@ralphwetzel/node-red-context-monitor 2.0.0

A Node-RED node to monitor a context.

npm install @ralphwetzel/node-red-context-monitor

@ralphwetzel/node-red-context-monitor

A Node-RED node to monitor a context.

What it does

This node allows to setup the reference to a context, then sends a message when this context is written to.

It sends a dedicated message on a separate port in case it detects that the value of the context was changed.

The message sent will carry the current value of the context as msg.payload, the context key as msg.topic.

Monitoring details will be provided as msg.monitoring:

• The monitoring setup: scope & key always, flow (id) / node (id) if applicable.
• The id of the node that wrote to the context as source.

The message sent off the change port carries an additional property in msg.monitoring:

• The value overwritten as previous.

It is possible to monitor an infinite number of contexts with each instance of this node.

This node supports the three context scope levels Global, Flow & Node.

Installation

Use the Node-RED palette manager to install this node.

Details

To monitor a Global scope context, set the scope to Global and provide the context key.

To monitor a Flow scope context, set the scope to Flow, then select the owning flow and provide the context key.

To monitor a Node scope context, set the scope to Node, then select flow & node and provide the context key.

Hint: This node doesn't create a context. It just tries to reference to those already existing. If you're referencing a non-existing context, no harm will happen.

Monitoring objects stored in context

You may of course define a setup that monitors objects stored in context.

If you create a reference to this object (stored in context) and write to its properties, this node issues its messages accordingly.

Disclaimer: Monitoring changes to elements of an Array currently is not supported.

Example:

Monitoring context definition:

Code in a function node:

    // suppose, test_flow = { prop: "value" }
    let obj = flow.get("test_flow");
    obj.prop = "new";

Message sent by the node:

Object property monitoring

You may define a setup that doesn't monitor the (whole) object, but only one of its properties:

Such a monitor will react only, when this property and - if it's an object - its child properties are written to.

Subflow considerations

You may use this node in subflows.

Monitoring a Node scope context of a node within a subflow is only supported if both nodes (the context monitor and the node to be monitored) belong to the same subflow. Please select Current Flow when defining the monitoring context reference.

Please be aware that all instances of a subflow commonly share a single Flow scope context. According to the Node-RED documentation, you can reference the context of the superior flow by prepending $parent to the context key.

Handling of context definition issues

If you edit this node's properties, it ensures that the context reference defined is always valid. By copy/paste-ing a node already configured its configuration yet might become invalid. Same applies if you delete flows or nodes that were referenced earlier. To ensure the setup, the node does a lazy validation of the context reference definition - when editing its properties and when it is (re)launched. You yet won't get a "red triangle" even if the reference became invalid - to allow a smooth flow editing experience.