Is Tornado a replacement to Django or are they complementary to each other?

To answer the question,

• NO, Tornado is not a replacement to Django. It’s an alternative.

• YES, they are complementary to each other but not in the same process (*)

I would use Django when it’s a big team effort and/or needs to run on a SQL database.

I would use Tornado for everything else. Ie. personal projects, WebSocket-related apps, or apps that use a NoSQL backend.

(*) Don’t bother running Django inside Tornado or the other way around unless you really have a strong requirement for that.