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converter_plugin_development.md

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Converter plugins

Converter plugins are used to convert between different storage serialization formats. A storage for rosbags in ROS 2 obtains and stores its data using a serialization format. By default, it will store a bag in the serialization format of the middleware it was recorded with (two middlewares may share the same serialization format). If messages should be read by a middleware with a different serialization format, the messages must be converted to that format. To simplify conversion between all formats each plugin provides functions to convert to and from generic ROS 2 messages.

Rosbag2 is shipped with a default converter plugin to convert between ROS 2 messages and serialized messages in the CDR format (this is the general serialization format specified by DDS).

Writing converter plugins for both serialization and deserialization

Most converter plugins are used to convert between a custom serialization format in both directions, i.e. serialization and deserialization. To write a plugin MyConverter supporting conversion to and from a custom serialization format, implement the interface rosbag2::converter_interfaces::SerializationFormatConverter.

The plugin interface provides two functions:

  • The function serialize takes an initialized SerializedBagMessage and a ROS 2 message together with information about its topic and type and should fill the SerializedBagMessage with the ROS 2 message contents.
  • The other function deserialize does the reverse, taking a full SerializedBagMessage to fill a preallocated ROS 2 message using the provided typesupport (which has to match the actual type of the SerializedBagMessage).

In order to find the plugin at runtime, it needs to be exported to the pluginlib. Add the following lines in the my_converter.cpp:

#include "pluginlib/class_list_macros.hpp"
PLUGINLIB_EXPORT_CLASS(MyConverter, rosbag2::converter_interfaces::SerializationFormatConverter)

Furthermore, we need some meta-information in the form of a plugin_description.xml file. In the case of MyConverter this would look like:

<library path="my_converter_library">
  <class
    name="my_storage_format_converter"
    type="MyConverter"
    base_class_type="rosbag2::converter_interfaces::SerializationFormatConverter"
  >
    <description>This is a converter plugin for my storage format.</description>
  </class>
</library>

where my_converter_library is the name of the library, while my_storage_format_converter is the identifier used by the pluginlib to load it. It must have the format <rmw storage format>_converter as described in the section "How to choose conversion at runtime".

In addition, in the CMakeLists.txt the plugin_description.xml file needs to be added to the index to be found at runtime:

pluginlib_export_plugin_description_file(rosbag2 plugin_description.xml)

The first argument rosbag2 denotes the ament index key we add our plugin to (this will always be rosbag2 for converter plugins), while the second argument is the path to the plugin description file.

Writing converter plugins for only serialization or deserialization

It is possible to write a plugin which only supports converting to OR from a custom serialization format. For example, the default plugin for legacy ROS1 rosbags supports only conversion from a ROS 1 to a ROS 2 message format (deserialization), since rosbag2 is not intended to be used to write rosbags in the old format. When a similar use case applies, it is possible to provide a plugin which supports only one direction (serialization or deserialization).

In order to write a plugin MyDeserializer supporting conversion from a custom serialization format, implement the interface rosbag2::converter_interfaces::SerializationFormatDeserializer. This interface only provides the deserializemethod mentioned above. Similarly, there exists arosbag2::converter_interfaces::SerializationFormatSerializer` for serialization plugins.

How to choose conversion at runtime

The conversion will be chosen automatically according to the storage format specified in the bagfile or specified by the user. To achieve this, rosbag2 searches for a converter plugin with name <storage format>_converter. For instance, the default plugin for conversion between CDR messages is named cdr_converter in its pluginlib.txt file. There should be no need for different converters for the same storage format hence this is not supported.