What is the difference between callback_group and callback_event when creating a publisher?

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The event callbacks are for middleware Quality of Service events such as a buffer being filled, a publication deadline being missed, or similar. You can see which events are available for publishers in the documentation here.

Callback groups are used to control how the callbacks are called. For example, they can be used to prevent two callbacks being entered at the same time if a multi-threaded executor is used. They are more important to subscribers, which have a callback for when data is received, than publishers, although publishers have QoS event callbacks that are relevant. Take a look at the documentation for the CallbackGroup object, which manages them, for more information about what they are and how to use them.