The queue starts here

Enterprise application integration solves many interoperability problems, but one of the most important involves time. Many mainframe applications rely on messaging servers, which allow transactions to be passed between applications in an asynchronous fashion, i.e. not time sensitive or reliant on a continuous session. The Internet Protocol, the language by which most e-commerce transactions are transmitted, assumes that synchronous connections can be established between a Web browser and a "merchant" server. This causes a problem: e-commerce payments can't wait in a queue until the mainframe wakes up, because the user can't wait that long for confirmation of the sale.

The solution, in this and many other EAI dilemmas, is to use a "broker". This application lies dormant on a server until a request is sent to it which ensures certain conditions are met, and then causes it to process a transaction. Under this model, instead of several e-commerce transactions clamouring for attention at once, a messaging broker organises them into an orderly queue, so that they can be handled by a mainframe without endangering the "transactional integrity" of the application by mixing up the data.