[Requesting Help] Support ContentFilteredTopic on rmw_connext

[fujitatomoya]

This is NOT exactly issue or bug in current source code, but we are not sure where should be the right place to discuss details about implementation. If this is inappropriate place, could you kindly guide us.

Description

Background

We Sony are now working on to enable ContentFilteredTopic on rmw_connext based on ros2/design for ContentFilteredTopic. Being optional to have this feature relies on the RMW implementation can lead a huge efficiency for network and CPU resource especially on edge IoT devices. Currently RTI Connext is the one to support ContentFilteredTopic, so we'd like to enable this feature based on RTI Connext.

Problem

Since there is specific dds writer/reader exists in,

rmw_connext/rmw_connext_cpp/src/rmw_publish.cpp

Lines 31 to 32 in 37117a9

	ConnextStaticSerializedDataDataWriter * data_writer =
	ConnextStaticSerializedDataDataWriter::narrow(dds_data_writer);

and

rmw_connext/rmw_connext_cpp/src/rmw_take.cpp

Lines 75 to 76 in 37117a9

	ConnextStaticSerializedDataDataReader * data_reader =
	ConnextStaticSerializedDataDataReader::narrow(dds_data_reader);

we are unable to use ContentFilteredTopic so far. If we use ContentFilteredTopic with filtering expression and expression parameters in reader side, rmw_take does not get event at all. It seems that ConnextStaticSerializedData has been introduced via #259. But we don't quiet understand how it works and why it is needed. Could anyone please elaborate a little bit?

Note

• We've confirmed that we can use ContentFilteredTopic on RTI Connext w/o any problems.

[fujitatomoya]

CC: @Barry-Xu-2018 @iuhilnehc-ynos

[fujitatomoya]

@Karsten1987
CC: @ivanpauno @wjwwood

It seems that you are the original author of #259, would you mind explaining a little kindly? This's been blocking us for days, we'd really appreciate for your help!

[Barry-Xu-2018]

@Karsten1987

Now we are implementing the feature of content filtered Topic. DDS is RTI connext for verification.

We encountered an issue that topic_reader created by a subscriber with content_filtered_topic can't not receive messages from a publisher of ROS2. read_condition is not be triggered by DDS::WaitSet.

After investigation, we find it seems to be related to #259. (During investigation, we created a issue #474. We think #474 is caused by #259).
We don't know the detail of implementation of RTi connext.
Below is our thought. We want to discuss with you to find out the solution to support content filtered topic.

1. rti sample (standard method)

# writer to filter a topic (use a data type, for example string.idl with a key member)

connext

    reader side
        set filter: key = 1

    writer side
        write_msg("key=1")
            # internal implementation (It's just guess.)
            for loop to check if the peer reader is filtered by key== 1
            if yes
                write_to_a_specific_reader
            else
                do nothing


2. ros2 (current)

connext

    reader side
        read_condition created by normal topic reader

    writer side
        write_msg(serialization_data(raw_data about "key=1"))
            just use the narrowed writer's socket to send data. (so the read_condition can be triggered)


3. ros2 with content filter topic feature (new feature)

# writer to filter a topic (data type: connext_static_serialized_data.idl)

connext

    reader side
        set filter: key = 1
        read_condition created by cft topic reader. 
        (We guess read_condition will be triggered inside only after receiving the filtered data, 
         it's totally different from the read_condition created by normal topic.)

    writer side
        write_msg(serialization_data(raw_data about "key=1"))
            Here is an issue.
            We think that it uses current writer's socket to send data
            and it will not check which peer reader this writer send because 
            this writer knows nothing about the raw_data "key=1"

[fujitatomoya]

@Karsten1987

friendly ping, we got really stuck here. in the meantime, we will be searching the solution.

[fujitatomoya]

we will give it a shot with https://github.com/rticommunity/rmw_connextdds/ instead.

[fujitatomoya]

closing this, since https://github.com/rticommunity/rmw_connextdds/ works okay.