Pull-style message retrieval means that the caller actively has to retrieve the response(s) after sending a request while with the push-style the caller will be sent any responses. In the first case, the caller may have to allocate a thread to retrieve the responses in the background, in order to be able to continue the work. In the second case, usually a callback (a function or method) will be invoked when a response is received (or, alternatively, when all responses have been received).