The Ash engine handles the parallelization/running of requests to Ash.

Much of the complexity of this doesn't come into play for simple requests. The way it works is that it accepts a list of Ash.Engine.Request structs. Some of values on those structs will be instances of Ash.Engine.Request.UnresolvedField. These unresolved fields can express a dependence on the field values from other requests. This allows the engine to wait on executing some code until it has its required inputs, or if all of its dependencies are met, it can execute it immediately. The engine's job is to resolve its unresolved fields in the proper order, potentially in parallel. It also has knowledge baked in about certain special fields, like data which is the field we are ultimately trying to resolve, and query which is the field that drives authorization for read requests. Authorization is done on a per engine request basis.

As the complexity of a system grows, it becomes very difficult to write code that is both imperative and performant. This is especially true of a framework that is designed to be configurable. What exactly is done, as well as the order it is done in, and wether or not is can be parallelized, varies wildly based on factors like how the resources are configured and what capabilities the datalayer has. By implementing a generic "parallel engine", we can let the engine solve for the optimization. We simply have to express the various operations that must happen, and what other pieces of data they need in order to happen, and the engine handles the rest.

Eventually, this module may (potentially) be used more explicitly, as a way to construct "sagas" or "multis" which represent a series of resource actions with linked up inputs. If each of those resource actions can be broken into its component requests, and the full set of requests can be processed, we can compose large series' of resource actions without having to figure out the most optimal way to do it. They will be done as fast as possible. But we have a long way to go before we get there.

Check out the docs for Ash.Engine.Request for some more information. This is a private interface at the moment, though, so this documentation is just here to explain how it works it is not intended to give you enough information to use the engine directly.