> ...there was no warning that between two lines of shutdown and delete BEAM could restart a child...

If you're talking about calling 'terminate_child/2' followed by 'delete_child/2', there is a very explicit warning. From <https://hexdocs.pm/elixir/1.18.3/Supervisor.html#terminate_c...>

> A non-temporary child process may later be restarted by the supervisor.

and from <https://www.erlang.org/doc/apps/stdlib/supervisor.html#termi...>

> The process, if any, is terminated and, unless it is a temporary child, the child specification is kept by the supervisor. The child process can later be restarted by the supervisor.

In the English-language docs, the warning is very clear: children that a supervisor will restart on termination may be restarted before you get around to calling 'delete_child'. Children with a restart type of 'permanent' will always cause a race between 't_c' and 'd_c'. Children with a restart type of 'transient' will cause a race if they terminate abnormally... which is a term defined by the same document that the warning comes from.

You keep insisting that this stuff isn't documented. Are you perhaps reading a poor translation of the docs?

> You’re coming from position of knowledge.

No, I'm not. I've forgotten most of the details I ever knew about how the system works and only remember broad strokes. My experience with Erlang/OTP was scattered throughout my spare time over an eight to twelve month period ten years ago. Unlike you, I've never been paid to work with it, and you've worked with it far more recently than I.

The reason I was able to direct you to the right part of the docs was because I said to myself "Wow, it would be fuckin stupid if you couldn't dynamically add children to a supervisor. I remember the Erlang docs being really, really good, so let's see if they failed to describe if and/or how this works.", and then spent like five minutes reading the docs and another couple cross-checking with the Elixir docs.

> I don’t memorize documentation because it changes often...

Do the rules for "+", "-", and (if present) "%" change often in most major languages when they are used with built-in types? [0] It's always well worth your time to learn the rules for commonly-used, bedrock parts of a language, major libraries, and runtime systems that you intend to use. Bedrock parts don't substantially change, because if they did, they would invalidate every program ever written against those parts.

OTP provides a collection of bedrock parts, and supervisors are one such part. Memorizing docs is really stupid, but having a solid understanding of how the major things you use work is always worth the time.

Seriously, how would you function as a programmer if you didn't know how addition or string concatenation worked? If you're using OTP's supervisors, having a good understanding of how they function is just as fundamental.

> One random recent day on Go I spent 95% time in E+C and 5% in documentation.

Sure, that makes some sense. OTP is far more complex and robust than anything Go offers, so it's quite a bit quicker to come up to speed with what's documented in the Go official docs than what's in the OTP docs. Also, as someone who has written Go professionally for the last five, ten years, I warn you that you're going to get turbofucked by the things that -if they are documented at all- are documented only in blog posts or random tutorials. Go is an absolute grab bag of poorly-documented sharp edges and surprising behavior.

> I rely on „map mode” (how I can find knowledge) and not on „collect mode” (i.e. I keep knowledge)... [and] I can show anecdotal data of the impact.

The impact of you failing to familiarize yourself with the necessarily-complex tools you chose [1] to use seems pretty clear to me. You got tripped up by documented behaviour that you didn't bother to understand, which caused you to spend hours looking for solutions that would have been clear after a ten minute trip to the documentation for the 'supervisor' module. Given what you've said elsewhere in this subthread about how your project is an "an enormous Jenga tower that's risky to breath around", I suspect that a significant number of your coworkers also refused to familiarize themselves with the tools they use.

[0] No, they do not.

[1] (or were obligated)