[Draft] ContentFilteredTopic Support

[fujitatomoya]

Feature request

Feature description

Draft overview and background is ros2/design#282,

Introduce dds contentfiltertopic to ROS, dds specification is DDS v1.4 2.2.2.3.3 ContentFilteredTopic Class. ContentFilteredTopic can be created on parent topic and DataReader only receives the messages that matches with filtering expression and expression parameters. If implementation does not support this feature, it returns not support.

• rmw implementation support.

Implementation	Supported	Comment
RTI Connext	Yes	https://community.rti.com/examples/content-filtered-topic, Not sure if filtering is done on writer or reader side.
eProsima Fast-DDS	No
ADLINK cyclonedds	No
ADLINK OpenSplice	Yes	E.O.L foxy or newer

• expected user for this feature.

  • action feedback / status topics.
  • parameter_events topic.
  • user defined topics.

Implementation considerations

• rmw and rcl implementation
  • ContentFilteredTopic requires parent topic, returning capability for ContentFilteredTopic with rmw_create_subscriotion rmw_subscription_t. So that rclcpp/rcl can tell if the implementation supports it or not.
  • Based on parent topic and subscription, it can create ContentFilteredTopic with subtopic name, filtering expression and expression parameters. Then implementation creates ContentFilteredTopic and replace the DataReader.
  • Expression parameter can be changed via setter dynamically, also getter should be provided. but filtering expression cannot be changed unless re-creating ContentFilteringTopic.
  • Test must be added to make sure ContentFIlteredTopic supported and functions.
• rcl and rclcpp
  • Action feedback and status to use ContentFilteredTopic.
  • Introduce new class for parameter_events filtering which calls ContentFilteredTopic.
  • Filtering in rcl based when rmw does not support ContentFilteredTopic.

[Barry-Xu-2018]

About implementation estimation.

1. Implement -- 18 ~ 23 MD

   Module	Estimation (man day)
   rmw & rmw_implementation	2
   rmw_connext	5~10
   rcl	4
   rclcpp	3
   rclpy	TBD
   rmw_fastrtps, rmw_cyclonedds	4

2. Integrate and test -- 5 MD

3. Review and modification -- 5~10 MD

[fujitatomoya]

@Barry-Xu-2018

I think that it would be really nice to break down into some steps, since huge single PR is really heavy work and hard to do review. the following is just idea.

1. rmw ContentFilteredTopic support

• is_cft_supported to ask rmw implementation if ContentFilteredTopic is supported or not.
• rmw_create_subscription with optional fields const char * filter_expression and const rcutils_string_array_t * expression_parameters
• test_rmw_content_filter to verify ContentFilteredTopic in rmw only.

2. rcl ContentFilteredTopic support

• basically bind rmw related interfaces to rcl, and verify ContentFilteredTopic in rcl.
• support internal filtering function on reader side if rmw does not support ContentFilteredTopic.

3. Action Topics with ContentFilteredTopic. (rcl action client mostly)

• issue ContentFilteredTopic based on current goal ids in action client.
• additional tests required.

4. Parameter Events with ContentFilteredTopic.

• user interface support to set filtering expression and expression parameters. (node name and parameter name)
• additional tests required.

let's discuss the details offline.

[Barry-Xu-2018]

@fujitatomoya

Completely agree.
Let's discuss it in more detail.

[fujitatomoya]

ros2/design#282 takes care of all activities content filtered topic. close this issue.