topic_relay_controller

Purpose

The node subscribes to a specified topic, remaps it, and republishes it. Additionally, it has the capability to continue publishing the last received value if the subscription stops, or to limit the output rate by dropping the messages that arrive too soon.

Inputs / Outputs

Input

Name	Type	Description
/<topic>	<specified message type>	Topic to be subscribed, as defined by the topic parameter.
/tf	tf2_msgs::msg::TFMessage	(Optional) If the topic is /tf, used for transform message relay.
/tf_static	tf2_msgs::msg::TFMessage	(Optional) If the topic is /tf_static, used for static transforms.

Output

Name	Type	Description
/<remap_topic>	<specified message type>	Republished topic after remapping, as defined by the remap_topic parameter.

Parameters

Variable	Type	Description
topic	string	The name of the input topic to subscribe to
remap_topic	string	The name of the output topic to publish to
topic_type	string	The type of messages being relayed
qos	integer	QoS profile to use for subscriptions and publications (default: 1)
transient_local	boolean	Enables transient local QoS for subscribers (default: false)
best_effort	boolean	Enables best-effort QoS for subscribers (default: false)
enable_relay_control	boolean	Allows dynamic relay control via a service (default: true)
srv_name	string	The service name for relay control when enable_relay_control is true
enable_keep_publishing	boolean	Keeps publishing the last received topic value when not subscribed (default: false)
update_rate	integer	The rate (Hz) for publishing the last topic value when enable_keep_publishing is true (optional)
frame_id	string	Frame ID for transform messages when subscribing to /tf or /tf_static (optional)
child_frame_id	string	Child frame ID for transform messages when subscribing to /tf or /tf_static (optional)
enable_throttle	boolean	Drops relayed messages so the output rate does not exceed msgs_per_sec (default: false)
msgs_per_sec	double	The upper bound (Hz) on the output rate when enable_throttle is true; must be greater than 0

Assumptions / Known limits

• The node assumes that the specified topic and remap_topic are valid and accessible within the ROS 2 environment.
• If enable_keep_publishing is true, the node continuously republishes the last received value even if no new messages are being received.
• enable_throttle and enable_keep_publishing both set the output rate and cannot be combined; enabling both logs an error and disables the throttle.
• Throttling only drops messages, so the output rate is at most msgs_per_sec and follows the input timing. Use enable_keep_publishing instead to emit at a fixed rate regardless of when messages arrive.
• For /tf and /tf_static, additional parameters like frame_id and child_frame_id are required for selective transformation relays.
• QoS settings must be carefully chosen to match the requirements of the subscribed and published topics.