10.3. Natural Flow Control in the form of Attachments

VDF includes a mechanism called attachments, which facilitates the passing of information from one deployer to the next. Attachments are implemented as a slightly-enhanced java.util.Map, whose entries each represent an attachment.

Some deployers are producers, while others are consumers. The same deployer can also perform both functions. Some deployers create metadata or utility instances, putting them into the attachments map. Other deployers only declare their need for these attachments and pull the data from the attachments map, before doing additional work on that data.

Natural order refers to the way that deployers are ordered. A common natural order uses the relative terms before and after. However, with the attachments mechanism already in place, you can order deployers by the way in which they produce and/or consume the attachments.

Each attachment has a key, and deployers pass keys to the attachments they produce. If the deployer produces an attachment, then that key is called output. If the deployer consumes an attachment, then that key is called input.

Deployers have ordinary inputs and required inputs. Ordinary inputs are only used to help determine the natural order. Required inputs also help determine order, but they have another function too. They help to determine if the deployer is actually relevant for the given deployment, by checking to see if an attachment corresponding to that required input exists in the attachments map.