Publisher

Port: :4050
Source: apps/workers/publisher/

The publisher delivers real-time MQTT notifications to connected clients. On every MongoDB change event it resolves the set of MQTT topics that should be notified based on the document's owner, shares, and clients fields, then publishes a message to each topic via EMQX's HTTP API.

Responsibilities

• Consume Kafka MongoDB change stream events (MongoSourcePayload).
• Derive MQTT topic paths from the document's access metadata.
• Bulk-publish QoS-2 messages to EMQX for each resolved topic.

Topic Resolution

For a document with owner, shares[], groups[] and clients[] in database db with id and collection, publish() derives one topic per ownership attribute and per cross-scoped pair — nine forms in all, listed once in realtime.md → Topics and Schemas. Duplicate topics are deduplicated before publishing.

Message Format

Each message is published with:

{
  "topic": "<resolved-topic>",
  "qos": 2,
  "retain": false,
  "payload_encoding": "plain",
  "payload": "{\"id\":\"<entity_id>\",\"op\":\"c|u|d\",\"src\":\"<collection-type>\"}"
}

The payload contains only the entity ID, operation type, and source collection — subscribers fetch the full entity from the API if needed.

Bulk Publishing

All resolved messages for a single change event are sent in one publishMessageBulk call to EMQX, minimizing HTTP round-trips.

Infrastructure Dependencies

Dependency	Usage
Kafka	Consumes MongoDB change stream events
EMQX	Receives bulk MQTT publish requests via HTTP API

Key Files

File	Purpose
app.service.ts	publish() — topic resolution and bulk EMQX publish
app.controller.ts	/status, /metrics