When to use rclcpp::spin or RCLCPP_COMPONENTS_REGISTER_NODE?

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if you compile your executable with a main function and rclcpp::spin(), It makes your executable, well, an executable. This will result in a binary file that you can execute directly.

RCLCPP_COMPONENTS_REGISTER_NODE macro generates the required code for registering the node to a node factory that is used by the class loader. This is exactly how plugins work, which means you are basically making your node a plugin. In order to use it, you need to compile your node as a shared library. This means there will be another program that can load your node from the shared library and execute it.

By using rclcpp_components_register_node in CMakeLists.txt, it essentially generates a cpp that instantiates your node and does rcp::spin. The generated cpp file uses the class loader to load the shared library, instead of compiling it statically against the source code. One of the advantages is that it minimized the boilerplate that makes a node an executable.

I would prefer to use RCLCPP_COMPONENTS_REGISTER_NODE because it allows you to have the option of loading multiple nodes to a single process (just like nodelet!) which enables zero-copy data transport.