Design Guide: Common patterns in ROS 2¶

Composable nodes as shared libraries¶

Context

You want to export composable nodes as a shared libraries from some packages and using those in another package that does link-time composition.

Solution

• Add code to the CMake file which imports the actual targets in downstream packages

  • Install the generated file

  • Export the generated file

Example

ROS Discourse - Ament best practice for sharing libraries

FastRTPS large data transfer¶

Context

You want to transfer large data via FastRTPS.

Problem

DDS/RTPS uses UDP with a maximum message size of 64k

Solution

Configure the middleware that it fragements large data into messages

Implementation

Use Asynchronous publication mode:

<publishMode>
  <kind>ASYNCHRONOUS</kind>
</publishMode>

ROS2 Fine Tuning

FastRTPS Best Effort Video Streaming¶

Context

You want to transfer video streams and provide up to date data. It is ok to loose some packages.

Problem

Acknowledged data transmission mechanisms prevent from being able to provide up to date packages.

Solution

Use “best effort” communication (instead of the usual acknowledgement based mechanism) and prioritize the last frame.

Implementation

• Configure “best effort” reliability mechanism

• Configure Quality of service history to keep last frame

<reliability>
  <kind>BEST_EFFORT</kind>
</reliability>

<historyQos>
  <kind>KEEP_LAST</kind>
  <depth>1</depth>
</historyQos>

ROS2 Fine Tuning

FastRTPS Reliable Video Streaming¶

Context

You want to transfer video streams in unreliable network settings.

Solution

Use a reliable communication mechanism. Use fast response by writer and reader.

Implementation

• Configure “reliable” reliability mechanism

• Configure NACK reponse delay and suppression duration of writer to 0

• Configure heartbeat response delay of reader to 0

<reliability>
  <kind>RELIABLE</kind>
</reliability>

# writer
<times>
  <nackResponseDelay>
    <durationbyname>ZERO</durationbyname>
  </nackResponseDelay>
  <nackSupressionDuration>
    <durationbyname>ZERO</durationbyname>
  </nackSupressionDuration>
</times>

# reader
<times>
  <heartbeatResponseDelay>
    <durationbyname>ZERO</durationbyname>
  </heartbeatResponseDelay>
</times>

ROS2 Fine Tuning