Flask app with long-running startup code, that blocks incoming requests until initialization is complete

flask_concurrent_blocking_initialization_example.py

import concurrent.futures
from functools import wraps
from flask import Flask


data = None


def long_running_initialization():
    # Long running initialization code goes here
    print('Initializing...')
    for i in range(1, 30):
        import time; time.sleep(1)
        print(f'  ... still initializing {i}s')

    global data; data = 42
    print('Initialization complete!')


def ensure_initialized(f):
    """
    A convenience view decorator that waits for the initialization to complete.
    """
    @wraps(f)
    def decorated_function(*args, **kwargs):
        # run initialization code, possibly blocking if necessary
        concurrent.futures.wait([initialization_future])

        # return back to decorated view function
        return f(*args, **kwargs)
    return decorated_function


# create Flask app -- this apparently needs to happen before the executor is created
app = Flask(__name__)

# create executor -- we only need a single worker, because we only want to run a single task on it ever
executor = concurrent.futures.ThreadPoolExecutor(max_workers=1)

# the initialization will take a long time, so kick it off asap, whether there's incoming requests or not!
initialization_future = executor.submit(long_running_initialization)

@app.route('/')
@ensure_initialized   # this ensures that any requests will block until the initialization has completed
def access_data():
    # now we can access the fully initialized data set
    global data
    return f'Data: {data}'