Are service callbacks queued?

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Dear all,

Reading the doc and tutorials I understand that when the topics are received (on the subscriber side) they are queued in a callback queue before being processed (on a call to spin(), spinOnce(), callAvailable(), callOne()...). Now what I did not get is weather the service callbacks (on the server side) were also queued in the same callback queue. Hence the question:

Are the service callbacks queued in the same callback queue than the topics or are they processed independently as soon as they are received (i.e. without waiting for a spin(), spinOnce(), callAvailable(), callOne()...))?

Thanks,

Antoine.