Node-RED, the "delay" node

In this article I will show you how to use the "delay" node to introduce a delay in the propagation of a message through a flow, and to limit the rate of messages coming through.

The rate-limiting function is very useful, for example, when you want to connect your flow to an external resource on the cloud. Typically, IoT resources impose a limit to how often you can "hit" them. If your application exceeds this limit, your account can be suspended, or at least made in-operable for an amount of time. With the "delay" node, you can ensure that your flow does not "hit" the IoT resource beyond a specific rate.

Setup the "delay" node

To explain how to use the "delay" node, I have create this simple flow:

I am using an "inject" node to send a timestamp to two "debug" nodes.

I'll be doing two experiments with the delay node. For both, the configuration of the inject node is the same, and it looks like this:

Experiment one: simple delay

In the first experiment, I want to use the "delay" node to hold the incoming message for 5 seconds, and then pass it on to the "debug" node.

To achieve that, I have set the "delay" node like this:

Deploy the flow, and click on the inject node button.

The first "debug" node will display the timestamp immediately.

However the second "debug" node is fed by the delay node, which holds the message for 5 seconds before it lets it through.

The second "debug" node will display the message exactly 5 seconds after the first one, as you can see from the message timestamps, marked by the yellow boxes:

Experiment two: rate limiter

In the second experiment, I want to use the "delay" node as a rate limiter.

The way that the flow works now, if you click on the "inject" button repeatedly and as fast as you can, you will see a timestamp for each click.

Very quickly, the debug pane will fill up with timestamps:

To reduce the number of messages that get through to the "debug" node to a small number, say one message per second, I can configure the delay node like this:

Now, no matter how fast I can click on the "inject" node button, only one message per second will appear in the debug pane. All intermediate timestamps will be dropped. 

The rate limiting capability of the "delay" node is something use in the terrarium controller project. For example, I have implemented a feature so that the gadget can notify me when the MCU or pump voltage drops below a threshold. I don't want to flood IFTTT (or my inbox) with such messages, so I have used the rate limiter to only send me one message every ten minutes.