Callbacks in world cycle

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I made a world plugin that spawns and deletes specific objects, but i don't fully understand the world update cycle. This question arose when comparing the ROS and Gazebo communication system. In ROS, we can decide when to dispatch callbacks using spinOnce, but in Gazebo I do not know when the callbacks are executed. So my question is: When are the callbacks executed? Is this process asynchronous due to Boost.Asio?