Hi, You should read this article (5 pages) Understanding scalability/distributed systems with example.

Along with sharding you will get to know other techniques and approaches that devs take to optimize and scale applications.

Let's say you have an account table in a single database containing user accounts, now after all the optimizations you've done, it's still taking strain.

Now we take users from US and place them in a separate account table on a seperate database and users from ZA into a separate account table on another database and you have some sort of router / loadbalancer reading / writing / updating rows in their relevant databases.

See this article from Percona: percona.com/blog/2015/03/19/choosing-a-good-shard…