This repository demonstrates Bazel configuration transitions and their limitations.

Specifically, it investigates the issue of targets being unnecessarily built multiple times when transitions are used.

Repository Strucutre

The repository is divided into four packages:

1. without_transitions
2. with_incoming
3. with_incoming_and_outgoing
4. with_incoming_and_path_mapping

Each package separately defines two rules: static_file and copy_file. static_file executes a single action with no inputs and a single output. copy_file executes a single action with a single input and which produces a single output by copying that input.

While we encourage you to read the README.md contained in each package to understand how they use transitions differently, we provide a brief summary of each package below.

While without_transitions defines the rules as you'd expect, with_incoming adds an incoming transition to demonstrate targets being built multiple times in separate output directories, due to the configuration under which they're built differing. Without the prescence of an outgoing transition or path mapping, Bazel isn't smart enought to realize that their output can be shared across configurations.

with_incoming_and_outgoing and with_incoming_and_path_mapping showcase two different approaches to solving this problem. with_incoming_and_outgoing adds an outgoing transition that resets configuration to its default state across dependency boundaries, ensuring that every target is built only once. with_incoming_and_outgoing leverages path mapping and action deduplication for a more elegant solution that still allows for a target to be built differently, depending on the configuration.