What does thread local mean in Flask framework?
It's not really a scalability issue, rather it can make it more difficult to maintain a complex code-base. This is because thread locals are used as a global data store, to avoid passing state via function arguments, which makes it harder to follow what's going on.
Speed-wise, thread locals are not particularly slow, but they do make it hard or impossible to use an asynchronous framework, where lots of tasks run for short amounts of time on a single thread.
I am evaluating python frameworks to build a REST API. I have gone through many frameworks including flask and found Flask pretty interesting and easy to use with all the features needed to build a REST web service.
One thing i don get is in flask docs it is mentioned that it uses "thread local and has issues with scalability".

Flask uses thread local objects (context local objects in fact, they
  support greenlet contexts as well) for request, session and an extra
  object you can put your own things on (g). Why is that and isn’t that
  a bad idea? Yes it is usually not such a bright idea to use thread
  locals. They cause troubles for servers that are not based on the
  concept of threads and make large applications harder to maintain.
  However Flask is just not designed for large applications or
  asynchronous servers. Flask wants to make it quick and easy to write a
  traditional web application.

Can someone help me understand this, what scalability issues is being mentioned and how other frameworks handle this??