Descriptions of Topics

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Hi, I was wondering if theres some documentation that describes the purpose / how to use the various topics that are available when you run gazebo.

For example, Ive not been able to find any documentation on the Request and Response topics. What are their purposes? What can be done with them?

Are all topics available to both publishers and subscribers? For instance, if I publish a message to the Factory topic to spawn some model, could I also have a listener subscribed to that factory topic to alert me when the model has been spawned? Is there some standard way of doing this?

Thank you.