Node-RED: Lecture 3 – Example 3.2 Using the switch node to handle a JSON object

This example continues to use the MQTT node we setup in Example 3.1. If you aren’t following these examples sequentially you may want to review example 3.1 as it shows how to use the free MQTT broker, HiveMQ which is needed for the MQTT node.

One of the nice features of having a JSON object is that you can easily act on its properties. A useful node for this is the switch node. Its role is to ‘switch’ or route messages depending on the incoming message properties.  For example, you can check the msg.payload.analyze property and, depending on its value (true/false), decide to route a message to one of the switch node’s outputs.

Drag a switch node and double-click on it. Configure it to evaluate the property “msg.payload.analyze”. If true, send the message to the first output; if false, send it to the second output as shown in in Fig 3.4.

Figure 3.4 Configuring the switch node to route based on a message property

Now you can connect two debug nodes as shown in Fig 3.5 – when you set up multiple outputs for a node, they are numbered from the top, so output 1 is the top output and output 2 is at the bottom in  Fig. 3.5.

Figure 3.5 Connecting a switch node to two debug nodes

If you now go back to the HiveMQ input page and send the MQTT message {“analyze”:true, “value”:6}, you will see that the first (top) output is activated and the incoming messages is routed, or ‘switched’, to output 1. If you send the original message {“analyze”:false, “value”:10}, the switch node will activate output 2 and the original debug node will fire. Hovering the pointer over the debug message will show which debug node is printing out the message as shown in Fig. 3.6.

 

Figure 3.6 Confirming the operation of the switch node

As you can see, this provides you with a built-in Node-RED node that allows you to quickly determine the contents of incoming messages and direct the message through to different parts of the flow depending on the input.

 

You can find the node-red description of this flow at:

https://raw.githubusercontent.com/SenseTecnic/nrguideflows/master/lesson3/3-2_switchnode.json

 

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© Lea, Blackstock, Calderon

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Author: Rodger Lea

Currently CEO of Internet of Things startup, Sense Tecnic, Dr. Lea has over 25 years experience spanning academic, large corporations and startups. For the last 10 years, he has started or helped start 4 new companies while managing an active research program (University of British Columbia, Canada and Lancaster University, UK) into distributed and ubiquitous computing, the IoT and Smart Cities.