Alternate Interfaces to Data#

The DDS-DCPS approach to data transfer using synchronization of strongly-typed caches (DataWriter and DataReader) is not appropriate for all applications. Therefore OpenDDS provides two different alternate interface approaches which are described in this section. These are not defined by OMG specifications and may change in future releases of OpenDDS, including minor updates. The two approaches are:

  • Recorder and Replayer

    • These interfaces allow the application to create untyped stand-ins for DataReaders and/or DataWriters

    • Recorder can be used with the Dynamic Language Binding XTypes features (Dynamic Language Binding) to access typed data samples through a reflection-based API

  • Observer

    • Observers play a role similar to the spec-defined Listeners (attached to DataReaders and/or DataWriters). Unlike the Listeners, Observers don’t need to interact with the DataReader/Writer caches to access the data samples.

The XTypes Dynamic Language Binding (Dynamic Language Binding) provides a set of related features that can be used to create DataWriters and DataReaders that work with a generic data container (DynamicData) instead of a specific IDL-generated data type.

Recorder and Replayer#

The Recorder feature of OpenDDS allows applications to record samples published on arbitrary topics without any prior knowledge of the data type used by that topic. Analogously, the Replayer feature allows these recorded samples to by re-published back into the same or other topics. What makes these features different from other Data Readers and Writers are their ability to work with any data type, even if unknown at application build time. Effectively, the samples are treated as if each one contains an opaque byte sequence.

The purpose of this section is to describe the public API for OpenDDS to enable the recording/replaying use-case.

API Structure#

Two new user-visible classes (that behave somewhat like their DDS Entity counterparts) are defined in the OpenDDS::DCPS namespace, along with the associated Listener interfaces. Listeners may be optionally implemented by the application. The Recorder class acts similarly to a DataReader and the Replayer class acts similarly to a DataWriter.

Both Recorder and Replayer make use of the underlying OpenDDS discovery and transport libraries as if they were DataReader and DataWriter, respectively. Regular OpenDDS applications in the domain will “see” the Recorder objects as if they were remote DataReaders and Replayers as if they were DataWriters.

Usage Model#

The application creates any number of Recorders and Replayers as necessary. This could be based on using the built-in topics to dynamically discover which topics are active in the Domain. Creating a Recorder or Replayer requires the application to provide a topic name and type name (as in DomainParticipant::create_topic()) and also the relevant QoS data structures. The Recorder requires SubscriberQos and DataReaderQos whereas the Replayer requires PublisherQos and DataWriterQos. These values are used in discovery’s reader/writer matching. See the section on QoS processing below for how the Recorder and Replayer use QoS. Here is the code needed to create a recorder:

OpenDDS::DCPS::Recorder_var recorder =

Data samples are made available to the application via the RecorderListener using a simple “one callback per sample” model. The sample is delivered as an OpenDDS::DCPS::RawDataSample object. This object includes the timestamp for that data sample as well as the marshaled sample value. Here is a class definition for a user-defined Recorder Listener.

class MessengerRecorderListener : public OpenDDS::DCPS::RecorderListener

  virtual void on_sample_data_received(OpenDDS::DCPS::Recorder*,
                                       const OpenDDS::DCPS::RawDataSample& sample);

  virtual void on_recorder_matched(OpenDDS::DCPS::Recorder*,
                                   const DDS::SubscriptionMatchedStatus& status );


The application can store the data wherever it sees fit (in memory, file system, database, etc.). At any later time, the application can provide that same sample to a Replayer object configured for the same topic. It’s the application’s responsibility to make sure the topic types match. Here is an example call that replays a sample to all readers connected on a replayer’s topic:


Because the stored data is dependent on the definition of the data structure, it can’t be used across different versions of OpenDDS or different versions of the IDL used by the OpenDDS participants.

QoS Processing#

The lack of detailed knowledge about the data sample complicates the use of many normal DDS QoS properties on the Replayer side. The properties can be divided into a few categories:

  • Supported

    • Liveliness

    • Time-Based Filter

    • Lifespan

    • Durability (transient local level, see details below)

    • Presentation (topic level only)

    • Transport Priority (pass-thru to transport)

  • Unsupported

    • Deadline (still used for reader/writer match)

    • History

    • Resource Limits

    • Durability Service

    • Ownership and Ownership Strength (still used for reader/writer match)

  • Affects reader/writer matching and built-in topics but otherwise ignored

    • Partition

    • Reliability (still used by transport negotiation)

    • Destination Order

    • Latency Budget

    • User/Group Data

Durability details#

On the Recorder side, transient local durability works just the same as any normal DataReader. Durable data is received from matched DataWriters. On the Replayerside there are some differences. As opposed to the normal DDS DataWriter, Replayer is not caching/storing any data samples (they are simply sent to the transport). Because instances are not known, storing data samples according to the usual History and Resource Limits rules is not possible. Instead, transient local durability can be supported with a “pull” model whereby the middleware invokes a method on the ReplayerListener when a new remote DataReader is discovered. The application can then call a method on the Replayer with any data samples that should be sent to that newly-joined DataReader. Determining which samples these are is left to the application.

