Open Control Architecture

Controls software is written in Node-RED.

Software works through packets of data (messages) passing along virtual (and real) wires, from one node to another.

A node can be:

• a function block within Node-RED
• an instance of Node-RED
• any point (server or device) in the communications network

Each instance of Node-RED has an identifier, stored under global.node

Communications between systems is performed using MQTT using a standardised 5 level topic structure. network / node / device / group / key.

Systems run a local MQTT broker to distribute data both internally and to enable external subscriptions.

The external network identifier, used for all data published to external MQTT brokers, is stored under global.networkId

Data from within a node or from connected devices is assigned a local network identifier of "local" (stored under global.localNetworkId) so that the node's external network identifier can be changed without affecting internal software. Separate internal and external identifiers also allows for separate message rates, where local traffic can be high speed. Node-RED is used to apply Reporting By Exception rules, to limit traffic to significant values (e.g. 0.25C), and save bandwidth.

Within Node-RED a Message Handler Node is used to handle data generated locally.

• The Node has two outputs - one for local traffic, and one for external traffic.
• Report by exception rules are applied to external traffic.
• The Node formats topics to the five level standard, adding group and device if missing.
• The Node stores data into global.readings, along with any attributes within the incoming message, as well as timestamps.
• Additional routing can be performed within Node-RED, including selection of retained vs fire & forget routes.

Where a number of nodes sit on a Local Area Network (LAN) it is common to use a central local MQTT broker.

Traffic can be routed to a Cloud MQTT broker by the use of MQTT Bridges. These run on an MQTT broker, and can re-publish messages filtered by topic (networkId) to another broker. Bridges are managed outside of Node-RED.

Additional Report By Exception rules can be applied inline.

On larger networks where data is shared between local nodes, Reporting By Exception can be centrally managed using a Node-RED instance, along with any additional MQTT broker functions such as providing backup routes.

Cloud based instances of Node-RED, as used on this Wiki, are often used to pull together data from various MQTT brokers into one place.

The topic group is used to define the type of data. Values include:

• dat (operational data)
• stat (statistics and status)
• settings
• cmd (a command)
• set (change a setting)
• system (software and networking)

Attributes for data may be loaded from a central index (GitHub hosted), providing additional information such as units and descriptions. This is to avoid the need to locally describe data, and to assist in compatibility.

Within Node-RED, data and attributes are stored within the global.readings object.

The current data can be viewed in the data explorer.

Historic data is saved into the global.readingsHistory object, in the same structure as global.readings, but containing an array of timestamped values.

Persistent settings for a node (or application) are held on disk and in the global.settings object. This is for local operational settings.

Settings have a type, may have maximum and minimum values, units and a title. Certain types require additional fields such as select from a list settings.

The snapshot below shows a variety of setting types including numeric/text values, html, or JSON objects and arrays.

Persistent settings for local devices are held on disk and in the global.deviceSettings object, and are specific to a device (or node).

Data within the readings object is nested according to the topic (excluding the node as more than one node may publish data on a device).

e.g. a message with a topic myNetwork/node123/myDevice/dat/outputTemperature  would be stored in (global).readings.myNetwork.myDevice.dat.outputTemperature.value

Internally, message topics can be truncated and the network and node will be filled in. The data group will be assumed to be "dat" if not supplied, representing operational data.

e.g. a message with a topic setpoint  would be stored in (global).readings.local.myNode.dat.setpoint.value

The following code could be used to read data and its attributes for output.

 var localNetworkId = global.get("localNetworkId");
 var node = global.get("node");

 var reading = global.get("readings." + localNetworkId + "." + node + ".dat.outputTemperature");

 var output = "The value of " + reading.title + " is " + reading.value + reading.units;  
 // "The value of Output Temperature is 35°C