Aggregators

The aggregators are the opposite of splitters—they combine multiple messages and present them as a single message to the next endpoint. This is a very complex operation, so let's start by a real life scenario. A news channel might have many correspondents who can upload articles and related images. It might happen that the text of the articles arrives much sooner than the associated images—but the article must be sent for publishing only when all relevant images have also arrived. This scenario throws up a lot of challenges; partial articles should be stored somewhere, there should be a way to correlate incoming components with existing ones, and also there should be a way to identify the completion of a message. Aggregators are there to handle all of these aspects—some of the relevant concepts that are used are MessageStore, CorrelationStrategy, and ReleaseStrategy. Let's start with a code sample and then we will dive down to explore each of these...