how does the request<-->response work?

edit

I don't think it's well-documented anywhere yet. It's kind of informal; there isn't a master list of things that can be requested. There's just a standard topic ~/request the request providers subscribe to and that requesters can advertise.

I grepped for Subscribe.*~/request in gazebo and found the following list of request providers:

./gazebo/gui/GLWidget.cc
./gazebo/gui/ModelListWidget.cc
./gazebo/gui/ModelRightMenu.cc
./gazebo/physics/HeightmapShape.cc
./gazebo/physics/PhysicsEngine.cc
./gazebo/physics/World.cc
./gazebo/rendering/Scene.cc

To see the requests provided by a given provider, you have to look at the code. For example, see World::ProcessRequestMsgs in World.cc to see some of the following:

    if ((*iter).request() == "entity_list")
…
    else if ((*iter).request() == "entity_delete")
…
    else if ((*iter).request() == "entity_info")
...

This is only a partial answer, but I'll add more if there are further questions.