add observer implementation

Makefile

@@ -1,7 +1,7 @@
 CWD = $(shell pwd)
 
 define HELP_TEXT
-Welcome to obkit!
+Welcome to okit!
 
 Targets:
 	help             provides help text

README.md

@@ -2,6 +2,9 @@
 
 Short for "observability kit", `okit` aims to provide an all-in-one solution to application observability.
 
+_**DISCLAIMER!** This is an early preview. There are significant portions of this repository that still need to be 
+implemented. Check out the [issues](https://github.com/mjpitz/okit/issues) to see how you can help._
+
 ## Why
 
 Traditional approaches to observability treat logging, application metrics, and tracing as independent operations, with
@@ -55,9 +58,8 @@ func main() {
 	okit.Emit("user_signup", tags...)
 
 	// Tracing
-	ctx, done := okit.Trace(context.Background(), tags...)
-	defer done()
-	_ = ctx.Err() // not needed, removes unused error
+	ctx := context.Background()
+	defer okit.Trace(&ctx, tags...).Done()
 
 	// Logging
 	okit.Debug("a message used for debugging", tags...)

api.go

@@ -22,9 +22,61 @@ package okit
 
 import (
 	"context"
-	"strings"
+	"fmt"
 )
 
+const (
+	// LevelTrace is used to enable bookend events for tracing.
+	LevelTrace Level = iota
+	// LevelDebug enables debug level logging for the application.
+	LevelDebug
+	// LevelInfo is the default level of logging for applications.
+	LevelInfo
+	// LevelWarn decreases log verbosity by removing informational messages from the stream.
+	LevelWarn
+	// LevelError further decreases log verbosity by only surfacing errors encountered by the system.
+	LevelError
+)
+
+// Level allows the logging level to be configured.
+type Level uint8
+
+// Set allows a given log level to be set to a certain value.
+func (l *Level) Set(val string) error {
+	switch val {
+	case "trace", "TRACE":
+		*l = LevelTrace
+	case "debug", "DEBUG":
+		*l = LevelTrace
+	case "info", "INFO":
+		*l = LevelTrace
+	case "warn", "WARN":
+		*l = LevelTrace
+	case "error", "ERROR":
+		*l = LevelTrace
+	}
+
+	return fmt.Errorf("unrecognized level")
+}
+
+// String returns the string representation of the Level.
+func (l *Level) String() string {
+	switch *l {
+	case LevelTrace:
+		return "trace"
+	case LevelDebug:
+		return "debug"
+	case LevelInfo:
+		return "info"
+	case LevelWarn:
+		return "warn"
+	case LevelError:
+		return "error"
+	}
+
+	return "unknown"
+}
+
 // Logger defines common operations for writing log messages.
 type Logger interface {
 	// Debug adds a debug message to the event stream, if configured.
@@ -55,7 +107,7 @@ type DoneFunc func()
 // Tracer allows method calls to be instrumented for debugging.
 type Tracer interface {
 	// Trace captures the method calling context and returns a function that can be invoked to complete the trace.
-	Trace(ctx context.Context, tags ...Tag) (context.Context, DoneFunc)
+	Trace(ctx *context.Context, tags ...Tag) ActiveTrace
 }
 
 // Span defines an internal structure for tracking traces across an application. This structure is only used for
@@ -83,48 +135,8 @@ type Interface[T any] interface {
 	Wither[T]
 }
 
-type Level int8
-
-const (
-	UnknownLevel = 0
-	TraceLevel   = 1
-	DebugLevel   = 2
-	InfoLevel    = 3
-	WarnLevel    = 4
-	ErrorLevel   = 5
-)
-
-func (l *Level) String() string {
-	switch *l {
-	case TraceLevel:
-		return "trace"
-	case DebugLevel:
-		return "debug"
-	case InfoLevel:
-		return "info"
-	case WarnLevel:
-		return "warn"
-	case ErrorLevel:
-		return "error"
-	}
-
-	return "unknown"
-}
-
-func (l *Level) Set(val string) error {
-	switch {
-	case strings.EqualFold("trace", val):
-		*l = TraceLevel
-	case strings.EqualFold("debug", val):
-		*l = DebugLevel
-	case strings.EqualFold("info", val):
-		*l = InfoLevel
-	case strings.EqualFold("warn", val):
-		*l = WarnLevel
-	case strings.EqualFold("error", val):
-		*l = ErrorLevel
-	}
-
-	*l = UnknownLevel
-	return nil
+// ActiveTrace defines an abstraction that allows traces to be completed using a Done function.
+type ActiveTrace interface {
+	// Done finishes the current tracing operation.
+	Done()
 }

client.go

@@ -26,88 +26,112 @@ import (
 	"encoding/base32"
 	"io"
 	"runtime"
+	"sync"
 	"time"
 
