Writing storage plugins

There are different interfaces for storage plugins depending on your need: The general ReadWriteStorage and the more specific ReadableStorage.

Writing a general plugin

Assume you write a plugin MyStorage which can both save messages and read messages. Its header file could be my_storage.hpp and MyStorage will derive from rosbag2_storage::storage_interfaces::ReadWriteInterface. Important: While implementing the interface provided by rosbag2_storage::storage_interfaces::ReadWriteInterface, make sure that all resources such as file handles or database connections are closed or destroyed in the destructor, no additional close call should be necessary.

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

#include "pluginlib/class_list_macros.hpp"
PLUGINLIB_EXPORT_CLASS(MyStorage, rosbag2_storage::storage_interfaces::ReadWriteInterface)

Furthermore, we need some meta-information in the form of a plugin_description.xml file. Here, it contains

<library path="my_storage_lib">
  <class name="my_storage" type="MyStorage" base_class_type="rosbag2_storage::storage_interfaces::ReadWriteInterface">
  </class>
</library>

my_storage_lib is the name of the library (ament package) while my_storage is an identifier used by the pluginlib to load it.

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_storage plugin_description.xml)

The first argument rosbag2_storage denotes the library we add our plugin to (this will always be rosbag2_storage), while the second argument is the path to the plugin description file.

Writing a plugin for reading only

When writing plugins to only provide functionality for reading, derive from rosbag2_storage::storage_interfaces::ReadOnlyInterface.

If the read-only plugin is called my_readonly_storage in a library my_storage_lib, it will be registered using

#include "pluginlib/class_list_macros.hpp"
PLUGINLIB_EXPORT_CLASS(MyReadonlyStorage, rosbag2_storage::storage_interfaces::ReadOnlyInterface)

with the plugin description

<library path="my_storage_lib">
  <class name="my_readonly_storage" type="MyReadonlyStorage" base_class_type="rosbag2_storage::storage_interfaces::ReadOnlyInterface">
  </class>
</library>

and the usual pluginlib export in the CMakeLists:

pluginlib_export_plugin_description_file(rosbag2_storage plugin_description.xml)