I doubt you will get a useful answer in a comment. Each of your questions is very broad. And you didn't even select a search engine yet. You didn't specify the

I doubt you will get a useful answer in a comment. Each of your questions is very broad. And you didn't even select a search engine yet. You didn't specify the scale you're dealing with. You didn't specify the number of reads/writes per second that you expect. Choose one system and learn it well.