Recorder With XTypes Dynamic Language Binding#

The Recorder class includes support for the Dynamic Language Binding from XTypes (Dynamic Language Binding). Type information for each matched DataWriter (that supports XTypes complete TypeObjects) is stored in the Recorder. Users can call Recorder::get_dynamic_data, passing a RawDataSample to get back a DynamicData object which includes type information – see DynamicData::type().

A tool called inspect, uses the Recorder and Dynamic Language Binding allow for the printing of any type, so long as the topic name, type name, and domain ID are known. The DataWriter must include code generation for complete TypeObjects. See tools/inspect/Inspect.cpp for this tool’s source code. It can be used as a standalone tool or an example for developing your own applications using these APIs.


To observe the most important events happening within OpenDDS, applications can create classes that derive from the Observer base class (in dds/DCPS/Observer.h). The design of Observer is intended to allow applications to have a single Observer object observing many Entities, however this is flexible to allow many different use cases. The following events can be observed:

  • DataWriter/Reader enabled, deleted

  • DataWriter/Reader QoS changed

  • DataWriter/Reader peer associated, disassociated

  • DataWriter sample sent, instance disposed, instance unregistered

  • DataReader sample received (enters the cache), read, taken, instance disposed, instance unregistered

Attaching Observers to Entities#

Entity is the spec-defined base interface of the following types:

  • DataWriter, DataReader

    • As seen above in Observer, the Observer events originate in the DataWriter and DataReader Entities

  • DomainParticipant, Publisher, Subscriber

    • Among their other roles, these Entities act as containers (either directly or indirectly) for DataWriters and DataReaders.

    • If a smaller-scoped Entity (such as a DataWriter) has no Observer for the event in question, its containing Entity (in this example, a Publisher) is checked for an Observer.

  • Topic

    • Although it is an Entity, no Observer events are generated by Topics or Entities they contain (since they don’t contain any Entities)

The class EntityImpl (in dds/DCPS/EntityImpl.h) is OpenDDS’s base class for all Entity types. EntityImpl includes public methods for Observer registration: set_observer and get_observer. These methods are not part of the IDL interfaces, so invoking them the requires a cast to the implementation (Impl) of Entity.

DDS::DataWriter_var dw = /* ... */;
EntityImpl* entity = dynamic_cast<EntityImpl*>(;
Observer_rch observer = make_rch<MyObserver>();
entity->set_observer(observer, Observer::e_SAMPLE_SENT);

Note that since the Observer class is an internal (not IDL) interface, it uses the “RCH” (Reference Counted Handle) smart pointer classes. Observer itself inherits from RcObject, and uses of Observer-derived classes should use the RcHandle template and its associated functions, as in the example above. See dds/DCPS/RcHandle_T.h for details.

Writing Observer-Derived Classes#

The virtual methods in the Observer class are divided into 3 groups based on the general category of events they observe:

  1. Operations on the observed Entity itself

    • on_enabled, on_deleted, on_qos_changed

    • The only parameter to these methods is the Entity, so the Observer implementation can use the public methods on the Entity.

  2. Events relating to associating with remote matched endpoints

    • on_associated, on_disassociated

    • In addition to the Entity, the Observer implementation receives a GUID_t structure which is the internal representation of remote Entity identity. The GUID_t values from on_associated could be stored or logged to correlate them with the values from on_disassociated.

  3. Events relating to data samples moving through the system

    • on_sample_sent, on_sample_received, on_sample_read, on_sample_taken, on_disposed, on_unregistered

    • In addition to the Entity, the Observer implementation receives an instance of the Sample structure. The definition of this structure is nested within Observer. See below for details.

The Observer::Sample structure#

The Observer::Sample structure contains the following fields:

  • instance and instance_state

    • Describe the instance that this sample belongs to, using the spec-defined types

  • timestamp and sequence_number

    • Attributes of the sample itself: timestamp uses a spec-defined type whereas sequence_number uses the OpenDDS internal type for DDSI-RTPS 64-bit sequence numbers.

  • data and data_dispatcher

    • Since Observer is an un-typed interface, the contents of the data sample itself are represented only as a void pointer

    • Implementations that need to process this data can use the data_dispatcher object to interpret it. See the class definition of ValueDispatcher in dds/DCPS/ValueDispatcher.h for more details.