Throughout the documentation, you may occasionally see references to Buf images. We'll go over what images are, how they are used, and the various options associated with them here.
How Protobuf plugins work
First we need to provide a short overview of how Protobuf plugins work.
When you invoke this command...
$ protoc -I . --go_out=gen/go foo.proto
...here's (roughly) what happens:
protoc
compiles the filefoo.proto
(and any imports) and internally produces aFileDescriptorSet
, which is a list ofFileDescriptorProto
messages. These messages contain all information about your.proto
files, including optional source code information such as the start/end line/column of each element of your.proto
file, as well as associated comments.- The
FileDescriptorSet
is turned into aCodeGeneratorRequest
, which contains theFileDescriptorProto
s thatprotoc
produced forfoo.proto
and any imports, a list of the files specified (justfoo.proto
in this example), as well as any options provided after the=
sign of--go_out
or with--go_opt
. protoc
then looks for a binary namedprotoc-gen-go
, and invokes it, giving the serialized CodeGeneratorRequest as stdin.protoc-gen-go
runs, and either errors or produces aCodeGeneratorResponse
, which specifies what files are to be generated and their content. The serialized CodeGeneratorResponse is written to stdout ofprotoc-gen-go
.- On success of
protoc-gen-go
,protoc
reads stdout and then writes these generated files.
The built-in generators to protoc
, such as --java_out
, --cpp_out
, etc., work in roughly the same manner, although
instead of executing an external binary, this is done internally to protoc
.
FileDescriptorSet
s are the core primitive used throughout the Protobuf ecosystem to represent a compiled Protobuf
schema. They are also the primary artifact that protoc produces.
That is to say that everything you do with protoc
, and any plugins you use, talk in terms of FileDescriptorSet
s.
gRPC Reflection uses them under the hood as well.
Creating FileDescriptorSet
s with protoc
protoc
provides the --descriptor_set_out
flag, aliased as -o
, to allow writing serialized FileDescriptorSet
s.
For example, given a single file foo.proto
, you can write a FileDescriptorSet
to stdout like this:
$ protoc -I . -o /dev/stdout foo.proto
The resulting FileDescriptorSet
contains a single FileDescriptorProto
with name foo.proto
.
By default, FileDescriptorSet
s don't include any imports not specified on the command line, and don't include source
code information. Source code information is useful for generating documentation inside your generated stubs, and for
things like linters and breaking change detectors. As an example, assume foo.proto
imports bar.proto
. To produce a
FileDescriptorSet
that includes both foo.proto
and bar.proto
, as well as source code information:
$ protoc -I . --include_imports --include_source_info -o /dev/stdout foo.proto
What are Buf images?
An image is Buf's custom extension to FileDescriptorSet
s. The actual definition is currently stored in the
bufbuild/buf repo as of this
writing.
Buf images are FileDescriptorSet
s, and FileDescriptorSet
s are images. Due to the forwards- and
backwards-compatible nature of Protobuf, we add a field to FileDescriptorSet
while maintaining compatibility in both
directions - existing Protobuf plugins drop this field, and buf
does not require this field to be set to work with
images.
Modules are the primitive of Buf, and Buf images represent the compiled artifact of a
module. In fact, images contain information about the module used to create it, which powers a variety of
BSR features. For clarity, the Image
Protobuf definition is shown below
(notice the ModuleName
in the ImageFileExtension
):
// Image is an extended FileDescriptorSet.
message Image {
repeated ImageFile file = 1;
}
// ImageFile is an extended FileDescriptorProto.
//
// Since FileDescriptorProto does not have extensions, we copy the fields from
// FileDescriptorProto, and then add our own extensions via the buf_extension
// field. This is compatible with a FileDescriptorProto.
message ImageFile {
optional string name = 1;
optional string package = 2;
repeated string dependency = 3;
repeated int32 public_dependency = 10;
repeated int32 weak_dependency = 11;
repeated google.protobuf.DescriptorProto message_type = 4;
repeated google.protobuf.EnumDescriptorProto enum_type = 5;
repeated google.protobuf.ServiceDescriptorProto service = 6;
repeated google.protobuf.FieldDescriptorProto extension = 7;
optional google.protobuf.FileOptions options = 8;
optional google.protobuf.SourceCodeInfo source_code_info = 9;
optional string syntax = 12;
// buf_extension contains buf-specific extensions to FileDescriptorProtos.
