bazel-build-graph-explosion

This repository demonstrates toolchains and configuration transitions in Bazel, and their limitations.

Repository Strucutre

The repository is divided into two packages:

• with_toolchains
• without_toolchains

Both packages contain separate of implementations of the greeting rule, which accepts a subject and outputs a file containing a greeting addressed to that subject:

$ bazel build //without_toolchains:hello-world
$ cat bazel-bin/without_toolchains/hello-world_greeting.txt
Hello, world!

Greetings can also have dependencies, in which case those dependencies' subjects will be added to the greeting:

$ bazel build //without_toolchains:hello-jaden-and-world
$ cat bazel-bin/without_toolchains/hello-jaden-and-world_greeting.txt
Hello, Jaden! Also, hello to world.

The core difference between without_toolchains and with_toolchains is that in the former, the subject is provided as an attribute, whereas in the latter, it's provided as a toolchain.

Build Graph Redundancies

Often, we talk about "multiple versions" of a package being built. To see what's meant by that, run:

bazel cquery 'kind("^greeting(_with_outgoing_transition)? rule$", //with_toolchains:*)' --output graph |
    dot -Grankdir=LR -Tsvg

to generate an SVG of the build graph for the with_toolchains package:

Note that this is distinct from the dependency graph, which can be viewed with:

bazel query 'kind("^greeting(_with_outgoing_transition)? rule$", //with_toolchains/...)' \
    --output graph \
    --nograph:factored

The build graph is built at analysis-time and takes configuration into account. The dependency graph is built at loading time and doesn't.

Notice how every target has a single node in the dependency graph, but potentially multipe targets in the build graph. with_toolchains/BUILD.bazel explains why some targets are unnecessarily built multiple times and how that can be mitigated. Also notice how when toolchains and configuration transitions aren't used, every target has a single node in the build graph:

bazel cquery 'kind("^greeting rule$", //without_toolchains:*)' --output graph | dot -Grankdir=LR -Tsvg