Python Tornado updating shared data between requests

Question

I have a Python Tornado app. The app contains request handlers, for which I am passing data to like (the code below is not complete, and is just to illustrate what I want):

Answer 1

Another strategy, in addition to what dano mentions above is to attach the shared data to the Application object.

class MyApplication(tornado.web.Application):

    def __init__(self):

       self.shared_attribute = foo;

        handlers = [#your handlers here]        
        settings = dict(#your application settings here)
        super().__init__(handlers, **settings)


server = tornado.httpserver.HTTPServer(MyApplication())
server.listen(8888)
tornado.ioloop.IOLoop.instance().start()

Next you can access shared_attribute defined above in all your request handlers using self.application.shared_attribute.

You update it at one place and it immediately reflects in all your subsequent calls to the request handlers.

Answer 2

Well, the simplest thing would be to pass the entire config dict to the handlers, rather than just the individual values inside the dict. Because dicts are mutable, any change you make to the values in the dict would then propagate to all the handlers:

import tornado.web
import tornado.httpserver

configs = {'some_data': 1, # etc.
          }

def update_configs():
    print("updating")
    configs['some_data'] += 1

class PageOneHandler(tornado.web.RequestHandler):
    def initialize(self, configs):
        self.configs = configs
    def get(self):
        self.write(str(self.configs) + "\n")


class PageTwoHandler(tornado.web.RequestHandler):
    def initialize(self, configs):
        self.configs = configs

    def get(self):
        self.write(str(self.configs) + "\n")


class Application(tornado.web.Application):
    def __init__(self):
        handlers = [('/pageone', PageOneHandler, {'configs' : configs}),
                ('/pagetwo', PageTwoHandler, {'configs': configs})]
        settings = dict(template_path='/templates',
                    static_path='/static', debug=False)
        tornado.web.Application.__init__(self, handlers, **settings)

# Run the instance
application = Application()
http_server = tornado.httpserver.HTTPServer(application)
http_server.listen(8888)

# Callback function to update configs
some_time_period = 1000 # Once an second
tornado.ioloop.PeriodicCallback(update_configs, some_time_period).start()
tornado.ioloop.IOLoop.instance().start()

Output:

dan@dantop:~> curl localhost:8888/pageone
{'some_data': 2}
dan@dantop:~> curl localhost:8888/pageone
{'some_data': 3}
dan@dantop:~> curl localhost:8888/pagetwo
{'some_data': 4}
dan@dantop:~> curl localhost:8888/pageone
{'some_data': 4}

To me this approach makes the most sense; the data contained in configs doesn't really belong to any one instance of a RequestHandler, it's global state shared by all RequsetHandlers, as well as your PeriodicCallback. So I don't think it makes sense to try to create X numbers of copies of that state, and then try to keep all those different copies in sync manually. Instead, just share the state across your whole process using either a custom object with class variables, or a dict, as shown above.