MQTT broker

The station's own MQTT broker: its row in the Communication column, turning it on and off, its port and the devices connected, client authentication, the entry This station it gives the MQTT driver, and what it does not do.

The MQTT broker takes the MQTT messages of the devices on your network, so a bench or a small laboratory reads its ESP32 boards, gateways and sensors without installing a broker of its own. It is the third row of the Servers folder, after the Modbus server, and it is not a driver: the devices connect to it and publish, and the station reads what they publish through the MQTT driver, as it reads any broker. Its panel says so under the title: "Takes the MQTT messages of the devices on your network, so they need no broker of their own. It receives connections and never makes one." Viewing the Connector reads the page; configuring the Connector changes it.

The row in the column

The row sits under the Servers folder of the Communication tree, below the Modbus server, with a hub icon where the drivers show a plug. A station has exactly one, and it is never created or deleted. The search box finds it by its name, and the state chips file it by its state: off while it is turned off, issues while its port is in use or it could not listen, OK otherwise. Beside the name it says what the broker is doing, and a dot sums it up:

The dot is the broker's alone: it takes no part in the Drivers folder's roll-up or in the status bar, which are about the equipment the station reads.

Turning it on and off

A role that may not configure the Connector is not drawn the two commands. Being on is a setting the station keeps, and listening follows the runtime: the broker opens its port once the runtime is up, closes it and drops every device when the runtime stops, and opens it again when the runtime comes back. A port it cannot open is its own state, said on the row and under the port field, and the station runs on without it.

The Port and clients card

A port the broker cannot move to is refused before anything is stored, said under the field, and the field goes back to the port in force: a number outside the range ("Port must be between 1 and 65535."), the port of the web server, of agent access, of the OPC UA server or of the Modbus server ("Port N is already the Modbus server. Pick another port, or move the Modbus server first."), and a port another program holds ("Port N is in use on this machine. Close what is holding it, or pick another port."). The station's other port fields refuse the broker's port in turn: "Port N is already the MQTT broker. Pick another port, or move the MQTT broker first."

Under the card a note says: "No TLS: devices connect in plain MQTT, at most 100 at once. Other machines reach this broker only if Windows Firewall lets TCP port 1883 in: allowing it is your decision, and this station never does it on its own. Retained messages are kept until the station restarts."

At most 100 devices are connected at once, the station's own session included: one more is refused with the MQTT reason for an exceeded quota, and a device that reconnects under its own client ID takes its old place back. Retained messages are kept in memory: they come back after the runtime stops and starts again, and they are gone when the station restarts.

Windows Firewall

The station never changes Windows Firewall by itself. Under the note, a dot and a sentence say whether its own inbound rule for the broker lets the port in: "Inbound MQTT connections are allowed on port 1883 for private and domain networks.", "Network clients may be blocked because no app-owned rule allows port 1883.", or, on a launch that may not manage the firewall, such as an isolated profile or a copy that is not the installed program, "This launch cannot manage Windows Firewall. Configure inbound access in the operating system if needed." Only while the rule is missing, Allow local network asks Windows, with your approval, for a rule named "Ganter Lab MQTT broker": inbound TCP on the port, private and domain networks, the local subnet, this program. It is pressed from the station's own app ("The firewall rule can only be applied from the station's desktop app."), and the feed says "Requesting Windows Firewall access for MQTT port 1883…", then "Windows Firewall now allows MQTT connections on port 1883."

Client authentication

The Client authentication card decides who may connect:

Under the choice stand Username and the password, whose label says only whether one is stored: Password (stored) or Password (none stored). The stored password is never drawn back into the page: the field starts empty, and "Leave it empty to keep the stored password." The choice, the username and the password are applied together with Apply, greyed while nothing changed ("Change the authentication to apply it."); the feed then says "MQTT broker authentication saved.", and, as the line beside the button warns, "A device already connected that the new authentication does not admit is disconnected."

Under Anonymous, the username and the password stay stored for the next time. The password is kept like the password of a broker the station reads, protected for this Windows account: a backup opened on another machine or account arrives without it, no device signs in until it is typed again, and the card says "Cannot be read on this Windows account: it only opens on the machine and the Windows account that entered it. Enter it again here."

This station

The panel's last card, This station, says what the broker gives the MQTT driver: "Turned on, the broker adds the entry This station under the MQTT driver, through which this station reads the devices that publish here: on 127.0.0.1, with the broker's port and authentication. Add those devices under This station, by a scan or by hand. Turning the broker off leaves This station and its devices in place, disconnected."

Nothing is added under the entry by itself: add the devices that publish to the broker with the broker's scan or by hand, as under any broker of the MQTT driver. The station's own session waits longer between attempts while the broker is off, so after the broker is turned on, the entry can take up to 30 seconds to read online. This station also carries the Publish card every broker does: what the station publishes of its own data to its own broker.

Agent access

Neither name begins with mqtt_broker_, the tools of the brokers the station reads; both are described with every other tool on Tools.

What it does not do

• No TLS and no WebSocket: devices connect in plain MQTT over TCP.
• One username and password for every device, never one per device.
• No persistent sessions, and no retained message survives a restart of the station.
• It stops with the station: devices that keep publishing while the station is off need a broker of their own (Installing a broker).