ROS Subscriber and Publisher node

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Hello ROS community,

I want to implement a ROS node that is both subscriber and publisher. The idea is that each time the subscribed topic changes, the node will publish to another topic. The node takes a detection command and gives a door command controlling a USB latch.

Here is the code :

#!/usr/bin/env python
import rospy
from std_msgs.msg import String
import serial

#Declaring USB port related to relay
relayport = serial.Serial(port = "/dev/ttyUSB0",
baudrate = 9600,
bytesize = serial.EIGHTBITS, 
parity = serial.PARITY_NONE,
stopbits = serial.STOPBITS_ONE, 
timeout = 1,
writeTimeout = 2)

#Declaring open and close commands in bytes used to control relay
open_cmd = b'\xA0\x01\x01\xA2'
close_cmd = b'\xA0\x01\x00\xA1'

door_state = ''

def Lock_callback(detection): #Callback function to be used when receiving detection topic data
    global door_state
    rospy.loginfo("Command is %s", detection.data)
    if detection.data == 'SH' or detection.data == 'MH' : #Close relay (ON) in Short High and Medium High states to lock door
        relayport.write(close_cmd)
        door_state = 'closed'
        rospy.loginfo("door_state %s", door_state)
    else : #Open relay (OFF) in other detection states
        relayport.write(open_cmd)
        door_state = 'opened'
        rospy.loginfo("door_state %s", door_state)

def Lock_decision(): #Lock decision function to subscribe to detection state topic and publish Lock topic
    Lock_pub = rospy.Publisher('/doorcmd', String, queue_size=10)
    rospy.init_node('Latch_decision', anonymous=True) #Create ROS node for relay
    Lock_sub = rospy.Subscriber('/detection', String, Lock_callback) #Subscribe node to relay command topic
    Lock_pub.publish(door_state)
    rospy.spin()

if __name__ == '__main__':
        Lock_decision()

The Lock_pub.publish only gives one value at the start and stops which is not what I want. I want to publish the /doorcmd topic everytime it is changed by the /detection topic. Any clues on how to do that ? What is the problem in my code ?

Thanks and regards. Lilia.