NoSQL definitely has its place if you need "web scale", I'm not sure mongodb is the right tool for that job though. Most people use redis/memcached for a quick lookup cache, but the data is usually always in a relational store as the source of truth.
Real web scale use cases usually will use cassandra or other big boy tools :)
I'm not too familiar with mongodb as i haven't used it in a long time but it seems like mongodb tries to replace mysql/postgresql.
The question is, do you start a project using nosql as your main datastore? Or do you go with something more traditional that has known scale-out options.
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