Add support for subscriptions for event based communication

[Rednas83]

Can you also add support for subscriptions like with opc ua and mqtt?

Something like

PlcProgram.Subscribe
{
"var": "\"MotorSpeed\""
}
MotorSpeed.on("changed", (dataValue: DataValue) => {
   console.log(" value has changed : ", dataValue.value.toString());
});

Returning
value has changed : 100

[KircMax]

@Rednas83 I can think of a "SubscriptionFaker" Service that would basically poll repeatedly and give an update when available with a configurable polling timer - it will be way less performant w.r.t load on the plc and of course less deterministic when running in the plc integrated but if it would help it could be a way.
what do you think of that?

[Rednas83]

This could be indeed an option if performance is not a requirement.

But why not send the changes directly from the PLC with MQTT which supports messages up to 8kb?

With some helper functions for sending the events
PublishAny for sending objects like reports as raw data which can be decoded with the Program.Browse api
PublishSensor for sending sensors with a deadband
PublishAlarm for sending alarms
SyncSettings for synchronizing settings

This would make the plc/webserver fully event based with only a few small PLC modifications.

[KircMax]

@Rednas83
I guess the concept makes sense but I would not provide anything within this lib for that for since the Lib is for the web-api "only" (not any MQTT-lib / Userprogram stuff in the plc). Also it will not generically work for anybody using the webserver/web-api but only for those who have just that implementation in the user program. I would see that use case as an application example to be provided via SIOS when theres enough customer requests / demand for it.

[Rednas83]

Fair enough!

I will be able to workaround it by using mqtt and webapi as seperate libraries.

• webapi for browsing
• mqtt for subscribing

What about your solution with the SubscriptionFaker to poll for changes?

I think this could also be a very nice idea if there is a need for subscriptions without the need of very fast performance.

[KircMax]

Hi @Rednas83
I guess that can be included. I also see a bit of optimization potential since the Bulk Requests can be used to make the request amount less stressful.
How urgent is your request in general about it or is it just for general purpose?
If so you can consider this issue as it being on the roadmap.
Also feel free to create a PR about it :)

[Rednas83]

I would only recommend polling for 1:1 connections like with HMI/SCADA
With multiple connections I would recommend UA for slow speed and MQTT for high speed. Advantage with MQTT is that you can run the broker externally like Mosquitto/Solace and you are in full control of the events and the content of the messages making it easily 10x faster then standard UA.

Perhaps WebAPI+SubscriptionFaker could be interesting but it's going to be about 10x slower then standard UA communication and probably 100x slower then custom MQTT communication. See also https://www.se.com/nl/nl/product-range/66388-ecostruxure-opc-ua-server-expert/#products which is a similar product

Protocol	Maturity	Speed	Discoverable	Event based	Broadcast	Rating
polling (default)	✅	1x	❌	❌	❌	⭐ Standard for 1:1 connections but can miss events because of the low speed
webapi	❓	1x	✅	❌	❌	⭐⭐ Based on polling with support for browsing
ua	✅	7x	✅	⚠️	✅	⭐⭐ Quick and easy event based messaging but not very fast and requires a license
rpc	⚠️	50x	⚠️	✅	❌	❌ Remote procedure calls but restricted to siemens functions
ua pubsub	✅	100x	❓	✅	✅	⭐⭐ Custom event messages based on OT standards but not easy to publish objects like reports
mqtt	✅	100x	⚠️	✅	✅	⭐⭐ Custom event messages based on IT standards but requires custom browsing
mqtt+webapi	❓	100x	✅	✅	✅	⭐⭐⭐ Custom event messages based on IT standards using the webapi for browsing