Control Message Structure

This page is to be used for design notes around the control channel.

The control channel is a separate Socket.IO channel to the data channels (The server channel sends from the server to the client, the client channel goes from the client to the server). Each instance of a uibuilder node gets it own Socket.IO session and you can limit messages to specific clients by using the _clientId (delete it if you want to send a message from a specific client to all clients connected to a node instance).

The control channel is there to keep control information separate to the general data.

The general structure of a control message is:

{ "type": "<text>" }

But other properties can be added as required.

Initially, only 2 control messages have been used.

• type = server connected (server -> client): Sent when Socket.IO connects to client. This message includes an extra property "debug": <boolean>
• type = shutdown (server -> client): Sent when the node instance is either redeployed or removed from Node-RED.

Questions

Please add any questions you want answering here - please start them with your initials

• JK: Should the server have a 2nd output port for passing on control messages? OR should they be passed out of the standard port. If using the standard port, should there be an option to turn them on/off?

  CDL: I suggest the 2nd output port solution. I can't see any reason to not do that, and if the user wants to use the messages then it is simpler if they appear on a different port.

  JK: Thanks Colin, I agree.

• JK: Should the server send out control messages from itself to the chosen output port? Generally, I'm assuming that control msgs go from client to server or server to client. Do we, for example, want a control message when the server loses touch with a client?

Ideas

Please add any ideas you have here for further discussion and design. Please start with your initials.

• CDL: Topic cache/repeater in the server. The suggestion is that (optionally) the uibuilder server will save the most recent payload for each topic it receives. Then on receipt of a 'reload' control message from the client it will send messages containing those topics/payloads back to the client. I think it would be useful if there were a short time delay before sending the messages back as under some circumstances the client may send multiple such requests in succession and by waiting then the requests can be combined. Maybe something like 250ms which will not be noticeable to the user. Probably there is a need for the flow to be able to tell the server to clear its cache, presumably by the flow sending a control message to the node. One further point is the the topic/payload messages sent back to the client should be identifiable as reloads rather than original in case the client app needs to know that. Perhaps add a 'uibuilder_reload' attribute to the messages.

  JK: QUESTION: Does there need to be an option to be able to cache ALL received messages? With the current suggestion, I don't think that will resolve Valter's startup issue where messages need to be retained until a client connects. This also raises some questions about sizes and how we ensure that Node-RED doesn't explode from too much data in the cache.