ROS 2 middleware interface

This article describes the rationale for using an abstract middleware interface between ROS and a specific middleware implementation. It will outline the targeted use cases as well as their requirements and constraints. Based on that the developed middleware interface is explained.

Original Author: Dirk Thomas

The middleware interface

Why does ROS 2 have a middleware interface

The ROS client library defines an API which exposes communication concepts like publish / subscribe to users.

In ROS 1 the implementation of these communication concepts was built on custom protocols (e.g., TCPROS).

For ROS 2 the decision has been made to build it on top of an existing middleware solution (namely DDS). The major advantage of this approach is that ROS 2 can leverage an existing and well developed implementation of that standard.

ROS could build on top of one specific implementation of DDS. But there are numerous different implementations available and each has its own pros and cons in terms of supported platforms, programming languages, performance characteristics, memory footprint, dependencies and licensing.

Therefore ROS aims to support multiple DDS implementations despite the fact that each of them differ slightly in their exact API. In order to abstract from the specifics of these APIs, an abstract interface is being introduced which can be implemented for different DDS implementations. This middleware interface defines the API between the ROS client library and any specific implementation.

Each implementation of the interface will usually be a thin adapter which maps the generic middleware interface to the specific API of the middleware implementation. In the following the common separation of the adapter and the actual middleware implementation will be omitted.

+-----------------------------------------------+
|                   user land                   |
+-----------------------------------------------+
|              ROS client library               |
+-----------------------------------------------+
|             middleware interface              |
+-----------------------------------------------+
| DDS adapter 1 | DDS adapter 2 | DDS adapter 3 |
+---------------+---------------+---------------+
|    DDS impl 1 |    DDS impl 2 |    DDS impl 3 |
+---------------+---------------+---------------+

Why should the middleware interface be agnostic to DDS

The ROS client library should not expose any DDS implementation specifics to the user. This is primarily to hide the intrinsic complexity of the DDS specification and API.

While ROS 2 only aims to support DDS based middleware implementations it can strive to keep the middleware interface free of DDS specific concepts to enable implementations of the interface using a different middleware. It would also be feasible to implement the interface by tying together several unrelated libraries providing the necessary functions of discovery, serialization and publish / subscribe.

+-----------------------------------+
|             user land             |   no middleware implementation specific code
+-----------------------------------+
|        ROS client library         |   above the interface
+-----------------------------------+
|       middleware interface        |   ---
+-----------------------------------+
| mw impl 1 | mw impl 2 | mw impl 3 |
+-----------+-----------+-----------+

How does the information flow through the middleware interface

One goal of the middleware interface is to not expose any DDS specific code to the user land code. Therefore the ROS client library “above” the middleware interface needs to only operate on ROS data structures. ROS 2 will continue to use ROS message files to define the structure of these data objects and derive the data structures for each supported programming language from them.

The middleware implementation “below” the middleware interface must convert the ROS data objects provided from the client library into its own custom data format before passing it to the DDS implementation. In reverse custom data objects coming from the DDS implementation must be converted into ROS data objects before being returned to the ROS client library.

The definition for the middleware specific data types can be derived from the information specified in the ROS message files. A defined mapping between the primitive data types of ROS message and middleware specific data types ensures that a bidirectional conversion is possible. The functionality to convert between ROS types and the implementation specific types or API is encapsulated in the type support (see below for different kind of type supports).

+----------------------+
|      user land       |   1) create a ROS message
+----------------------+      v
|  ROS client library  |   2) publish the ROS message
+----------------------+      v
| middleware interface |      v
+----------------------+      v
|      mw impl N       |   3) convert the ROS message into a DDS sample and publish the DDS sample
+----------------------+
+----------------------+
|      user land       |   3) use the ROS message
+----------------------+      ^
|  ROS client library  |   2) callback passing a ROS message
+----------------------+      ^
| middleware interface |      ^
+----------------------+      ^
|      mw impl N       |   1) convert the DDS sample into a ROS message and invoke subscriber callback
+----------------------+

Depending on the middleware implementation the extra conversion can be avoided by implementing serialization functions directly from ROS messages as well as deserialization functions into ROS messages.

Considered use cases

The following use cases have been considered when designing the middleware interface:

Single middleware implementation

ROS applications are not built in a monolithic way but distributed across several packages. Even with a middleware interface in place the decision of which middleware implementation to use will affect significant parts of the code.

For example, a package defining a ROS message will need to provide the mapping to and from the middleware specific data type. Naively each package defining ROS messages might contain custom (usually generated) code for the specific middleware implementation.

In the context of providing binary packages of ROS (e.g., Debian packages) this implies that a significant part of them (at least all packages containing message definitions) would be specific to the selected middleware implementation.

