A change detection node for Node-RED

Change detecting works by keeping track of

• when the last message was processed (typically Date.now()) and
• what the value of the last message was (typically msg.payload)

By keeping track of timestamp and value by identifier (typically msg.topic) the node is able to determine if a message timestamp or value has changed enough according to the criteria defined.

Individual change detection nodes- if more than one is used- are independent. Each node builds its own cache of message identifiers and time/value history. Note that this map will continue to grow with each new identifier found.

Message that fulfill the change detection criteria are forwarded to the pass output, all other messages are forwarded on the skip output.

To use this node configure the following settings.

minTimeDelta specifies the minimum time resolution in milliseconds. If another message of the same identifier arrives within minTimeDelta it will be removed.

minValueDelta specifies the minimum value resolution. If another message of the same identifier arrives that is not different from the previous message by at least minValueDelta it will be removed.

maxTimeDelta can be used to ensure that once in a while a message is passed even if it's value did not change. Specifying maxTimeDelta is only helpful if matching mode is AND.

Conditions matching mode indicates under what condition messages are allowed to pass:

• OR: messages pass if either the minValueDelta or minTimeDelta conditions are true. This is the default.
• AND: messages only pass if both the minValueDelta and minTimeDelta conditions are true