, as declared in the catalog."
    },
    "subject": {
      "type": ["string", "null"],
      "description": "What the event is ABOUT within the topic -- an order id, a list id. Null for a topic that has no subject. A subject-scoped subscription sees only its own."
    },
    "occurred_at": {
      "type": "string",
      "format": "date-time",
      "description": "When the operator's state actually changed, RFC 3339 in UTC. Not when the message was delivered: a replayed event carries the time of the transition, which is what a subscriber reasons about."
    },
    "data": {
      "description": "The payload, validating against the topic's payload_schema."
    }
  },
  "$defs": {
    "subscribed": {
      "type": "object",
      "description": "The first message of an accepted subscription (Section 8.5.4).",
      "required": ["type", "topic", "subject", "head", "truncated"],
      "additionalProperties": false,
      "properties": {
        "type": { "const": "subscribed" },
        "topic": { "type": "string", "pattern": "^[a-z][a-z0-9_]*$", "description": "The topic of the subscription." },
        "subject": { "type": ["string", "null"], "description": "The subject of the subscription. Null for a subscription that names no subject." },
        "head": { "type": "integer", "minimum": 0, "maximum": 9007199254740991, "description": "The origin's current event id: the cursor that the subscriber records." },
        "truncated": { "type": "boolean", "description": "True when the operator cannot prove that the subscriber saw every event after its `since`. The subscriber then reads the state one time through the verb that owns it." }
      }
    },
    "unsubscribed": {
      "type": "object",
      "description": "The message that ends a subscription when the operator withdraws the subscriber's reach (Section 8.5.6).",
      "required": ["type", "topic", "reason"],
      "additionalProperties": false,
      "properties": {
        "type": { "const": "unsubscribed" },
        "topic": { "type": "string", "pattern": "^[a-z][a-z0-9_]*$", "description": "The topic of the subscription." },
        "reason": { "type": "string", "description": "Why the subscription ended: `reach_revoked` when the subscriber's reach to the subject is withdrawn." }
      }
    }
  }
}