nemo-mqtt-bridge

Knowing a tool's status (interlock enabled or disabled) is critical in most labs using NEMO, but many setups only indicate this via NEMO itself or a simple LED. This project enables NEMO to send MQTT messages to displays on each tool, providing detailed, real-time status information such as current user, start time, and previous user.

The hardware, firmware, and broker code associated with this project can be found at: https://github.com/alexanderenrique/NEMO-Tool-Display

This is a Django plugin that publishes NEMO tool usage events to MQTT (tool enable/disable, tool saves). Uses PostgreSQL LISTEN/NOTIFY and a separate bridge process to keep broker connections out of Django.

Architecture

┌─────────────────┐    ┌──────────────────┐    ┌─────────────────────┐    ┌─────────────┐
│   Django NEMO   │───▶│   PostgreSQL     │───▶│ PostgreSQL–MQTT      │───▶│ MQTT Broker │
│  (signals)      │    │  (event queue)   │    │ Bridge (standalone)  │    │             │
└─────────────────┘    └──────────────────┘    └─────────────────────┘    └─────────────┘

• Django: Signal handlers (Tool save, UsageEvent, Tool operational status) insert into MQTTEventQueue and use pg_notify to wake the bridge.
• Bridge: Separate process runs python -m NEMO_mqtt_bridge.postgres_mqtt_bridge; it LISTENs for notifications, fetches events, and publishes to the MQTT broker with QoS 1.
• Topics (usage status): nemo/tools/{id}/enabled, nemo/tools/{id}/disabled
• Topics (operational status): nemo/tools/{id}/operational, nemo/tools/{id}/non-operational

Configuration is stored in Django (e.g. /customization/mqtt/) and loaded by the bridge on each connection.

Stakeholders and PostgreSQL NOTIFY

Two channels coordinate Django and the standalone bridge:

Event streams: There are no per-category toggles. Every supported signal path enqueues when its NEMO event occurs (while MQTT is enabled and PostgreSQL is available).

Multi-worker note: Django’s mqtt_active_config entry lives in your configured cache backend. With per-process caches (e.g. LocMem), workers other than the one that saved a config may see stale broker settings until the TTL expires or they read fresh data; use a shared cache (Redis/Memcached) if all web workers must see broker changes immediately. The bridge process always reloads from the DB on reconnect after a reload notification or fingerprint change.

Tool operational vs. down (per-tool status)

In addition to usage events (who enabled/disabled the tool), the plugin publishes operational status so displays can show when a tool is marked down or back up:

• nemo/tools/{id}/operational — emitted when the tool becomes operational again (e.g. problem cleared, forced-shutdown task resolved).
• nemo/tools/{id}/non-operational — emitted when the tool is marked non-operational (e.g. a task with “force shutdown” is created).

These events use NEMO’s tool_enabled / tool_disabled signals and are independent of who is currently using the tool. Payloads include event, tool_id, tool_name, operational (boolean), and timestamp (ISO). See src/NEMO_mqtt_bridge/monitoring/README.md for payload examples.

Installation

Prerequisites: Python 3.8+, Django 3.2+, NEMO-CE 4.0+, MQTT broker (e.g. Mosquitto), PostgreSQL 12+ (NEMO's database; 15, 16, 17, 18 tested). The plugin uses the same PostgreSQL database as NEMO; no Redis required.

Simplified deployment: The plugin package is NEMO_mqtt_bridge. Add 'NEMO_mqtt_bridge' to INSTALLED_APPS, then run python manage.py setup_nemo_integration (use --write-urls to add the URL include to NEMO/urls.py) and python manage.py migrate NEMO_mqtt_bridge.

From PyPI (recommended)

pip install nemo-mqtt-bridge
cd /path/to/your/nemo-ce
# Add 'NEMO_mqtt_bridge' to INSTALLED_APPS in your settings (see Manual below).
python manage.py setup_nemo_integration
python manage.py migrate NEMO_mqtt_bridge

Local / testing: The command above only prints integration steps (no file changes). Add NEMO_mqtt_bridge to INSTALLED_APPS and any logging config yourself. Use --write-urls to add the MQTT URL include to NEMO/urls.py.

