How to implement social elements?

Question

MySQL

Now the site has a "private messages" functionality, it works on the basis of mysql and it is very expensive (now there are 10 mysql servers only for this feature and the load is growing). You need to look for alternatives (or hear your ideas), yesterday I tried the kphp lists engine for this, it suits it, but VK abandoned it immediately after publication, and open source does not support it much (for 3 years, 60 commits in the fork and even then of dubious quality).

Answer 1

Why is it expensive? Where do the brakes go? on the base? Why? Heavy requests? Indexes? Large volume stretches?
Why instead of optimizing, do you want alternatives?
inhale, exhale and calmly and in great detail describe the problem with all the details ...