+	"go.pitz.tech/okit/observer"
 	"go.pitz.tech/okit/pb"
 )
 
-var encoding = base32.StdEncoding.WithPadding(base32.NoPadding)
+var (
+	encoding = base32.StdEncoding.WithPadding(base32.NoPadding)
+
+	defaultUUID = func() string {
+		buf := make([]byte, 16)
+		n, err := io.ReadFull(rand.Reader, buf)
+		if err != nil {
+			panic(err)
+		}
+
+		return encoding.EncodeToString(buf[:n])
+	}
+)
 
 // NewClient produces a new default client implementation for emitting metrics.
 func NewClient() Client {
 	return Client{
-		now: time.Now,
-		uuid: func() string {
-			buf := make([]byte, 16)
-			n, err := io.ReadFull(rand.Reader, buf)
-			if err != nil {
-				panic(err)
-			}
-
-			return encoding.EncodeToString(buf[:n])
-		},
+		observer:   observer.Local(),
+		level:      LevelInfo,
+		now:        time.Now,
+		uuid:       defaultUUID,
 		callerSkip: 2,
 	}
 }
 
 // Client provides a default implementation.
 type Client struct {
+	observer   observer.Observer
+	level      Level
 	now        func() time.Time
 	uuid       func() string
 	tags       []Tag
 	callerSkip int
 }
 
+// WithObservers configures the new client with the provided observers.
+func (c Client) WithObservers(observers ...observer.Observer) Client {
+	c.observer = observer.Composite(observers)
+	return c
+}
+
+// WithLevel allows the current logging level to be tuned. This does not prevent traces from being emit, or metrics
+// from being reported. It only modifies the behavior of logs.
+func (c Client) WithLevel(level Level) Client {
+	c.level = level
+	return c
+}
+
 // WithNow configures the function that's used to obtain the current timestamp.
-func (o Client) WithNow(now func() time.Time) Client {
-	o.now = now
-	return o
+func (c Client) WithNow(now func() time.Time) Client {
+	c.now = now
+	return c
 }
 
 // WithUUID returns a new uuid that uniquely identifies traces, spans, and tags.
-func (o Client) WithUUID(uuid func() string) Client {
-	o.uuid = uuid
-	return o
+func (c Client) WithUUID(uuid func() string) Client {
+	c.uuid = uuid
+	return c
 }
 
 // WithCallerSkip is used to configure the number of frames to skip when determining the caller. Callers are
 // predominantly used when performing traces.
-func (o Client) WithCallerSkip(callerSkip int) Client {
-	o.callerSkip = callerSkip
-	return o
+func (c Client) WithCallerSkip(callerSkip int) Client {
+	c.callerSkip = callerSkip
+	return c
 }
 
 // With appends tags to the current client that will automatically be added to all events, metrics, logs, and traces.
-func (o Client) With(tags ...Tag) Client {
-	o.tags = append(o.tags, tags...)
-	return o
+func (c Client) With(tags ...Tag) Client {
+	c.tags = append(c.tags, tags...)
+	return c
 }
 
 // Emit allows for the emission of an event which has now value and only associated tags.
-func (o Client) Emit(event string, tags ...Tag) {
-	o.With(tags...).emit(&pb.Entry{
+func (c Client) Emit(event string, tags ...Tag) {
+	c.With(tags...).emit(&pb.Entry{
 		Kind:  pb.Kind_Event,
 		Scope: event,
 	})
 }
 
 // Observe reports a metric and it's associated value.
-func (o Client) Observe(metric string, value float64, tags ...Tag) {
-	o.With(tags...).emit(&pb.Entry{
-		Value: &pb.Entry_Double{Double: value},
+func (c Client) Observe(metric string, value float64, tags ...Tag) {
+	c.With(tags...).With(Float64("value", value)).emit(&pb.Entry{
 		Kind:  pb.Kind_Metric,
 		Scope: metric,
 	})
 }
 
 // Trace allows a method to be traced, and it's execution time recorded and reported for viewing.
-func (o Client) Trace(ctx context.Context, tags ...Tag) (context.Context, DoneFunc) {
-	name, _ := caller(o.callerSkip)
-	start := o.now()
+func (c Client) Trace(ctxp *context.Context, tags ...Tag) ActiveTrace {
+	ctx := *ctxp
+
+	name, _ := caller(c.callerSkip)
+	start := c.now()
 
 	span := Span{
-		TraceID: o.uuid(),
-		ID:      o.uuid(),
+		TraceID: c.uuid(),
+		ID:      c.uuid(),
 	}
 
 	v := ctx.Value(SpanKey)
@@ -120,101 +144,142 @@ func (o Client) Trace(ctx context.Context, tags ...Tag) (context.Context, DoneFu
 	}
 
