SoD: "how it works"

[oliver-sanders]

It would be good to have an in-depth description of the implementation of SoD in the docs, partly for the determined reader, but mostly for our own benefit.

The original proposal is a useful document, however, it is now out of date. It would be great to keep this essential information alive as a living document. The target audience of the document would be ourselves, new contributors and enthusiastic users.

For a taste of the kind of content I think this document should include:

• Pool management (e.g. partially satisfied prereqs & incomplete tasks).
• Flow specifics (e.g. merge conditions, db lookups on task spawning, triggering).
• Graph execution (e.g. branching, stalling & play/pause interaction).

The best place for this document under the current structure would be "reference".