Flask global variables [duplicate]

Question

This question already has answers here:

Are global variables thread-safe in Flask? How do I share data between requests? (4 answers)

Closed 4 years ago.

I am trying to find out how to work with global variables in Flask:

gl = {'name': 'Default'}

@app.route('/store/<name>')
def store_var(name=None):
    gl['name'] = name
    return "Storing " + gl['name']

@app.route("/retrieve")
def retrieve_var():
    n = gl['name']
    return "Retrieved: " + n

Storing the name and retrieving it from another client works fine. However, this doesn't feel right: a simple global dictionary where any session pretty much simultaneously can throw in complex objects, does that really work without any dire consequences?

Answer 1

No, it doesn't work, not outside the simple Flask development server.

WSGI servers scale in two ways; by using threads or by forking the process. A global dictionary is not a thread-safe storage, and when using multi-processing changes to globals are not going to be shared. If you run this on a PAAS provider like Google App Server, the processes aren't even forked; they are run on entirely separate machines even.

Use some kind of backend storage instead; a memcached server, a database server, something to control concurrent access and share the data across processes.