Proposal: nested Schedulers

[sfeilmeier]

Bug Report or Feature Request (mark with an x)

- [ ] bug report -> please search issues before submitting
- [x] feature request

Bug description or desired functionality.

One of the main principles of OpenEMS is, that there is one global "Cycle", that executes "Controllers" in the order given by a "Scheduler". See https://openems.github.io/openems.io/openems/latest/edge/architecture.html#_cycle. There are different implementations of Schedulers, like the AllAlphabetically (executing all Controllers in alphabetic order or their Component-IDs), a DailyScheduler (executing Controllers on defined hours of a day) and a WeeklyScheduler (like 7 x DailyScheduler) The execution order of Controllers is important to apply inherent prioritization. A later executed Controller can not overwrite a setting that was done by a Controller with higher priority.

The current logic is: The Cycle gets a list of all configured Schedulers (there is usually only one configured). Each Scheduler can have its individual Cycle-Time (defaults to 1 second). The CycleWorker asks every Scheduler (sorted alphabetically by the Scheduler Component-ID) for the Controllers that should be executed within a Cycle - and executes these Controllers.

This logic is ok for basic use cases, but has certain limitations:

• It is always required to configure a Scheduler; typically AllAlphabetically, as in the Getting Started Guide
• Configuration of more advanced Schedulers quickly becomes complicated; for example DailyScheduler requires a complex JSON-format which is hard to set right in Apache Felix Console
• All Controllers need to be mentioned explicitely or they just do not get executed. It's easy to forget basic Controllers like Rest-Api, Websocket-Api, DebugLog,...
• When using multiple Schedulers it could easily happen, that Controllers get executed twice per Cycle if not explicitely excluded.

I am proposing the following new Scheduler logic:

• The Cycle defines one global Cycle-Time and holds a list of all Schedulers (like before)
• A new interface "Schedulable" is introduced. Controller and Scheduler implement Schedulable.
• Each Scheduler returns two sorted lists:
  • Schedulables (Controller or Scheduler) that should be executed ("include") in a Cycle
  • Schedulables that should NOT be executed ("exclude") in a Cycle.
• The CycleWorker goes through the tree of Schedulables and creates an ordered list of all Controllers that should be executed.
• Controllers that were not explicitely included, but also not excluded, are added alphabetically to the the end of the list.

This allows:

• In normal use-cases no Scheduler configuration is required; AllAlphabetically Scheduler becomes obsolete and new users of OpenEMS don't need to care about Schedulers
• Advanced Schedulers only take care of their specific tasks; no need to take care of basic Controllers like Rest-Api,...
• Schedulers can be nested and reused; e.g. the WeeklyScheduler defines 7 DailySchedulers
• New, simple Schedulers can be developed, like "SingleShot", which executes a Controller (or Schedulable) at a specific time; or "CronScheduler",...
• Just like for Controllers, specific UI-Widgets can be designed to easily configure Schedulers via UI.

What do you think? Do you have use-cases that are not covered by this proposal?

[sfeilmeier]

This is also discuess in OpenEMS Community Forum: https://community.openems.io/t/proposal-nested-schedulers/164