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Design guide: common patterns in ROS 2

Composable nodes as shared libraries

Context

You want to export a composable node as a shared library from a package and use that node 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

Fast RTPS large data transfer

Context

You want to transfer large data via Fast RTPS.

Problem

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

Solution

Configure the middleware so that it fragments large data into messages

Implementation

Use Asynchronous publication mode:

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

ROS2 Fine Tuning

Fast RTPS Best Effort Video Streaming

Context

You want to transfer video streams and provide up-to-date data. It is OK to lose some of the images.

Problem

Acknowledged data transmission mechanisms (also known as “reliable” delivery) ensure every packet is delivered, which can cause the video stream to fall behind.

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

Fast RTPS 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 response 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