OpenDDS FAQ

last updated on: 11/19/2007
Send updates, additions, and corrections to faq@ociweb.com



General


Q:

What's a PRF?

Q:

Is there a mailing list or forum or usenet group for discussion of OpenDDS ?

Q:

Which platforms does OpenDDS support?

Q:

Is OpenDDS interoperable with other DDS implementations?

Q:

How does OpenDDS use CORBA?

Q:

Summarize the network connections established by OpenDDS in a two participant scenario?


Obtaining and building OpenDDS


Q:

How do I obtain, configure, and build OpenDDS?

Q:

Why the tao_idl generated stub code did not define *KeylessThan struct ?

Q:

What could be causing std lib related build failures on Linux?


Configuring OpenDDS


Q:

Why "SimpleTcp" is not automatically registered ?

Q:

Does OpenDDS support broadcast or multicast? If so, how do you set it up?

Q:

Why a TCP subscriber did not receive all samples sent by the publisher?

Q:

Why can't my publisher establish connection with the remote subscriber?


Built In Topics


Q:

What is the status of BIT support in OpenDDS?

Q:

How do I turn off BIT support at build-time?

Q:

How do I turn off BIT support at run-time?




General


Q:

What's a PRF?

A:

"PRF" refers to the PROBLEM-REPORT-FORM, i.e., $DDS_ROOT/PROBLEM-REPORT-FORM.

The odds of getting questions answered, bugs fixed, etc., goes up significantly when you report problems using the PRF because it insures that developers and support personnel get the most commonly required information right away. Failure to use the PRF means that those folks have to spend valuable time (yours and theirs) having a conversation to get that information just to start debugging. Unless you've got a paid support contract, there's even a chance that nobody will answer!

So, always use the PRF!


Q:

Is there a mailing list or forum or usenet group for discussion of OpenDDS ?

A:

Yes. The DDS mailing lists are available through the OpenDDS SourceForge project page. The archives of previous messages are searchable. Please remember to use the Problem Report Form (See What's a PRF?) when posting questions (and not just "problems").

Q:

Which platforms does OpenDDS support?

A:

Currently the following platforms are fully supported:

  • Windows with VC71 and VC8 compiler
  • Linux with gcc compiler
  • Solaris with gcc compiler

    The following platforms have experimental support:

  • Solaris with sunCC compiler
  • QNX

  • Q:

    Is OpenDDS interoperable with other DDS implementations?

    A:

    Currently no. OpenDDS currently doesn't have an implementation of RTPS (Real Time Publish-Subscribe) transport layer required for interoperability.

    Q:

    How does OpenDDS use CORBA?

    A:

    OpenDDS uses CORBA to administer and control the establishment of data connections. CORBA is not used in the distribution of application data.

    Q:

    Summarize the network connections established by OpenDDS in a two participant scenario?

    A:

    Two participants (one only publishes and the other only subscribes) using TCP with no BITs. Here we would expect to see three established TCP connections at each participant:
    1. The participant contacts the InfoRepo over IIOP (CORBA). The endpoint used for this connection can be supplied as:
         -DCPSInfoRepo corbaloc:iiop:HOST:PORT/DCPSInfoRepo
      (or a similar setting in the .ini file)
      The HOST is obviously the host where the InfoRepo is running. The PORT for the InfoRepo can be controlled by passing an argument to the InfoRepo process:
         -ORBListenEndpoints iiop://:PORT
      Another option that's useful for port-forwarding solutions such as NAT traversal and SSH tunneling is hostname_in_ior. This gets appended to the endpoint like so
         -ORBListenEndpoints iiop://:PORT/hostname_in_ior=HOST_ALIAS
      Where HOST_ALIAS is a hostname (or dotted-decimal IP) that can be used by the client to contact the server. In the NAT traversal example, the HOST_ALIAS would be the external side of the NAT and the PORT would be the port that's forwarded to the host running the server.
      You can also specify more than one endpoint (just repeating the -ORBListenEndpoints will work) so you can provide endpoints that will work for both "inside" and "outside" clients.

    2. The InfoRepo will make an IIOP connection *back* to the participant. This is just the inverse of connection #1, but there is no equivalent to the -DCPSInfoRepo corbaloc:... option. It's not necessary because the InfoRepo knows how to find the participant already (from data transmitted over connection #1). To control the endpoint here, use the same -ORBListenEndpoints ... option but this time with the participant and not the InfoRepo.

    3. Once the publishing participant and the subscribing participant have been associated, they connect directly using the SimpleTCP transport instance in each one. Here the endpoint is determined by the subscriber's transport config (the "local_address" value). Unfortunately there is no equivalent to the hostname_in_ior for DDS transports.



    Obtaining and building OpenDDS


    Q:

    How do I obtain, configure, and build OpenDDS?

