A new Go API for Protocol Buffers
02 Mar 2020
Tags: protobuf, technical

Joe Tsai, Damien Neil, and Herbie Ong

* Introduction

We are pleased to announce the release of a major revision of the Go API for
[[https://developers.google.com/protocol-buffers][protocol buffers]],
Google's language-neutral data interchange format.

* Motivations for a new API

The first protocol buffer bindings for Go were
[[https://blog.golang.org/third-party-libraries-goprotobuf-and][announced by Rob Pike]]
in March of 2010. Go 1 would not be released for another two years.

In the decade since that first release, the package has grown and
developed along with Go. Its users' requirements have grown too.

Many people want to write programs that use reflection to examine protocol
buffer messages. The
[[https://pkg.go.dev/reflect][`reflect`]]
package provides a view of Go types and
values, but omits information from the protocol buffer type system. For
example, we might want to write a function that traverses a log entry and
clears any field annotated as containing sensitive data. The annotations
are not part of the Go type system.

Another common desire is to use data structures other than the ones
generated by the protocol buffer compiler, such as a dynamic message type
capable of representing messages whose type is not known at compile time.

We also observed that a frequent source of problems was that the
[[https://pkg.go.dev/github.com/golang/protobuf/proto?tab=doc#Message][`proto.Message`]]
interface, which identifies values of generated message types, does very
little to describe the behavior of those types. When users create types
that implement that interface (often inadvertently by embedding a message
in another struct) and pass values of those types to functions expecting
a generated message value, programs crash or behave unpredictably.

All three of these problems have a common cause, and a common solution:
The `Message` interface should fully specify the behavior of a message,
and functions operating on `Message` values should freely accept any
type that correctly implements the interface.

Since it is not possible to change the existing definition of the
`Message` type while keeping the package API compatible, we decided that
it was time to begin work on a new, incompatible major version of the
protobuf module.

Today, we're pleased to release that new module. We hope you like it.

* Reflection

Reflection is the flagship feature of the new implementation. Similar
to how the `reflect` package provides a view of Go types and values, the
[[https://pkg.go.dev/google.golang.org/protobuf/reflect/protoreflect?tab=doc][`google.golang.org/protobuf/reflect/protoreflect`]]
package provides a view of values according to the protocol buffer
type system.

A complete description of the `protoreflect` package would run too long
for this post, but let's look at how we might write the log-scrubbing
function we mentioned previously.

First, we'll write a `.proto` file defining an extension of the
[[https://github.com/protocolbuffers/protobuf/blob/b96241b1b716781f5bc4dc25e1ebb0003dfaba6a/src/google/protobuf/descriptor.proto#L509][`google.protobuf.FieldOptions`]]
type so we can annotate fields as containing
sensitive information or not.

	syntax = "proto3";
	import "google/protobuf/descriptor.proto";
	package golang.example.policy;
	extend google.protobuf.FieldOptions {
		bool non_sensitive = 50000;
	}

We can use this option to mark certain fields as non-sensitive.

	message MyMessage {
		string public_name = 1 [(golang.example.policy.non_sensitive) = true];
	}

Next, we will write a Go function which accepts an arbitrary message
value and removes all the sensitive fields.

	// Redact clears every sensitive field in pb.
	func Redact(pb proto.Message) {
	   // ...
	}

This function accepts a
[[https://pkg.go.dev/google.golang.org/protobuf/proto?tab=doc#Message][`proto.Message`]],
an interface type implemented by all generated message types. This type
is an alias for one defined in the `protoreflect` package:

	type ProtoMessage interface{
		ProtoReflect() Message
	}

To avoid filling up the namespace of generated
messages, the interface contains only a single method returning a
[[https://pkg.go.dev/google.golang.org/protobuf/reflect/protoreflect?tab=doc#Message][`protoreflect.Message`]],
which provides access to the message contents.

(Why an alias? Because `protoreflect.Message` has a corresponding
method returning the original `proto.Message`, and we need to avoid an
import cycle between the two packages.)

The
[[https://pkg.go.dev/google.golang.org/protobuf/reflect/protoreflect?tab=doc#Message.Range][`protoreflect.Message.Range`]]
method calls a function for every populated field in a message.

	m := pb.ProtoReflect()
	m.Range(func(fd protoreflect.FieldDescriptor, v protoreflect.Value) bool {
		// ...
		return true
	})

The range function is called with a
[[https://pkg.go.dev/google.golang.org/protobuf/reflect/protoreflect?tab=doc#FieldDescriptor][`protoreflect.FieldDescriptor`]]
describing the protocol buffer type of the field, and a
[[https://pkg.go.dev/google.golang.org/protobuf/reflect/protoreflect?tab=doc#Value][`protoreflect.Value`]]
containing the field value.

The
[[https://pkg.go.dev/google.golang.org/protobuf/reflect/protoreflect?tab=doc#Descriptor.Options][`protoreflect.FieldDescriptor.Options`]]
method returns the field options as a `google.protobuf.FieldOptions`
message.

	opts := fd.Options().(*descriptorpb.FieldOptions)

(Why the type assertion? Since the generated `descriptorpb` package
depends on `protoreflect`, the `protoreflect` package can't return the
concrete options type without causing an import cycle.)

We can then check the options to see the value of our extension boolean:

	if proto.GetExtension(opts, policypb.E_NonSensitive).(bool) {
		return true // don't redact non-sensitive fields
	}

Note that we are looking at the field _descriptor_ here, not the field
_value_. The information we're interested in lies in the protocol
buffer type system, not the Go one.

