Running the DAG
In our previous sections we said things like "...the application asks power framework to prepare the system...". An "ask" in this context means the application requests a lease. A lease is for a power level.
Power levels above "OFF" levels are only active when there are active leases in the system. If there are no active leases all power elements are at their lowest level.
Let's consider the DAG in our example from the coordination section and assume that we have power elements with only two power levels, which we simplify here by not showing the levels.
Here we read the arrow direction as, for example, "Application needs CPU".
If the application asks power framework to prepare the system for downloading, then the power framework walks the DAG starting at the application and figures out the order to create the valid system state.
The power framework examines system resources to see if they are in the right state and tells them to change if they are not. In this case it talks to the resource owner of the CPU, network driver, and network stack in that order and finally the application when everything is ready. The power framework waits for each dependency to report it is ready before asking its dependent to change its state.
After the application receives the file contents it can tell the power framework it no longer needs the system to be ready to download a file. The power framework then walks the DAG again, informing resources they can power down.
