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.
The repository is divided into four packages:
without_transitionswith_incomingwith_incoming_and_outgoingwith_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.