ROS2 Built-In QoS

For Foxy version^[1]

The basic QoS settings available:

• History
  • keep last: only store up to N samples, configurable via the queue depth option.
  • keep all: store all samples, subject to the configured resource limits (resource limit is one of the QoS as well) of the underlying middleware.
• Depth
  • Queue size: only honored if the "history" policy is set to "keep last"
• Reliability
  • Best effort: attempt to deliver samples, but may lose them if the network is not robust
  • Reliable: guarantee that samples are delivered, may retry multiple times.
• Durability
  • Transient local: the publisher becomes responsible for persisting samples for "late-joining" subscription.
  • Volatile: no attempt is made to persist samples.
• Deadline
  • Duration: the expected maximum amount of time between subsequent message being published to topic.
• Lifespan
  • Duration: the maximum amount of time between the publishing and the reception of a message without the message being considered stale or expired (expired messages are silent dropped and are effectively never received)
• Liveliness
  • Automatic: the system will consider all of the node's publishers to be alive for another "lease duration" when any one of it's publishers has published a message.
  • Manual by topic: the system will consider the publisher to be alive for another "lease duration" if it manually asserts that it's sill alive (via call to the publisher API)
• Lease Duration
  • Duration: the maximum period of time a publisher has to indicate that it is alive before the system considers it to have lost liveliness (losing liveliness could be an indicate of a failure)

How rmw_cyclonedds_cpp create default QoS ?

This sections records how rmw_cyclonedds_cpp create QoS on foxy version ^[1:1]

History and Depth

• RMW_QOS_POLICY_HISTORY_SYSTEM_DEFAULT is RMW_QOS_POLICY_HISTORY_KEEP_LAST
• RMW_QOS_POLICY_DEPTH_SYSTEM_DEFAULT is 1
• when history is set to keep all, depth is set to unlimited

Reliable

• RMW_QOS_POLICY_RELIABILITY_SYSTEM_DEFAULT is RMW_QOS_POLICY_RELIABILITY_RELIABLE
• when reliable is set to reliable, blocking time is set to unlimited
• when reliable is set to best effort, blocking time is set to zero

Durability

• RMW_QOS_POLICY_DURABILITY_SYSTEM_DEFAULT is RMW_QOS_POLICY_DURABILITY_VOLATILE
• if durability is set to transient local, it will respect to history QoS setting and set to:
  • service_cleanup_delay: 0 seconds
  • history kind: respect to history kind
  • history depth: respect to history depth
  • max samples: unlimited
  • max instances: unlimited
  • max samples per instance: unlimited

LifeSpan

• respect to QoS option passed from rcl

Deadline

• respect to QoS option passed from rcl

Liveliness and Lease Time

• if QoS option's lease time is 0, then lease time is set to unlimited
• RMW_QOS_POLICY_LIVELINESS_SYSTEM_DEFAULT is RMW_QOS_POLICY_LIVELINESS_AUTOMATIC
• we can also set to RMW_QOS_POLICY_LIVELINESS_MANUAL_BY_TOPIC by external QoS option.

LifeCycle

data lifecycle is set to false always , which means disable automatic disposal of unregister instances.