    A:

    Please refer to the Building OpenDDS documentation for complete requirements and build steps.

    Q:

    Why the tao_idl generated stub code did not define *KeylessThan struct ?

    A:

    The *KeylessThan struct is DCPS specific code, the -Gdcps needs be passed to tao_idl command line to generate those specific code.
      e.g. $(ACE_ROOT)\bin\tao_idl -Gdcps -I$(DDS_ROOT) Messenger.idl
    


    Q:

    What could be causing std lib related build failures on Linux?

    A:

    OpenDDS makes more extensive use of the C++ standard library, compared to ACE+TAO. You may be uncovering issues that ACE+TAO may have managed to avoid.

    We have seen gcc 4.0.x-series compiler users to hit this problem more often. The visibility feature in this compiler still seems to be in a state of flux. We therefore recommend turning it off by adding "no_hidden_visibility=1" to your platform_macros.GNU file.


    Configuring OpenDDS


    Q:

    Why "SimpleTcp" is not automatically registered ?

    A:

    Starting with the 0.11 release, the SimpleTcp transport was separated the from DCPS core library to reduce footprint. Please see the section on "Pluggable Transports" in the DDS DevGuide (http://download.ociweb.com/OpenDDS/OpenDDS-latest.pdf) for how to load the SimpleTcp library.Or you can look at the $DDS_ROOT/DevGuideExamples/DDS/Messenger example. Test script "run_test.pl" runs the test with SimpleTcp transport configuration.

    On a related note the DCPSInfoRepo uses the SimpleTcp transport solely to transmit BIT (Built In Topics) information. If your application isn't subscribing to BITS, then the InfoRepo doesn't need the transport library. Just start up the InfoRepo with the -NOBITS option and you will be OK.

    Q:

    Does OpenDDS support broadcast or multicast? If so, how do you set it up?

    A:

    Reliable multicast has been available in OpenDDS since the 0.12 release (unreliable multicast since 0.11 release). A broadcast based transport is currently not available. Please see the section on "Pluggable Transports" in the DDS DevGuide (http://download.ociweb.com/OpenDDS/OpenDDS-latest.pdf) for how to setup using the unreliable multicast transport. Or you can look the $DDS_ROOT/DevGuideExamples/DDS/Messenger. The "run_test.pl mcast" runs the test with SimpleMcast transport configuration.

    Q:

    Why a TCP subscriber did not receive all samples sent by the publisher?

    A:

    While there can be multiple reasons that the samples are lost or dropped, a common problem is that the publisher starts sending samples before the subscriber is ready to receive samples. To synchronize the two make the publisher call get_matched_subscriptions() before writing. This ensures the subscriber/datareader is ready to receive samples. Here is an example code.
        ::DDS::InstanceHandleSeq handles;
        while (1)
        {
          writer_->get_matched_subscriptions(handles);
          if (handles.length() > 0)
            break;
          else
            ACE_OS::sleep(ACE_Time_Value(0,200000));
        }
    


    Q:

    Why can't my publisher establish connection with the remote subscriber?

    A:

    While there can be multiple reasons for inter-host communication failure, this entry deals with connection establishment failure due to improper endpoint configuration The configuration option "local_address" represents the acceptor endpoint. This host/port tuple is used by the publisher to initiate an active connection establishment. In order to allow inter-host communications, make sure the endpoint interface is publicly visible. A common mistake is to re-use the configuration provided in the exercises without modifications. Since the tests are intended to be run on an intra-host environment, they generally use the 'localhost' interface. This configuration will fail when the test is run on an inter-host setting.


    Built In Topics


    Q:

    What is the status of BIT support in OpenDDS?

    A:

    Support for Built-In-Topics is complete since the 0.12 release. This means that BITs will now work as specified. BIT support is now turned on by default.

    BIT associates are created via TCP and therefore require support of the SimpleTcp transport library. When using BITs make sure the SimpleTcp transport has been properly configured in both the InfoRepo as well as the participants.

    Q:

    How do I turn off BIT support at build-time?

    A:

    It is possible to build OpenDDS without BIT support which will reduce overall footprint. For this you will need to generate new project files:
         mwc.pl -type <yourtype> -features built_in_topics=0 DDS.mwc
    
    If <yourtype> happens to be gnuace, add "built_in_topics=0" to the platform_macros.GNU file or the MAKEFLAGS environment variable.

    Q:

    How do I turn off BIT support at run-time?

    A:

    Turning off BIT support involves it being turned off in the DCPS Information Repository and any associated participants.

    The DCPSInfoRepo option -NOBITS disables BIT support for the InfoRepo.

    The option '-DCPSBit 0' will disable BIT support for all participants in the process.


    Send updates, additions, and corrections to faq@ociweb.com