Change of technology. There's one time that the team can't decide what tech to use. So after 2 years of production suddenly we moved from Nodejs to Elixir for server side and MySql to Cassandra for database ... It is crazy that time...

So learning a new language it was so difficult as elixir is functional ... (And I was sucks back then) , and Cassandra is something new to me , which is difficult.

(Hey bit now it is ok )