                +-----------+
                | user land |
                +-----------+
                     |||
      +--------------+|+-----------------+
      |               |                  |
      v               v                  v
+-----------+   +-----------+   +-----------------+   All three packages
| msg pkg 1 |   | msg pkg 2 |   | middleware impl |   on this level contain
+-----------+   +-----------+   +-----------------+   middleware implementation specific code

Static vs. dynamic message types with DDS

DDS has two different ways to use and interact with messages.

On the one hand the message can be specified in an IDL file from which usually a DDS implementation specific program will generate source code. The generated code for C++, e.g., contains types specifically generated for the message.

On the other hand the message can be specified programmatically using the DynamicData API of the XTypes specification. Neither an IDL file nor a code generation step is necessary for this case.

Some custom code must still map the message definition available in the ROS .msg files to invocations of the DynamicData API. But it is possible to write generic code which performs the task for any ROS .msg specification passed in.

                +-----------+
                | user land |
                +-----------+
                     |||
      +--------------+|+----------------+
      |               |                 |
      v               v                 |
+-----------+   +-----------+           |            Each message provides its specification
| msg pkg 1 |   | msg pkg 2 |           |            in a way which allows a generic mapping
+-----------+   +-----------+           |            to the DynamicData API
      |               |                 |
      +-------+-------+                 |
              |                         |
              v                         v
 +-------------------------+   +-----------------+   Only the packages
 | msg spec to DynamicData |   | middleware impl |   on this level contain
 +-------------------------+   +-----------------+   middleware implementation specific code

However the performance using the DynamicData API will likely always be lower compared to the statically generated code.

Switch between different implementations

When ROS supports different middleware implementations it should be as easy and low effort as possible for users to switch between them.

Decide at compile time

One obvious way will be for a user to build all ROS packages from source selecting a specific middleware implementation. While the workflow won’t be too difficult (probably half a dozen command-line invocations), it still requires quite some build time.

To avoid the need to build from source a set of binary packages could be provided which chose the middleware implementation at build time. While this would reduce the effort for the user the buildfarm would need to build a completely separate set of binary packages. The effort to support N packages with M different middleware implementations would require significant resources (M * N) for maintaining the service as well as the necessary computing power.

Decide at runtime

The alternative is to support selecting a specific middleware implementation at runtime. This requires that the to be selected middleware implementation was available at compile time in order for the middleware specific type support to be generated for every message package.

When building ROS with a single middleware implementation the result should follow the design criteria:

Any features that you do not use you do not pay for.

This implies that there should be no overhead for neither the build time nor the runtime due to the ability to support different middleware implementations. However the additional abstraction due to the middleware interface is still valid in order to hide implementation details from the user.

Design of the middleware interface

The API is designed as a pure function-based interface in order to be implemented in C. A pure C interface can be used in ROS Client Libraries for most other languages including Python, Java, and C++ preventing the need to reimplement the core logic.

Publisher interface

Based on the general structure of ROS nodes, publishers and messages for the case of publishing messages the ROS client library need to invoke three functions on the middleware interface:

  • create_node()
  • create_publisher()
  • publish()

Essential signature of create_node

Subsequent invocations of create_publisher need to refer to the specific node they should be created in. Therefore the create_node function needs to return a node handle which can be used to identify the node.

NodeHandle create_node();

Essential signature of create_publisher

Besides the node handle the create_publisher function needs to know the topic name as well as the topic type. The type of the topic type argument is left unspecified for now.

Subsequent invocations of publish need to refer to the specific publisher they should send messages on. Therefore the create_publisher function needs to return a publisher handle which can be used to identify the publisher.

PublisherHandle create_publisher(NodeHandle node_handle, String topic_name, .. topic_type);

The information encapsulated by the topic type argument is highly dependent on the middleware implementation.

Topic type information for the DynamicData API

In the case of using the DynamicData API in the implementation there is no C / C++ type which could represent the type information. Instead the topic type must contain all information needed to describe the format of the message. This information includes:

  • the name of the package in which the message is defined

  • the name of the message

  • the list of fields of the message where each includes:

    • the name for the message field
    • the type of the message field (which can be either a built-in type or another message type), optionally the type might be an unbounded, bounded or fixed size array
    • the default value
  • the list of constants defined in the message (again consisting of name, type and value)

In the case of using DDS this information enables one to:

  • programmatically create a DDS TypeCode which represents the message structure
  • register the DDS TypeCode with the DDS Participant
  • create a DDS Publisher, DDS Topic and DDS DataWriter
  • convert data from a ROS message into a DDS DynamicData instance
  • write the DDS DynamicData to the DDS DataWriter
Topic type information for statically generated code

