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I am trying MQTT on my Raspberry Pi running Raspbian.
The basic setup works, but I can't seem to configure persistence.

Following https://tutorials-raspberrypi.com/raspberry-pi-mqtt-broker-client-wireless-communication/

I installed mosquitto mosquitto-clients

I installed paho-mqtt and am running the following script in a background process to publish temperature and humidity readings from an attached sensor.

#!/usr/bin/python3

import os

import time

import sys

import Adafruit_DHT as dht

import paho.mqtt.client as mqtt

import json

import datetime

#

# Sensor should be set to Adafruit_DHT.DHT11, Adafruit_DHT.DHT22, or Adafruit_DHT.AM2302.

sensor = dht.DHT11

pin = 4



BROKER = 'localhost'



# Data capture and upload interval in seconds.

INTERVAL=15



sensor_data = {'date': 0, 'temperature': 0, 'humidity': 0}



next_reading = time.time()



client = mqtt.Client()



# Connect to BROKER using default MQTT port and 60 seconds keepalive interval

client.connect(BROKER, 1883, 60)



client.loop_start()



try:

    while True:

        humidity,temperature = dht.read_retry(sensor, pin)

        humidity = round(humidity, 2)

        temperature = round(temperature, 2)

        print(u"Temperature: {:g}u00b0C, Humidity: {:g}%".format(temperature, humidity))

        sensor_data['temperature'] = temperature

        sensor_data['humidity'] = humidity

        sensor_data['date'] = datetime.datetime.now().replace(microsecond=0).isoformat()



#         client.publish('test_channel', json.dumps(sensor_data), 1)

        client.publish('test_channel', json.dumps(sensor_data), 2)



        next_reading += INTERVAL

        sleep_time = next_reading-time.time()

        if sleep_time > 0:

            time.sleep(sleep_time)

except KeyboardInterrupt:

    pass



client.loop_stop()

client.disconnect()

This works and I can see the messages on another Pi with mosquitto_sub -h IPaddress -v -t test_channel but this only works while the process is running. I want the Broker to save messages until the subscriber connects.

I added "persistence true" to a file /etc/mosquitto/conf.d/Milliways.conf and the log file shows messages like

1547597521: Saving in-memory database to /var/lib/mosquitto/mosquitto.db.

The database does not seem to contain any relevant data.

I have read the Documentation on https://pypi.org/project/paho-mqtt/#publishing and the man for mosquitto.

Can anyone help?