protoslog

protoslog provides utilities for using protocol buffer messages with the log/slog package introduced in Go 1.21.

Example

protoslog operates against Protocol Buffer messages. Below, one might have such a User message:

syntax="proto3";

import "google/protobuf/timestamp.proto";

message User {
  fixed64 id = 1;
  string email = 2 [debug_redact=true];
  Status status = 3;
  google.protobuf.Timestamp updated = 4;
}

enum Status {
  UNSPECIFIED = 0;
  ACTIVE = 1;
  INACTIVE = 2;
}

protoslog does NOT require any code generation (beyond the output of protoc-gen-go) to properly log a message:

package main

import (
	"log/slog"

	"github.com/rodaine/protoslog"
	"github.com/rodaine/protoslog/internal/gen"
)

func main() {
	msg := &gen.User{
		Id:      123,
		Email:   "rodaine@github.com",
		Status:  gen.ACTIVE,
		Updated: time.Now(),
	}

	slog.Info("hello", protoslog.Message("user", msg))
}

Outputs:

2022/11/08 15:28:26 INFO hello user.id=123 user.email=REDACTED user.status=ACTIVE user.updated=2022-11-08T15:28:26.000Z

Field Value Types

Messages are lazily converted into a slog.GroupValue with each of its populated field converted into a slog.Attr with the field name as the key and value produced based on its type (similar to the canonical JSON encoding rules)

Scalar Types

• bool: slog.BoolValue
• floats: slog.Float64Value
• bytes: base64 encoded in a slog.StringValue
• string: slog.StringValue
• enum: slog.StringValue of the value name if it's defined, or slog.Int64Value otherwise
• signed integer: slog.Int64Value
• unsigned integer: slog.Uint64Value

Composite Types

Populated composite fields are encoded as a slog.GroupValue:

• message: each field converted into a slog.Attr with its name as the key and the value recursively applying these rules
• repeated: each item converted into a slog.Attr with its index string-ified as the key and the value recursively applying these rules
• map: each entry converted into a slog.Attr with its key string-ified and the value recursively applying these rules

Well-Known Types (WKTs)

Similar to the canonical JSON encoding, some of the WKTs produce special-cased slog.Value:

• google.protobuf.NullValue: empty slog.Value{} (equivalent of nil)\
• google.protobuf.Timestamp: slog.TimeValue
• google.protobuf.Duration: slog.DurationValue
• wrappers: it's value field, applying these rules
• google.protobuf.ListValue: its values field, applying the repeated rule above
• google.protobuf.Struct: its fields field, applying the map rule above
• google.protobuf.Value: the field set in its kind oneof, applying these rules
• google.protobuf.Any: see [Any WKT Resolution] below

Redaction

Messages may contain personal identifiable information (PII), secrets, or similar data that should not be written into a log. Message fields can be annotated with the debug_redact option to identify such values. By default, protoslog will redact these fields, with the behavior customizable via options.

Populated redacted fields are replaced with a slog.StringValue("REDACTED"):

msg := &gen.User{Email: "personal@identifiable.info"}
slog.Info("default", protoslog.Message("user", msg))
// Stderr: 2022/11/08 15:28:26 INFO default user.email=REDACTED

To elide redacted fields instead of including them, WithElideRedactions can be used:

slog.Info("elide", protoslog.Message("user", msg, protoslog.WithElideRedactions()))
// Stderr: 2022/11/08 15:28:26 INFO elide

Redaction may also be disabled via WithDisableRedactions:

slog.Info("disable", protoslog.Message("user", msg, protoslog.WithDisableRedactions()))
// Stderr: 2022/11/08 15:28:26 INFO disable email=personal@identifiable.info

All Fields

By default, protoslog only emits fields that are populated on the message (via the behavior of protoreflect.Message#Has):

msg := &gen.Location{Latitude: 1.23}
slog.Info("default", protoslog.Message("loc", msg))
// Stderr: 2022/11/08 15:28:26 INFO default loc.latitude=1.23

To emit all fields regardless of presence, use WithAllFields:

slog.Info("all", protoslog.Message("loc", msg, protoslog.WithAllFields()))
// Stderr: 2022/11/08 15:28:26 INFO all loc.latitude=1.23 loc.longitude=0

For unpopulated "nullable," repeated, and map fields, the zero slog.Value is emitted (which is equivalent to nil). All other fields emit their default values.

Any WKT Resolution

protoslog emits the Any field's type_url with the key @type. By default, protoslog attempts to resolve the field's value and on success emits it:

msg := &gen.User{Id: 123}
anyPB, _ := anypb.New(msg)
slog.Info("success", protoslog.Message("any", anyPB))
// Stderr: 2022/11/08 15:28:26 INFO success any.@type=type.googleapis.com/User any.id=123

If the inner value does not resolve to a slog.GroupValue (e.g., it's a WKT), the result is added as @value:

msg := durationpb.New(5*time.Second)
anyPB, _ := anypb.New(msg)
slog.Info("wkt", protoslog.Message("any", anyPB))
// Stderr: 2022/11/08 15:28:26 INFO wkt any.@type=type.googleapis.com/google.protobuf.Duration any.@value=5s

If the value cannot be resolved (either unknown or an error occurs), only the @type attribute will be present:

anyPB := &anypb.Any{TypeUrl: "foobar"}
slog.Info("unknown", protoslog.Message("any", anyPB))
// Stderr: 2022/11/08 15:28:26 INFO unknown any.@type=foobar

By default, protoslog uses protoregistry.GlobalTypes to resolve Any WKTs. A custom resolver can be provided via WithAnyResolver:

slog.Info("custom", protoslog.Message("any", anyPB, protoslog.WithAnyResolver(myResolver)))

To skip resolving Any WKTs, use WithSkipAnys. Only the @type attribute will be emitted:

slog.Info("skip", protoslog.Message("any", anyPB, protoslog.WithSkipAnys()))

slog Handler

If a message is not wrapped via protoslog, it will be presented in the logs with the behavior of slog.AnyValue. To ensure all messages are resolved correctly regardless, a protoslog.Handler can wrap a slog.Handler:

handler := protoslog.NewHandler(slog.Default().Handler())
logger := slog.New(handler)

msg := &gen.User{Id: 123}
logger.Info("handler", "user", msg)
// Stderr: 2022/11/08 15:28:26 INFO handler user.id=123

The options on protoslog.Handler supersede those on messages wrapped via other protoslog functions.

protoc-gen-slog

To make the generated message types produced by protoc-gen-go implement slog.LogValuer, protoc-gen-slog can be used to generate LogValue methods.

go install github.com/rodaine/protoslog/protoc-gen-slog

Buf CLI

When using buf, ensure the out path and opt values are equivalent for both protoc-gen-go and protoc-gen-slog plugins:

# buf.gen.yaml
version: v1
plugins:
  - plugin: buf.build/protocolbuffers/go:v1.32.0
    out: gen
    opt:
      - paths=source_relative
  - plugin: slog
    out: gen
    opt:
      - paths=source_relative

protoc

When using protoc, ensure both plugin options and output path are equivalent:

protoc \
  --go_out="$OUT" \
  --slog_out="$OUT" \
  $PROTOS