Allow "randomization of schedules" for schedule adapters

[Apollon77]

We should find a way to randomize the defaut schedule for schedule adapters, if the start is not required to be exactly at a special time; and especially when external systems are queried (see yr and tankerkoenig case).

We could allow placeholders like "X * * * *" in the io-package config where X is then on instance creation being replaced by a random number. We also could be able to find solutions to allow "every 5 minutes" schedules.

We could also allow (or by default?) delay the start within the minute by some random seconds on start to make sure the run is also spread in the respective minute.

[AlCalzone]

My thoughts, as I mentioned offline:

• simple placeholders don't allow specifying how often an adapter should run. What's X? Every 5 minutes? Every 15 minutes?
• randomization would allow us to spread out the start:
  • On first start after controller start, figure out how long until the next scheduled start and delay a random amount of seconds between 0 and that duration
  • remember the duration and delay all following starts by that
  • needs an opt-out for "precise" adapter starts
• "every 5 minutes" schedules would be a simple way IMO, but they would have to override the CRON somehow

[klein0r]

https://man.openbsd.org/crontab.5

A random value (within the legal range) may be obtained by using the ‘~’ character in a field. The interval of the random value may be specified explicitly, for example “0~30” will result in a random value between 0 and 30 inclusive. If either (or both) of the numbers on either side of the ‘~’ are omitted, the appropriate limit (low or high) for the field will be used.

[AlCalzone]

I don't think that's supported by node-scheduler, which uses cron-parser internally: https://github.com/harrisiirak/cron-parser/blob/f687555f25ff0e4e12a951f3a4827b382436692f/lib/expression.js#L125-L135

[foxriver76]

Unfortunately not, would have been too easy 😀

[AlCalzone]

Came up in Discord today: Randomization shouldn't be done for manual starts. If the user wants to test something, adding a random delay of several seconds to minutes will be confusing and prolong the test iterations.