What is the QoS policy used for ROS2 /rosout?

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The QoS for the rosout topic is configurable per node, though in most cases you can probably expect users to be using the default, defined here:

https://github.com/ros2/rcl/blob/7a25...

RMW_QOS_POLICY_HISTORY_KEEP_LAST,
1000,  // History depth
RMW_QOS_POLICY_RELIABILITY_RELIABLE,
RMW_QOS_POLICY_DURABILITY_TRANSIENT_LOCAL,
RMW_QOS_DEADLINE_DEFAULT,
{10, 0},  // Lifespan
RMW_QOS_POLICY_LIVELINESS_SYSTEM_DEFAULT,
RMW_QOS_LIVELINESS_LEASE_DURATION_DEFAULT

You can also introspect the QoS settings for each publisher and subscription to the topic at runtime with ros2 topic info --verbose /rosout. Expect to see output like this:

Type: rcl_interfaces/msg/Log

Publisher count: 2

Node name: _ros2cli_daemon_16
Node namespace: /
Topic type: rcl_interfaces/msg/Log
Endpoint type: PUBLISHER
GID: 7a.57.10.01.35.c5.a7.5c.9f.bc.fa.d3.00.00.06.03.00.00.00.00.00.00.00.00
QoS profile:
  Reliability: RELIABLE
  Durability: TRANSIENT_LOCAL
  Lifespan: 10000000000 nanoseconds
  Deadline: 9223372036854775807 nanoseconds
  Liveliness: AUTOMATIC
  Liveliness lease duration: 9223372036854775807 nanoseconds

Node name: _ros2cli_5495
Node namespace: /
Topic type: rcl_interfaces/msg/Log
Endpoint type: PUBLISHER
GID: f0.a8.10.01.7d.2f.7f.0e.d6.03.27.4a.00.00.06.03.00.00.00.00.00.00.00.00
QoS profile:
  Reliability: RELIABLE
  Durability: TRANSIENT_LOCAL
  Lifespan: 10000000000 nanoseconds
  Deadline: 9223372036854775807 nanoseconds
  Liveliness: AUTOMATIC
  Liveliness lease duration: 9223372036854775807 nanoseconds

Subscription count: 0