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's (roughly) what happens:

  • protoc compiles the file foo.proto (and any imports) and internally produces a FileDescriptorSet, which is a list of FileDescriptorProto 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 a CodeGeneratorRequest, which contains the FileDescriptorProtos that protoc produced for foo.proto and any imports, a list of the files specified (just foo.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 named protoc-gen-go, and invokes it, giving the serialized CodeGeneratorRequest as stdin.
  • protoc-gen-go runs, and either errors or produces a CodeGeneratorResponse, which specifies what files are to be generated and their content. The serialized CodeGeneratorResponse is written to stdout of protoc-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.

FileDescriptorSets 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 FileDescriptorSets. gRPC Reflection uses them under the hood as well.

Creating FileDescriptorSets with protoc

protoc provides the --descriptor_set_out flag, aliased as -o, to allow writing serialized FileDescriptorSets. 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, FileDescriptorSets 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 FileDescriptorSets. The actual definition is currently stored in the bufbuild/buf repo as of this writing.

Buf images are FileDescriptorSets, and FileDescriptorSets 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 ModuleInfo 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 build an image with this command:

$ buf build -o image.binpb

The resulting Buf image is written to the image.binpb file. Of note, the ordering of the FileDescriptorProtos 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.binpb

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 three formats:

Any 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.binpb
$ buf build -o image.binpb.gz
$ buf build -o image.binpb.zst
$ buf build -o image.json
$ buf build -o image.json.gz
$ buf build -o image.json.zst
$ buf build -o image.txtpb
$ buf build -o image.txtpb.gz
$ buf build -o image.txtpb.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
"google.actions.type" "" "" "" "" "" "" "" "" ""

Images always include the ImageFileExtension field. If you want a pure FileDescriptorSet without this field set, to mimic protoc entirely:

$ buf build -o image.binpb --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" FileDescriptorSets, we can provide plugins protoc-gen-buf-lint and protoc-gen-buf-breaking as standard Protobuf plugins as well.