Production with GitLab/Ansible: If your config is in version control and deployed by GitLab or Ansible, run with --gitlab so no files are changed on the server; the command will print the snippets to add to your repo:

python manage.py setup_nemo_integration --gitlab
# Add the printed snippets to your repo (INSTALLED_APPS and URLs; configure logging as needed for your environment), commit, and deploy. Then on the server:
python manage.py migrate NEMO_mqtt_bridge

Manual

1. pip install nemo-mqtt-bridge
2. Add 'NEMO_mqtt_bridge' to INSTALLED_APPS in your settings.
3. (Optional) If you use Django's LOGGING setting, add a NEMO_mqtt_bridge logger with your preferred level and handlers (e.g. DEBUG in dev/test, INFO or WARNING in production). To correlate lines across Gunicorn/Uvicorn workers, include %(process)d (and optionally %(thread)d) in your log format string for the relevant handlers. Bridge lifecycle messages from this plugin also include a [NEMO_mqtt_bridge pid=… thread=…] prefix.
4. Add path("mqtt/", include("NEMO_mqtt_bridge.urls")) to NEMO/urls.py (or run python manage.py setup_nemo_integration --write-urls). Skip this step for Docker/pip installs—NEMO auto-includes plugin URLs (see Plugin URLs).
5. Run python manage.py migrate NEMO_mqtt_bridge.

After install

1. Configure: Open /customization/mqtt/ in NEMO, set broker host/port (and auth if needed), enable the config.
2. Run the PostgreSQL–MQTT bridge so queued events reach the broker. By default the bridge does not run inside Django; when NEMO starts, the plugin auto-spawns a detached bridge_supervisor process (which runs postgres_mqtt_bridge with auto-restart). Disable with NEMO_MQTT_BRIDGE_SPAWN_SUBPROCESS=0 if you prefer a separate service, e.g. python -m NEMO_mqtt_bridge.postgres_mqtt_bridge (second terminal, systemd unit, or a dedicated Docker Compose service—see Docker below). For simple single-process dev only, you may set NEMO_MQTT_BRIDGE_RUN_IN_DJANGO=1 so AppConfig.ready() embeds the bridge in the same process as runserver (avoid with Gunicorn/Uvicorn and multiple workers).
3. Start NEMO (e.g. python manage.py runserver). With the default AUTO mode, the bridge can use an embedded MQTT broker (mqttools, pure Python) for development. No separate Mosquitto binary required in that mode.

Production: Use EXTERNAL mode so the plugin does not start or kill brokers. Set NEMO_MQTT_BRIDGE_AUTO_START=0 (env) or NEMO_MQTT_BRIDGE_AUTO_START = False in Django settings. Then start the MQTT broker yourself, and run the bridge separately (e.g. python -m NEMO_mqtt_bridge.postgres_mqtt_bridge or as a systemd service).

Bridge in Django vs separate process: Django writes to MQTTEventQueue and uses pg_notify (via the DB publisher); it does not need a long-lived MQTT connection in the web workers. The standalone bridge process LISTENs, drains the queue, and publishes to MQTT. Default: the bridge is not started inside Django. Set NEMO_MQTT_BRIDGE_RUN_IN_DJANGO=1 (or true / yes / on) in the environment, or NEMO_MQTT_BRIDGE_RUN_IN_DJANGO = True in Django settings, to embed the bridge in-process (dev/simple installs). Set 0 / false / no / off to force off. The bridge can idle without an enabled MQTT config and pick up settings when you enable them, without restarting Django.

Single container / auto-spawn (default on): With NEMO_MQTT_BRIDGE_RUN_IN_DJANGO=0 (default), NEMO_MQTT_BRIDGE_SPAWN_SUBPROCESS defaults to on: each Gunicorn worker schedules a short background task with random jitter and a launcher file lock so typically one worker starts a detached bridge_supervisor; others see the bridge lock PID and skip. Set NEMO_MQTT_BRIDGE_SPAWN_SUBPROCESS=0 to turn this off. If spawn and RUN_IN_DJANGO are both enabled, the plugin logs an error and uses detached subprocess only. Spawning is skipped for manage.py migrate, test, shell, etc., and when NEMO_MQTT_BRIDGE_SPAWN_SKIP=1. Optional: NEMO_MQTT_BRIDGE_SPAWN_USE_SUPERVISOR=0 (plain bridge module), NEMO_MQTT_BRIDGE_SPAWN_JITTER_SEC, NEMO_MQTT_BRIDGE_SPAWN_LOCK_WAIT_SEC. Horizontally scaled deployments (many app replicas) can run one bridge per replica filesystem—you may get duplicate MQTT publishes; prefer a dedicated bridge service or a single replica if that matters.

Docker: Use the same image as the web app for a second Compose service that runs only the bridge so docker compose up starts NEMO and the bridge together. Mirror database-related environment (and DJANGO_SETTINGS_MODULE) so the bridge loads the same Django settings and can reach Postgres.

services:
  db:
    image: postgres:15
    # ... your Postgres config ...

  nemo:
    image: your-nemo-image
    environment:
      DJANGO_SETTINGS_MODULE: settings
      # Do not embed the bridge in Gunicorn workers (default since 2.2.0):
      NEMO_MQTT_BRIDGE_RUN_IN_DJANGO: "0"
    command: ["gunicorn", "..."]
    depends_on:
      - db

  nemo_mqtt_bridge:
    image: your-nemo-image
    environment:
      DJANGO_SETTINGS_MODULE: settings
      NEMO_MQTT_BRIDGE_RUN_IN_DJANGO: "0"
    command: ["python", "-m", "NEMO_mqtt_bridge.postgres_mqtt_bridge"]
    depends_on:
      - db

The bridge only needs Postgres + settings (not HTTP); depends_on: [nemo] is optional if you want start ordering. Run exactly one bridge replica: do not use docker compose up --scale nemo_mqtt_bridge=2. A file lock in the container still enforces a single bridge process per host if something starts a duplicate by mistake. If the web container would also auto-spawn a bridge, set NEMO_MQTT_BRIDGE_SPAWN_SUBPROCESS=0 there so only the dedicated service runs the bridge.

