Question

One of the things I like about Google App Engine development environment is, I don’t have to restart the server every time I make changes to any python source files, other static files or even configuration files. It has spoilt me and I forget to restart the server when I am working with other server environments (tornadoweb, web.py, node.js).

Can anyone explain how GAE does that? How difficult is it to make other servers (at least python based) to achieve the same thing?

Answer 1

You can view the source for dev_appserver.py(link). Looks like ModuleManager makes a copy of sys.modules and monitors each module to track changes based on time:

class ModuleManager(object):
  """Manages loaded modules in the runtime.

  Responsible for monitoring and reporting about file modification times.
  Modules can be loaded from source or precompiled byte-code files.  When a
  file has source code, the ModuleManager monitors the modification time of
  the source file even if the module itself is loaded from byte-code.
  """

http://code.google.com/p/googleappengine/source/browse/trunk/python/google/appengine/tools/dev_appserver.py#3636