Intermediate Rubyist here who's been mostly making simple postgres/mongo-backed Rails apps for the past 2-3 years. I started at the very front end of web dev and have been learning my way down the stack.

Here's my question. In 3 years of web development, I have never had mock, dupe(dup),** shard, or **stub anything. I have read through the documentation and have a basic understanding of what each of these terms means, what I don't know is why or when you would do them?

For example how does making dupe of an object help you? When and why would you shard a db (and what does this really mean, that the db table gets broken into smaller, easier-to-search chunks?). Why would I make a stub or a mock?

I guess this more of a best practices and methadologies in OOP question, but I want to make sure that I'm using all the tools available.

tl;dr: Why & where would you mock or dupe an object, shard a db, or stub a method?