This is also an example of an area where we
have simplified the `proto` package API. The original
[[https://pkg.go.dev/github.com/golang/protobuf/proto?tab=doc#GetExtension][`proto.GetExtension`]]
returned both a value and an error. The new
[[https://pkg.go.dev/google.golang.org/protobuf/proto?tab=doc#GetExtension][`proto.GetExtension`]]
returns just a value, returning the default value for the field if it is
not present. Extension decoding errors are reported at `Unmarshal` time.

Once we have identified a field that needs redaction, clearing it is simple:

	m.Clear(fd)

Putting all the above together, our complete redaction function is:

	// Redact clears every sensitive field in pb.
	func Redact(pb proto.Message) {
		m := pb.ProtoReflect()
		m.Range(func(fd protoreflect.FieldDescriptor, v protoreflect.Value) bool {
			opts := fd.Options().(*descriptorpb.FieldOptions)
			if proto.GetExtension(opts, policypb.E_NonSensitive).(bool) {
				return true
			}
			m.Clear(fd)
			return true
		})
	}

A more complete implementation might recursively descend into
message-valued fields. We hope that this simple example gives a
taste of protocol buffer reflection and its uses.

* Versions

We call the original version of Go protocol buffers APIv1, and the
new one APIv2. Because APIv2 is not backwards compatible with APIv1,
we need to use different module paths for each.

(These API versions are not the same as the versions of the protocol
buffer language: `proto1`, `proto2`, and `proto3`. APIv1 and APIv2
are concrete implementations in Go that both support the `proto2` and
`proto3` language versions.)

The
[[https://pkg.go.dev/github.com/golang/protobuf?tab=overview][`github.com/golang/protobuf`]]
module is APIv1.

The
[[https://pkg.go.dev/google.golang.org/protobuf?tab=overview][`google.golang.org/protobuf`]]
module is APIv2. We have taken advantage of the need to change the
import path to switch to one that is not tied to a specific hosting
provider. (We considered `google.golang.org/protobuf/v2`, to make it
clear that this is the second major version of the API, but settled on
the shorter path as being the better choice in the long term.)

We know that not all users will move to a new major version of a package
at the same rate. Some will switch quickly; others may remain on the old
version indefinitely. Even within a single program, some parts may use
one API while others use another. It is essential, therefore, that we
continue to support programs that use APIv1.

- `github.com/golang/protobuf@v1.3.4` is the most recent pre-APIv2 version of APIv1.

- `github.com/golang/protobuf@v1.4.0` is a version of APIv1 implemented in terms of APIv2.
  The API is the same, but the underlying implementation is backed by the new one.
  This version contains functions to convert between the APIv1 and APIv2 `proto.Message`
  interfaces to ease the transition between the two.

- `google.golang.org/protobuf@v1.20.0` is APIv2.
  This module depends upon `github.com/golang/protobuf@v1.4.0`,
  so any program which uses APIv2 will automatically pick a version of APIv1
  which integrates with it.

(Why start at version `v1.20.0`? To provide clarity.
We do not anticipate APIv1 to ever reach `v1.20.0`,
so the version number alone should be enough to unambiguously differentiate
between APIv1 and APIv2.)

We intend to maintain support for APIv1 indefinitely.

This organization ensures that any given program will use only a single
protocol buffer implementation, regardless of which API version it uses.
It permits programs to adopt the new API gradually, or not at all, while
still gaining the advantages of the new implementation. The principle of
minimum version selection means that programs may remain on the old
implementation until the maintainers choose to update to the new one
(either directly, or by updating a dependency).

* Additional features of note

The
[[https://pkg.go.dev/google.golang.org/protobuf/encoding/protojson][`google.golang.org/protobuf/encoding/protojson`]]
package converts protocol buffer messages to and from JSON using the
[[https://developers.google.com/protocol-buffers/docs/proto3#json][canonical JSON mapping]],
and fixes a number of issues with the old `jsonpb` package
that were difficult to change without causing problems for existing users.

The
[[https://pkg.go.dev/google.golang.org/protobuf/types/dynamicpb][`google.golang.org/protobuf/types/dynamicpb`]]
package provides an implementation of `proto.Message` for messages whose
protocol buffer type is derived at runtime.

The
[[https://pkg.go.dev/google.golang.org/protobuf/testing/protocmp][`google.golang.org/protobuf/testing/protocmp`]]
package provides functions to compare protocol buffer messages with the
[[https://pkg.go.dev/github.com/google/go-cmp/cmp][`github.com/google/cmp`]]
package.

The
[[https://pkg.go.dev/google.golang.org/protobuf/compiler/protogen?tab=doc][`google.golang.org/protobuf/compiler/protogen`]]
package provides support for writing protocol compiler plugins.

* Conclusion

The `google.golang.org/protobuf` module is a major overhaul of
Go's support for protocol buffers, providing first-class support
for reflection, custom message implementations, and a cleaned up API
surface. We intend to maintain the previous API indefinitely as a wrapper
of the new one, allowing users to adopt the new API incrementally at
their own pace.

Our goal in this update is to improve upon the benefits of the old
API while addressing its shortcomings. As we completed each component of
the new implementation, we put it into use within Google's codebase. This
incremental rollout has given us confidence in both the usability of the new
API and the performance and correctness of the new implementation. We believe
it is production ready.

We are excited about this release and hope that it will serve the Go
ecosystem for the next ten years and beyond!