Project Depth
TL;DR - Low depth values mean faster builds and better all-around scalability.
It's often useful to think of module dependencies as a directed tree or directed acyclic graph. If a module is a node, then each module dependency is a child node, and the dependencies of those dependencies are grand-child nodes.
This is especially useful when thinking about build performance, because the parent-child relationship is clear: child nodes must build before parent nodes.
In the above example,
:lib-1and:lib-2must be built before:screen-1.:lib-2must be build before:screen-2.:screen-1and:screen-2must be built before:app.
It's worth pointing out that this relationship is recursive, as well. Grand-child nodes must build before their parents.
Dependencies and Build Concurrency
Individual module builds are always done single-threaded, but multiple modules may build in parallel so long as no module in the set depends upon another module in that set. In the above graph,
:lib-1and:lib-2may build in parallel:lib-1and:screen-2may build in parallel:scren-1and:screen-2may build in parallel
The maximum number of parallel module builds is determined by the structure of the dependency graph and the number of available processor cores on the machine which is performing the build.
Depth
Depth refers to the maximum number of edges between a module and each of its leaf nodes in the project dependency graph.
Low depth values indicate a shallow or flat project structure with loose (or no) coupling between modules. In a full build, these projects scale well with hardware upgrades because they're able to build all those independent modules in parallel.
On the other hand, "deep" projects do not offer many opportunities for parallelization. They have project dependencies which must be built sequentially. They also perform poorly in incremental builds, because a single change to even a mid-level module invalidates cached builds for half of the project.