Adjust image names, env (secrets, DATABASE_URL / Django DB vars), and networks to match your stack. For production with an external MQTT broker, set NEMO_MQTT_BRIDGE_AUTO_START=0 and point MQTT customization at that broker (e.g. broker_host=mqtt for a Compose service named mqtt). For local-style AUTO mode, the embedded broker (mqttools) can run inside the bridge container.

Supervisor (auto-restart): To restart the bridge automatically when the process exits (and optionally when the DB heartbeat goes stale), run python -m NEMO_mqtt_bridge.bridge_supervisor or the nemo-mqtt-bridge-supervisor console script instead of invoking postgres_mqtt_bridge directly. Use --db-health (or NEMO_MQTT_SUPERVISOR_DB_HEALTH=1) only when DJANGO_SETTINGS_MODULE and DB access match the bridge; tune intervals via NEMO_MQTT_SUPERVISOR_INTERVAL, NEMO_MQTT_SUPERVISOR_STALE_SEC, NEMO_MQTT_SUPERVISOR_GRACE_SEC, etc. Do not run both a plain bridge service and a supervisor that spawns the same module on the same host—they compete for the bridge lock. After upgrading, run python manage.py migrate NEMO_mqtt_bridge so last_heartbeat exists.

Troubleshooting (cyclic bridge / workers)

• Gunicorn workers restarting or “multiple bridges”: Ensure NEMO_MQTT_BRIDGE_RUN_IN_DJANGO=0 (or unset with no True in settings). Embedding the bridge with multiple workers causes lock contention; since 2.2.1, extra workers skip starting the in-process bridge instead of exiting, but you should still run one standalone bridge and keep Django out of the bridge entirely.
• Standalone bridge dies when something uses AUTO mode: Older builds ran pkill -f postgres_mqtt_bridge during AUTO cleanup. That is now disabled by default; enable only for local debugging with NEMO_MQTT_BRIDGE_DEV_PKILL=1.

Plugin URLs

Where to find them depends on how NEMO loads the plugin:

• Docker / pip-installed NEMO: The app is often mounted at the site root, so use /mqtt_bridge_status/ (and /mqtt_monitor/ redirects there).
• Source install with --write-urls: If you add path("mqtt/", include("NEMO_mqtt_bridge.urls")) to NEMO/urls.py, paths are under /mqtt/: e.g. /mqtt/mqtt_bridge_status/, /mqtt/mqtt_monitor/ → redirect.

All of these require login. If you get a 404, check which URL scheme your NEMO uses (auto-include vs manual).

Response fields (summary): status and updated_at describe the MQTTBridgeStatus DB row (broker connection as recorded by the bridge). bridge_status_row_updated_at is the same timestamp, named explicitly. last_heartbeat is the bridge consumption-loop heartbeat. bridge_last_reload is when the bridge process last reapplied configuration from the database (at + reason); this is not the same as saving the MQTT customization form. mqtt_configuration is the current MQTTConfiguration row (updated_at = last save in NEMO). applied_mqtt_configuration mirrors diagnostics.applied_fingerprint (the config version last applied by the bridge). queue has pending_count and optional pending timestamps. package_version is the installed nemo-mqtt-bridge version (distribution metadata when available); plugin_version is the same string (alias). diagnostics never includes applied_config_snapshot. See mqtt_bridge_status_payload() in utils.py for the full docstring.

• Robustness roadmap: Phases 1–5 in docs/ROBUSTNESS_PLAN.md are implemented in 2.1.5 (idle bridge until MQTT enabled, processed-only-on-publish, LISTEN reconnect, close_old_connections, NEMO_MQTT_BRIDGE_RUN_IN_DJANGO). 2.2.0 changes the default so the bridge does not run in Django unless opted in. Phase 6 items in the robustness doc remain optional.
• Monitoring: Connection and diagnostics JSON at /mqtt_bridge_status/ (or /mqtt/mqtt_bridge_status/ when under mqtt/); CLI tools in NEMO_mqtt_bridge.monitoring (see src/NEMO_mqtt_bridge/monitoring/README.md).
• License: MIT. Issues · Discussions