slog-schema

A Go library for structured logging with log/slog using struct tags. Define your log schema once on a struct and pass it directly to any slog logger — no manual attribute building.

Installation

go get github.com/silvercory/slog-schema

Quick start

type Request struct {
    TraceID  string        `slog:"trace_id"`
    Method   string        `slog:"method"`
    Path     string        `slog:"path"`
    Status   int           `slog:"status"`
    Duration time.Duration `slog:"duration"`
    Error    string        `slog:"error,omitempty"`
    Email    string        `slog:"email,hashed"`
}

req := Request{
    TraceID:  "abc-123",
    Method:   "POST",
    Path:     "/api/orders",
    Status:   201,
    Duration: 18 * time.Millisecond,
    Email:    "alice@example.com",
}

logger.LogAttrs(ctx, slog.LevelInfo, "request", slogschema.ToAttrs(req)...)
// time=... level=INFO msg=request trace_id=abc-123 method=POST path=/api/orders status=201 duration=18ms email=sha256:...

Struct tags

Tag	Effect
slog:"name"	Log this field with the given key
slog:"-"	Skip this field entirely
(no tag)	Skip this field entirely
slog:"name,omitempty"	Skip when the value is a zero value
slog:"name,hashed"	Replace the value with its SHA-256 hash
slog:"name,omitempty,hashed"	Both — skip if zero, hash otherwise

Supported types

All types natively supported by log/slog are handled directly:

Go type	slog kind
string	StringValue
bool	BoolValue
int, int8 … int64	Int64Value
uint, uint8 … uint64	Uint64Value
float32, float64	Float64Value
time.Time	TimeValue
time.Duration	DurationValue
Pointer	Dereferenced; nil → AnyValue(nil)
Struct	GroupValue (recursive)
Slice / Array	GroupValue with "0", "1" … keys
Map	GroupValue with stringified keys
slog.LogValuer	.LogValue() called
Everything else	AnyValue

Embedded structs

Embedded structs without a tag are inlined — their fields appear at the top level:

type Meta struct {
    TraceID string `slog:"trace_id"`
    Region  string `slog:"region"`
}

type Request struct {
    Meta            // inlined
    Method string   `slog:"method"`
}

// Output: trace_id=... region=... method=...

Give the embedded field a tag to emit it as a named group instead:

type Request struct {
    Meta   `slog:"meta"` // grouped
    Method string        `slog:"method"`
}

// Output: meta.trace_id=... meta.region=... method=...

PII and hashed fields

Use hashed for PII such as emails, phone numbers, and user identifiers. The SHA-256 digest is deterministic, so the same value produces the same hash across log lines — enabling correlation without exposing raw data.

type User struct {
    ID    uint64 `slog:"id,hashed"`
    Email string `slog:"email,hashed"`
    Role  string `slog:"role"`
}

Custom marshalling

Implement SLogMarshaler to take full control of how a type is logged:

type Money struct {
    Units    int64
    Currency string
}

func (m Money) MarshalSLog() []slog.Attr {
    return []slog.Attr{
        slog.String("amount", fmt.Sprintf("%d %s", m.Units, m.Currency)),
    }
}

Any type implementing SLogMarshaler is handled automatically, both at the top level and as a nested field.

API

// ToAttrs converts a struct to []slog.Attr for use with logger.LogAttrs.
func ToAttrs(v any) []slog.Attr

// ToAttrsAny converts a struct to []any for use with logger.Info, logger.Error, etc.
func ToAttrsAny(v any) []any

Protobuf

protoc-gen-slogschema generates MarshalSLog() methods directly on your protoc-gen-go message types, so the same struct is used for both gRPC and logging with no mapping required.

See cmd/protoc-gen-slogschema for details.

Examples

• basic — tags, omitempty, embedded structs, slices, maps
• pii — hashed fields for PII
• marshal — custom MarshalSLog implementations
• protobuf — generated methods on protoc-gen-go types