Design documents are not descriptions of the current functionality of Bazel. Always go to the documentation for current information.
Author: Klaus Aehlig
Design document published: 11 October 2016
Bazel depends on a protobuffer compiler to generate code, especially
java code, from an abstract description of the protocol buffer;
in particular, files generated by
protoc are machine-independent.
In fact, Bazel most of the time uses the latest version of
New versions of
protoc that contain incompatible changes to the
programming interface are released frequently.
The current approach to the
protoc dependency is to have checked-in
statically-linked executables for all the supported platforms (where
some platforms, like FreeBSD, have to use Linux-compatibility features).
The full source tree of the protobuf compiler is also part of the repository.
However, for generating files, the committed binaries are always used.
The current approach as certain shortcomings.
Having up-to-date binaries for all the supported platforms does not scale well as the number of platforms Bazel should run on is increasing.
The requirement of having a suitable executable in the code base adds additional complexity to the process of bootstrapping a new architecture.
Binaries in the code base do not follow standard open-source principles; in fact, meaningful reviews for changes updating them are hard and in practise often boil down to a question of trust in the person making the change.
Committed binaries make the "source" repository unnecessary big. Currently,
a checkout at head contains over 250MB in committed
BUILD file for the
third_party/protobuf is changed in such a
way, that the
protoc is compiled from source instead of selecting from
the committed pre-built binaries; the pre-built binaries are removed from
the source tree. As the
protoc sources are already part of the repository,
this is not a huge change; also, as
protoc is written in
C++, no additional
dependencies are introduced that way.
Note that then, every user who already has a working (bootstrap)
build bazel from source, without depending on committed binaries or having
protoc already on the machine. The problem of building your first
will be addressed in the next sections.
This change also removes an internal consistency requirement from the code base. It was always assumed that the binaries actually match the accompanying sources.
A new target
//:bazel-distfile will be added. This will be an archive
all source files in their respective places, including the files
scripts, etc, as well as
under a subdirectory
derived all the files generated by
are needed to compile a bootstrap version of
For convenience, the
derived subdirectory may also contain other
generated architecture-independent files, like an HTML-version of the
documentation for local browsing. A corrollary of the archive layout is that
by removing the
derived directory a checkout of the upstream sources is
This new artifact will be built for every release and made available together along with the other release artifacts (like packages, installers, executables). The same means of certifying integrity (like hashes, SSL-certificates) will be used.
compile.sh will be modified to first check if a
derived directory exists
and if this is the case assume that all the files generated by
already present there; only if not present, it will try to generate the needed
protoc for bootstrapping, assuming that the
variable points to a good
So, there will be three ways to build
If one has an old
bazel binary already, a new one can be built from a
checkout of the source repository. This approach is useful for developpers.
It might also be used by users who want to upgrade their old
to the next release.
By downloading the distribution artifact, the
compile.sh script can be
used to build bazel. Again, no
protoc has to be installed ahead of time.
This approach is useful for source distributions, as well as for bringing
Bazel to a new platform.
If one already has the correct version of
protoc on the machine, the
compile.sh script can be used by setting the
PROTOC environment variable.
This approach is useful for distributions that want to provide snapshots
bazel inbetween official releases and maintain a
protoc package anyway.
This would be the standard open-source approach of requiring the user to have
the required dependencies installed ahead of time. Unfortunately,
contains incompatible changes too frequently, so that this would be an
burden. Note that the bootstrapping from your own
protoc and a repository
checkout is still possible with the suggested approach.
Another approach would be to make the output of
protoc part of the versioned
sources instead of generating them for the distribution file. As with all
approaches based on committing generated files, this would
introduce another consistency requirement to the repository. In this case, the
requirement would be that the generated files be up-to-date with respect to the
.proto files. Of course, such a consistency could be verified by
an appropriate test. Nevertheless, it seems the cleaner and probably more
managable to only version true source files and generate derived files from
the respective sources.