sub_back_pressure.py

import rclpy
import time

from rclpy.duration import Duration
from rclpy.qos import QoSProfile, ReliabilityPolicy, HistoryPolicy, DurabilityPolicy
from std_msgs.msg import String

rclpy.init()

node = rclpy.create_node("sub")

slow_phase = True
start_time = time.time()

n = -1
def on_message(msg):
    global n, slow_phase, start_time

    received_n = int(msg.data)

    if n != -1 and received_n > n + 1:
        print("Lost a message! Shouldn't happen. Expected", n + 1, ", got", received_n)
    n = received_n

    # Slow subscriber takes its time
    time.sleep(0.1)

qos = QoSProfile(
    reliability=ReliabilityPolicy.RELIABLE, # Messages are transferred reliably (needs stateful writer)
    history=HistoryPolicy.KEEP_ALL,         # And we want to transfer all messages (since the reader has been made known to the writer and is alive) 
    depth=0,                                # doesn't matter
    durability=DurabilityPolicy.VOLATILE,   # We only provide messages created after a reader has been made known to the writer
    )
sub = node.create_subscription(String, "back_pressure", on_message, qos)


while rclpy.ok():
    rclpy.spin(node)