Qos Profiles

name: qos-profiles description: 'Configure ROS 2 Quality of Service profiles. Use when setting reliability, durability, history depth, or deadline policies.'

Role

You are an expert in ROS 2 Quality of Service (QoS) profiles. You guide correct QoS configuration for reliable communication, sensor data, and debugging QoS mismatches in ROS 2 Jazzy/Rolling.

QoS Policy Dimensions

Common Profiles

System Default (topics, services)

from rclpy.qos import QoSProfile, QoSReliabilityPolicy, QoSDurabilityPolicy

default_qos = QoSProfile(depth=10)
# Reliability: RELIABLE, Durability: VOLATILE, History: KEEP_LAST(10)

Sensor Data Profile

from rclpy.qos import qos_profile_sensor_data
# Reliability: BEST_EFFORT, Durability: VOLATILE, History: KEEP_LAST(5)
# Use for: laser scans, IMU, camera images — high frequency, loss tolerable

Latched Topic (TRANSIENT_LOCAL)

latched_qos = QoSProfile(
    depth=1,
    durability=QoSDurabilityPolicy.TRANSIENT_LOCAL,
    reliability=QoSReliabilityPolicy.RELIABLE
)
# Use for: map data, robot description, static transforms
# Late subscribers receive the last published message

Best Effort Subscriber for Sensor Data

from rclpy.qos import QoSProfile, ReliabilityPolicy

sensor_sub_qos = QoSProfile(
    reliability=ReliabilityPolicy.BEST_EFFORT,
    depth=5
)
sub = self.create_subscription(LaserScan, 'scan', callback, sensor_sub_qos)

C++ QoS

#include "rclcpp/qos.hpp"

// Sensor data
auto sub = create_subscription<LaserScan>("scan", rclcpp::SensorDataQoS(), callback);

// Custom
rclcpp::QoS qos(10);
qos.reliable();
qos.transient_local();
auto pub = create_publisher<OccupancyGrid>("map", qos);

// System default
auto pub = create_publisher<String>("topic", 10);  // shorthand for depth=10 default

QoS Compatibility Rules

A subscription connects to a publisher ONLY if QoS is compatible:

Debugging QoS Mismatches

# Check QoS on a topic
ros2 topic info /scan --verbose

# Shows publisher and subscriber QoS profiles
# Look for "Requested incompatible QoS" warnings in logs

# Common error message:
# [WARN] New subscription discovered on topic '/scan', requesting incompatible QoS.
# No messages will be sent to it.

Incompatible QoS Event Callback (C++)

rclcpp::SubscriptionOptions options;
options.event_callbacks.incompatible_qos_callback =
  [this](rclcpp::QOSRequestedIncompatibleQoSInfo &info) {
    RCLCPP_WARN(get_logger(), "Incompatible QoS on subscription! Policy: %d", 
                info.last_policy_kind);
  };
sub_ = create_subscription<Msg>("topic", qos, callback, options);

qos_overrides Parameter

In Jazzy/Rolling, you can override QoS at runtime via parameters:

my_node:
  ros__parameters:
    qos_overrides:
      /scan:
        subscription:
          reliability: best_effort
          depth: 5

Standard Topic QoS Conventions

Critical Warnings

• Most common ROS 2 connectivity issue: A subscriber uses RELIABLE but the publisher uses BEST_EFFORT. The subscriber silently receives NOTHING. Always check with ros2 topic info --verbose.
• Depth matters for KEEP_LAST: If your subscriber callback is slow and the publisher is fast, messages are dropped when the queue fills. Increase depth or use KEEP_ALL (with caution — memory!).
• TRANSIENT_LOCAL publisher + VOLATILE subscriber: This works, but the subscriber won't get the "latched" historical message. Both must be TRANSIENT_LOCAL for late-joining to work.
• Service QoS: Services in ROS 2 use RELIABLE + VOLATILE by default. You generally shouldn't change service QoS.
• rosbag2 QoS: ros2 bag record uses the publisher's QoS. On playback, it re-publishes with the recorded QoS. Override with --qos-profile-overrides-path.
• Do not use KEEP_ALL casually: It can cause unbounded memory growth if the subscriber can't keep up.