ROS2 QoS - Reliable vs Best Effort

🧠 Concept: QoS — Quality of Service

QoS controls how messages are delivered between publishers and subscribers. Two critical settings:

Reliability:

• RELIABLE — guarantees delivery (retransmits lost messages). Use for: commands, goals, service calls.
• BEST_EFFORT — faster, no retransmission. Use for: sensor streams (camera, LiDAR) where old data is useless anyway.

History / Durability:

• VOLATILE (default) — subscribers only get messages published after they subscribe.
• TRANSIENT_LOCAL — subscribers get the last N messages even if they subscribed late. Use for: robot_description, map.

Mismatch = silent failure: If a publisher uses RELIABLE and a subscriber uses BEST_EFFORT, they won't communicate. ROS2 silently drops the connection.

from rclpy.qos import QoSProfile, ReliabilityPolicy
qos = QoSProfile(depth=10, reliability=ReliabilityPolicy.BEST_EFFORT)
self.sub = self.create_subscription(LaserScan, '/scan', self.cb, qos)

Problem Statement

Understand Quality of Service (QoS) policies. Create a publisher with Reliable QoS and one with Best Effort.

Requirements

• Create node qos_test
• Publisher 1: /reliable_topic, QoS: Reliable
• Publisher 2: /best_effort_topic, QoS: Best Effort
• Message type: std_msgs/String

Input/Output Format

Output:

• Node with 2 publishers configured correctly.

⚠️ Common Pitfalls

• Mismatched QoS: A Reliable subscriber cannot receive data from a Best Effort publisher (ROS2 compatibility rules).

📚 Helpful Resources

• ROS2 QoS Policies