Whenever two systems need to "talk", the nature of the speech acts to be used raises its head.
Do I talk while you listen?
Do I ask you a question and you answer me?
Do I state a fact and broadcast it to anybody who happens to be listening?
Do I sit back and say "go on, ask me a question"?
Some of these choices can be safely relegated to implementation detail. i.e. technically speaking, numerous options can be made to work.
However, some are directly impacted by the boundaries - both physical and organisational - between the communicating parties.
In the Public Services Broker - an SOA based integration infrastructure under construction in Ireland, many of the services connected to the PSB will be hosted by agencies separated from the PSB both physically and organisationally.
As a consequence, it is necessary to analyse the various possible meessage exchange choreographies and take these "business" issues into account. The document Push and Pull Message Delivery in the PSB Architecture is a position paper on the topic.
Although the document is specific to the PSB, the questions discused have a wider applicability in any SOA based on an asynchronous messaging substrate.