In the case of using statically generated code derived from an IDL there is are C / C++ types which represent the type information. The generated code contains functions to:

  • create a DDS TypeCode which represents the message structure

Since the specific types must be defined at compile time the other functionalities can not be implemented in a generic (not specific to the actual message) way. Therefore for each message the code to perform the following tasks must be generated separately:

  • register the DDS TypeCode with the DDS Participant
  • create a DDS Publisher, DDS Topic and DDS DataWriter
  • convert data from a ROS message into a DDS DynamicData instance
  • write the DDS DynamicData to the DDS DataWriter

The information encapsulated by the topic type must include the function pointers to invoke these functions.

get_type_support_handle

Since the information encapsulated by the topic type argument is so fundamentally different for each middleware implementation it is actually retrieved through an additional function of the middleware interface:

MessageTypeSupportHandle get_type_support_handle();

Currently this function is a template function specialized on the specific ROS message type. For C compatibility an approach using a macro will be used instead which mangles the type name into the function name.

Essential signature of publish

Beside the publisher handle the publish function needs to know the ROS message to send.

publish(PublisherHandle publisher_handle, .. ros_message);

Since ROS messages do not have a common base class the signature of the function can not use a known type for the passed ROS message. Instead it is passed as a void pointer and the actual implementation reinterprets it according to the previously registered type.

Type of the returned handles

The returned handles need to encapsulate arbitrary content for each middleware implementation. Therefore these handles are just opaque objects from a user point of view. Only the middleware implementation which created it knows how to interpret it.

In order to ensure that these information are passed back to the same middleware implementation each handle encodes a unique identifier which the middleware implementation can check before interpreting the handles payload.

Subscriber interface

The details of the interface necessary for the subscriber side are not (yet) described in this document.

Optionally exposing native handles

The RMW interface only exposes middleware agnostic handles. But the middleware implementation can optionally provide additional API to provide native handles. E.g. for a given ROS publisher handle a specific implementation can provide an API to access publisher related handles specific to the implementation.

While using such a feature would make the user land code specific to the middleware implementation it provides a way to use features of a middleware implementation which are not exposed through the ROS interface. See this demo for an example.

Current implementation

The described concept has been implemented in the following packages:

  • The package rmw defines the middleware interface.

  • The package rosidl_typesupport_introspection_cpp generates code which encapsulated the information from each ROS msg file in a way which makes the data structures introspectable from C++ code.

  • The package rmw_fastrtps_cpp implements the middleware interface using eProsima Fast-RTPS based on the introspection type support.

  • The package rosidl_generator_dds_idl generates DDS IDL files based on ROS msg files which are being used by all DDS-based RMW implementations which use static / compile type message types.

  • The package rmw_connext_cpp implements the middleware interface using RTI Connext DDS based on statically generated code.

    • The package rosidl_typesupport_connext_cpp generates:

      • the DDS specific code based on IDL files for each message
      • additional code to enable invoking the register/create/convert/write functions for each message type
  • The package rmw_connext_dynamic_cpp implements the middleware interface using RTI Connext DDS based on the DynamicData API of Connext and the introspection type support.

  • The package rmw_opensplice_cpp implements the middleware interface using PrismTech OpenSplice DDS based on statically generated code.

    • The package rosidl_typesupport_opensplice_cpp generates:

      • the DDS specific code based on IDL files for each message
      • additional code to enable invoking the register/create/convert/write functions for each message type
  • The package rmw_implementation provides the mechanism to switch between compile time and runtime selection of the middleware implementation.

    • If only one implementation is available at compile time it links directly against it.
    • If multiple implementations are available at compile time it implements the middleware interface itself and acts according to the strategy pattern by loading the shared library of a specific middleware implementation identified by an environment variable at runtime and pass all calls along.

One or multiple type support generators

The packages contributing to the message generation process of each message are called type support generators. Their package name starts with the prefix rosidl_typesupport_.

Each message package will contain the generated code from all type support generators which are available when the package is configured. This can be only one (when building against a single middleware implementation) or multiple type support generators.

Mapping between DDS and ROS concepts

Every ROS node is one DDS participant. If multiple ROS nodes are being run in a single process they are still mapped to separate DDS participants. If the containing process exposes its own ROS interface (e.g. to load nodes into the process at runtime) it is acting as a ROS node itself and is therefore also mapped to a separate DDS participant.

The ROS publishers and subscribers are mapped to DDS publishers and subscribers. The DDS DataReader and DataWriter as well as DDS topics are not exposed through the ROS API.

The ROS API defines queue sizes and a few Quality of Service parameters which are being mapped to their DDS equivalent. The other DDS QoS parameters are not being exposed through the ROS API.