If a large map is not safe, then where to keep ws connections?

Question

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You say that map and a couple of million keys + mutex are not safe. Where then to store links to ws connections?

Answer 1

I read your previous questions ...
Do you even understand that there are search engines? Google, Yandex, Bing, etc.
You seem to know how to write, why not look first?
Take redis, stop building bicycles, which in your case are with a square wheel, one and without a steering wheel.

Answer 2

Why is it unsafe?
With numbers like "a couple of million", it's time for you to horizontally scale the service easily.