How does the rules engine work. where can I start making changes

[rlkoshak (Rich Koshak)]

@rpwong

Rules DSL is what most people use, and it will be deprecated in OH3. Rules built this way will still work, but the intention is to transition to what’s called the Next Generation Rules Engine.

It’s a little more nuanced than that.

Right now there are two rule engines, Rules DSL Engine and NGRE.

Rules DSL Engine:

• only run Rules DSL Rules
• has a limit of only five Rules that can run at the same time (the likely cause of OP’s problem)
• does not have a REST API (i.e. doesn’t appear in PaperUI or what will be it’s replacement)

NGRE:

• currently supports Jython, JavaScript or Groovy
• in OH 3 will also support Rules DSL (seems to be the plan at least)
• supports building rules using text source files or building rules through the REST API (i.e. through a GUI)
• unlike Rules DSL Engine, will only run one instance of a given rule at a time
• unlike Rules DSL Engine, there is no limit to the number of different rules that can run simultaneously

Rules DSL Engine is likely going away. Rules DSL as a language to write rules itself is likely going to be around for a bit pending overcoming technical difficulties.

As with all things OH 3, stuff is in flux and is likely to change several times before the first beta release.

@hannaPolaskus

I had instances were the openHab logs shows an events but the rules doesn’t trigger.

@Senseh_B

what do you mean by hogging rsources? also my rules works fine 99% of the times but sometimes I get a missing events. I just want to know what part of the code trigger the rules and why sometimes logs show the event without the rules triggering?

As Andrew pointed out with the link to that thread, Rules DSL Engine can only run up to five Rules at the same time. If you have long running Rules (anything more than a couple hundred milliseconds is considered long running) you may encounter a situation where all five threads are in use meaning that later rule triggers either have to wait or get dropped.

This is a limitation of the Rules DSL Engine and, as mentioned, that engine is going away. This is not a problem with NGRE and it shouldn’t be a problem with NGRE even when using Rules DSL. Though I can’t say that for sure.