Flask update a global variable with value from Redis

Issue

I have a python flask app with a global variable reading some data from Redis. I have another process that’s updating data in Redis independently, and it will ping the flask app (i.e. endpoint /reload-redis-data) to tell flask app to update that global variable with the latest data in Redis.

There are 3 files involved:

  • app.py
from re import A
from flask import Flask
from util import load_redis_config
from function import f

app = Flask(__name__)

@app.route("/")
def index():
    return "Hello world!"

@app.route("/test")
def test():
    f()
    return ""

@app.route("/reload-redis-config")
def reload_redis_config():
    load_redis_config()
    return "reloaded"

if __name__ == "__main__":
    app.run("localhost", 3001)
  • util.py
import redis

global a
a = ""

def load_redis_config():
    con = redis.Redis("localhost", 6379)
    global a
    a = eval(con.get("a"))
    print(f"Updated {a=}")

load_redis_config()
print(f"At server start {a=}")
  • function.py
from util import a

def f():
    global a
    print(f"In function f {a=}")

The setup is:

  1. in Redis set a 1
  2. at server start, load_redis_config() will be called and load a -> print 1
  3. /test -> print 1
  4. in Redis set a 2
  5. ping /reload-redis-config, load_redis_config() will print a is changed to 2
  6. /test -> I’m expecting it to be changed to 2 since a is a global variable which is updated in step 5, but within function f a is still 1.

My trace:

Updated a=1
At server start a=1
 * Serving Flask app 'app'
 * Debug mode: off
WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
 * Running on http://localhost:3001
Press CTRL+C to quit
In function f a=1
127.0.0.1 - - [28/Oct/2022 14:22:21] "GET /test HTTP/1.1" 200 -
Updated a=2
127.0.0.1 - - [28/Oct/2022 14:22:32] "GET /reload-redis-config HTTP/1.1" 200 -
In function f a=1
127.0.0.1 - - [28/Oct/2022 14:22:35] "GET /test HTTP/1.1" 200 -

Basically my question is, why function isn’t picking up the updated global variable a? Is there anything or any concept that I’m missing?

Solution

So, your function.py file is rendered once when the server starts, and after that, only the function f() is rendered when /test is called. Since you only import the variable a once in the function.py script and that is also rendered only when the server starts, the script does not check if it is updated in the util.py again when f() is called.

To solve this, in your util.py:

import redis

global a
a = ""

def load_redis_config():
    con = redis.Redis("localhost", 6379)
    global a
    a = eval(con.get("a"))
    print(f"Updated {a=}")

def get_a():
    return a

load_redis_config()
print(f"At server start {a=}")

In You function.py

from util import get_a

def f():
    a = get_a()
    print(f"In function f {a=}")

Answered By – Divyessh

This Answer collected from stackoverflow, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0

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