Question: I've created a Flask app that retrieves data from a queue that is updated in a separate thread. I'm not sure why the Queue is empty when I retrieve it from the Flask GET endpoint, and am a bit ignorant of what Queues being thread-safe is supposed to mean, since my example doesn't appear to reflect that. In the example below, the queue in the Flask @app.route('/measurements') measurements route is empty even though it's updated in the TCP message handler. If anyone can enlighten me I'd appreciate it. I am running this on Ubuntu with python3 in case that's relevant. from flask import Flask, render_template import socket from threading import Thread import os from wyze_sdk import Client from dotenv import load_dotenv from wyze_sdk.errors import WyzeApiError import time from queue import Queue load_dotenv() app = Flask(__name__) start_time = time.time() response = Client().login( email=os.environ['WYZE_EMAIL'], password=os.environ['WYZE_PASSWORD'], key_id=os.environ['WYZE_KEY_ID'], api_key=os.environ['WYZE_API_KEY'] ) client = Client(token=response['access_token']) HOST = '192.168.1.207' # Listen on all network interfaces PORT = 9000 # The same port as in your ESP8266 sketch PORT_UI = 7001 MIN_WATER_DIST = 20 # minimum distance from sensor to water in cm MAX_WATER_DIST = 45 # maximum distance from sensor to water in cm MAX_TIME_PLUG_ON = 600 # maximum amount of time plug should be on # Initialize state variables plug_on_time = None # Track when the plug was turned on measurements = Queue(maxsize=86400) @app.route('/measurements') def measurements_api(): current_time = time.time() recent_measurements = [m for m in list(measurements.queue) if current_time - m['timestamp'] <= 86400] return {'measurements': recent_measurements} # empty queue, no measurements returned # This function will handle incoming TCP messages def handle_tcp_connection(client_socket, client_address, measurements): try: data = client_socket.recv(1024) # Buffer size of 1024 bytes if data: distance_str = data.decode('utf-8') dist = int(distance_str) print(f"Received message: {distance_str}") timestamp = time.time() print(len(measurements.queue)) # prints the correct number of measurements measurements.get() measurements.put({'value': value, 'timestamp': timestamp}) client_socket.close() except Exception as e: print(f"Error: {e}") client_socket.close() # This function runs the TCP server in a separate thread def run_tcp_server(measurements): with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as server_socket: server_socket.bind((HOST, PORT)) server_socket.listen(5) print(f"Listening on {HOST}:{PORT} for TCP connections...") while True: client_socket, client_address = server_socket.accept() # Handle each incoming client connection in a new thread Thread(target=handle_tcp_connection, args=(client_socket, client_address, measurements)).start() # Start the TCP server in a separate thread tcp_server_thread = Thread(target=run_tcp_server, daemon=True, args=(measurements,)) tcp_server_thread.start() @app.route('/') def index(): return render_template('index.html') if __name__ == "__main__": app.run(host='0.0.0.0', port=PORT_UI, debug=True) 

Answer: Queue is thread-safe but not process-shared. Could you please show your uvicorn command for running server? Also, I see you using debug=True. This command involves reloading, which can create two processes. I could suggest you:  Use debug=False:  app.run(host='0.0.0.0', port=PORT_UI, debug=False)   Confirming it’s a single process in router handlers  print(os.getpid())