Protobuf is the most stable and widely adopted interface description language available today - it's why Buf is concentrating its initial efforts on Protobuf. But Protobuf has never had an officially published Protobuf grammar - there are proto2 and proto3 specs published, but neither actually cover all edge cases, of which there are many (especially around options).
In effect, the official Protobuf "grammar" is the
protoc implementation - this has been the only codified
representation of what Protobuf is, and the only way to properly parse Protobuf messages and produce
FileDescriptorSets suitable for stub generation.
Additionally, there are many situations outside of stub generation that rely on proper Protobuf parsing, such as linting and breaking change detection. All existing Protobuf tooling has gone one of two routes:
Use a third-party Protobuf parser instead of
protocthat produces non-
FileDescriptorSetresults. There are many third-party Protobuf parsers in existence, however no parser has been able to reliably cover all edge cases of the grammar, inevitably there are breakdowns that either result in parse errors, or an invalid representation of Protobuf sources. The edge cases in the Protobuf grammar are so numerous, that some of the most popular third-party parsers actually get around the problem by happily parsing invalid Protobuf, resulting in being unable to make a decision from these parsers as to whether or not a file is valid.
Shell out to (or build against)
protoc. This results in both accurate parsing, and
FileDescriptorSetproduction, however this method presents a number of issues. First, actually managing external
protocinstalls becomes problematic - it makes any tooling reliant on either managing
protocinstallation itself, or relying on
protocbeing deterministically installed. Second, parsing
protoc's output is difficult, as there is no structured output format, both warnings and errors are printed to stderr, and the warning and error output changes between minor releases. To accurately parse
protoc, tooling needs to handle every release of
protocas it comes out, which makes any such tooling unmaintainable. Additionally,
protochas different behavior depending on the location of the Well-Known Types.
We find neither of these solutions to be tenable in the long-term for a tool that aims to manage your Protobuf schema. Therefore, we've taken a different route.
The internal compiler quite literally replaces
of the built-in plugins (
--cpp_out, etc.). The resulting
FileDescriptorSets are tested for equivalence
protoc, including both
proto3 definitions, imports,
custom options. (We even took what we learned from building this compiler to write a
comprehensive language specification!)
FileDescriptorSets are almost byte equivalent to
protoc, in fact - under most scenarios without
SourceCodeInfo, you can actually compare the byte representation of a serialized
FileDescriptorSet produced by
protoc, and they are equal. There are two known exceptions that make this not always the case:
bufproduces slightly different SourceCodeInfo for the special
defaultfield options, to be consistent with SourceCodeInfo of other options. In
protoc, the SourceCodeInfo for these special options is inconsistent.
bufrepresents custom/unknown options slightly differently on the wire, although when deserialized, the result is equivalent for consumers of
FileDescriptorSets. The underlying compiler provides a work-around (see method
bufdoes not enable it because it has both a performance and usability cost, and it has zero effect on any actual usage.
Besides removing the need to manually manage
protoc and the
Well-Known Types (which
in all cases),
buf's compiler is faster than
protoc in most scenarios.
buf parses your
.proto files across all
available cores, and re-orders the result to match
protoc's ordering as a post-processing task. As an example,
can compile all 3,944
.proto files in
googleapis in about 0.9s, on a
2022 MacBook Pro with an M1 Max chip, as opposed to about 1.6s for
protoc v21.7 on the same machine.
We know this is all a series of big claims. There have been many claims in the Protobuf community about producing
protoc-based parsing, so this is one of the reasons that we enable
protoc output to be
buf input. If you don't
trust us, then use
protoc as your compiler instead, no problem.
It's also one of the reasons we've exposed
buf build as we have - you can produce
FileDescriptorSets yourself and pass them to your Protobuf plugins to verify that the resulting stubs are equivalent.
There is one known exception with docs generated based on
this issue to track this being updated within
Instead of this...
$ rm -rf java
$ mkdir java
$ protoc -I . --java_out=java $(find . -name '*.proto')
...you can use
buf's compiler to generate your stubs with the
--descriptor_set_in flag of
protoc. We need to do
buf build | buf ls-files - instead of
buf ls-files to make sure that the filenames are root.
$ rm -rf java
$ mkdir java
$ buf build -o - | protoc --descriptor_set_in=/dev/stdin --java_out=java $(buf ls-files)
This results in protoc's internal parser not being used at all, so you can verify our claims further. If you do find an issue, contact us.
Having this new compiler is a key component of Buf's future. Right now, it enables reliable linting, breaking change detection, generation, and the BSR. In the future, it enables a lot of other real-time possibilities for us.