//
// The prefixed name and high tag value is used to all but guarantee there
// will never be any conflict with Google's FileDescriptorProto definition.
// The definition of a FileDescriptorProto has not changed in years, so
// we're not too worried about a conflict here.
optional ImageFileExtension buf_extension = 8042;
}
message ImageFileExtension {
// is_import denotes whether this file is considered an "import".
optional bool is_import = 1;
// ModuleInfo contains information about the Buf module this file belongs to.
optional ModuleInfo module_info = 2;
// is_syntax_unspecified denotes whether the file did not have a syntax explicitly specified.
optional bool is_syntax_unspecified = 3;
// unused_dependency are the indexes within the dependency field on
// FileDescriptorProto for those dependencies that are not used.
repeated int32 unused_dependency = 4;
}
Linting and breaking change detection
Linting and breaking change detection internally operate on Buf images that the buf
CLI either produces on the fly or
reads from an external location. They represent a stable, widely used method to represent a compiled Protobuf schema.
For the breaking change detector, images are the storage format used if you want to manually store the state of your
Protobuf schema. See the input documentation for more details.
Creating images
You can create Buf images using buf build
. If the current directory contains a valid
buf.yaml
, you can building an image with this command:
$ buf build -o image.bin
The resulting Buf image is written to the image.bin
file. Of note, the ordering of the FileDescriptorProto
s is
carefully written to mimic the ordering that protoc
would produce, for both the cases where imports are and are not
written.
By default, buf
produces a Buf image with both imports and source code info. You can strip each of these:
$ buf build --exclude-imports --exclude-source-info -o image.bin
In general, we do not recommend stripping these, as this information can be useful for various operations. Source code info, however, takes up a lot of additional space (generally ~5x more space), so if you know you do not need this data, it can be useful to strip source code info.
Images can be outputted in one of two formats:
- Binary
- JSON
Either format can be compressed using Gzip or Zstandard.
Per the Buf input documentation, buf build
can deduce the format from the file extension:
$ buf build -o image.bin
$ buf build -o image.bin.gz
$ buf build -o image.bin.zst
$ buf build -o image.json
$ buf build -o image.json.gz
$ buf build -o image.json.zst
The special value -
is used to denote stdout. You can manually set the format. For example:
$ buf build -o -#format=json
You can combine this with jq to introspect the built image. To see a list of all packages:
$ buf build -o -#format=json | jq '.file[] | .package' | sort | uniq | head
Output"google.actions.type" "google.ads.admob.v1" "google.ads.googleads.v1.common" "google.ads.googleads.v1.enums" "google.ads.googleads.v1.errors" "google.ads.googleads.v1.resources" "google.ads.googleads.v1.services" "google.ads.googleads.v2.common" "google.ads.googleads.v2.enums" "google.ads.googleads.v2.errors"
Images always include the ImageFileExtension
field. If you want a pure FileDescriptorSet
without this field set, to
mimic protoc
entirely:
$ buf build -o image.bin --as-file-descriptor-set
The ImageFileExtension
field doesn't affect Protobuf plugins or any other operations; they merely see this as an
unknown field. But we provide this option in case you need it.
Using protoc output as buf
input
Since buf
speaks in terms of Buf images and [FileDescriptorSet
][filescriptorset]s are images, we can
useprotoc
output as buf
input. Here's an example for buf lint
:
$ protoc -I . --include_source_info -o /dev/stdout foo.proto | buf lint -
Protoc lint and breaking change detection plugins
Since buf
"understands" FileDescriptorSet
s, we can provide plugins
protoc-gen-buf-lint
and protoc-gen-buf-breaking
as standard
Protobuf plugins as well.