Ros::spinOnce(), how does it work?

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Hi,

I have a servo and I am trying to call a service from a client code. Then, based on the software architecture of the client, the call is supposed to be taken up by a service server. In the launch file I specified both the client and server code name. In both client and server node, I have used ros::spinOnce to use looping. The client code is written in a manner that it checks for any input given from keyboard. I noticed that if I don't give any input, it still goes to the server node.

I used print statements to check it. Specifically, I mean to say that although the client.call function is not accessed in the client code, it still goes into the while loop section of the ROS::Rate in the server code and keeps on printing the test string for a while. Then, again it goes back to the while loop of ros::Rate section in client code. This process keeps on repeating.

My question is that how does ros run the server code if it has not been called yet?

thanks,