 	//goland:noinspection GoAssignmentToReceiver
-	o = o.With(
-		append(
-			[]Tag{
-				String("traceId", span.TraceID),
-				String("traceSpanId", span.ID),
-				String("traceParentId", span.ParentID),
-			},
-			tags...,
-		)...,
-	)
+	c = c.
+		With(
+			String("traceId", span.TraceID),
+			String("traceSpanId", span.ID),
+			String("traceParentId", span.ParentID),
+		).
+		With(tags...)
 
-	// bookend
-	o.With(String("bookend", "start")).emit(&pb.Entry{
-		Kind:  pb.Kind_Log,
-		Scope: name,
-		Value: &pb.Entry_String_{String_: "trace"},
-	})
+	if c.level <= LevelTrace {
+		c.With(String("bookend", "start"), level(LevelTrace)).
+			emit(&pb.Entry{
+				Kind:  pb.Kind_Log,
+				Scope: name,
+			})
+	}
 
-	return context.WithValue(ctx, SpanKey, span), func() {
-		duration := o.now().Sub(start)
+	ctx = context.WithValue(ctx, SpanKey, span)
+	*ctxp = ctx
 
-		// bookend
-		o.With(String("bookend", "end")).emit(&pb.Entry{
-			Kind:  pb.Kind_Log,
-			Scope: name,
-			Value: &pb.Entry_String_{String_: "trace"},
-		})
+	return &active{name, start, c, sync.Once{}}
+}
+
+type active struct {
+	name  string
+	start time.Time
+	c     Client
+	once  sync.Once
+}
+
+func (a *active) Done() {
+	a.once.Do(func() {
+		duration := a.c.now().Sub(a.start)
+
+		//goland:noinspection GoAssignmentToReceiver
+		a.c = a.c.With(Duration("duration", duration))
+
+		if a.c.level <= LevelTrace {
+			a.c.With(String("bookend", "end"), level(LevelTrace)).
+				emit(
+					&pb.Entry{
+						Kind:  pb.Kind_Log,
+						Scope: a.name,
+					},
+				)
+		}
 
 		// trace
-		o.emit(
-			&pb.Entry{
-				Kind:  pb.Kind_Trace,
-				Scope: name,
-				Value: &pb.Entry_Duration{Duration: pb.DurationPB(duration)},
-			},
-		)
-	}
+		a.c.emit(&pb.Entry{
+			Kind:  pb.Kind_Trace,
+			Scope: a.name,
+		})
+	})
 }
 
 // Debug produces a debug log event.
-func (o Client) Debug(msg string, tags ...Tag) {
-	o.With(tags...).emit(&pb.Entry{
-		Kind:  pb.Kind_Log,
-		Scope: msg,
-		Value: &pb.Entry_String_{String_: "debug"},
-	})
+func (c Client) Debug(msg string, tags ...Tag) {
+	if c.level > LevelDebug {
+		return
+	}
+
+	name, _ := caller(c.callerSkip)
+
+	c.With(tags...).
+		With(message(msg), level(LevelDebug)).
+		emit(&pb.Entry{
+			Kind:  pb.Kind_Log,
+			Scope: name,
+		})
 }
 
 // Info produces an information log event.
-func (o Client) Info(msg string, tags ...Tag) {
-	o.With(tags...).emit(&pb.Entry{
-		Kind:  pb.Kind_Log,
-		Scope: msg,
-		Value: &pb.Entry_String_{String_: "info"},
-	})
+func (c Client) Info(msg string, tags ...Tag) {
+	if c.level > LevelInfo {
+		return
+	}
+
+	name, _ := caller(c.callerSkip)
+
+	c.With(tags...).
+		With(message(msg), level(LevelInfo)).
+		emit(&pb.Entry{
+			Kind:  pb.Kind_Log,
+			Scope: name,
+		})
 }
 
 // Warn produces a warning that is surfaced to operators.
-func (o Client) Warn(msg string, tags ...Tag) {
-	o.With(tags...).emit(&pb.Entry{
-		Kind:  pb.Kind_Log,
-		Scope: msg,
-		Value: &pb.Entry_String_{String_: "warn"},
-	})
+func (c Client) Warn(msg string, tags ...Tag) {
+	if c.level > LevelWarn {
+		return
+	}
+
+	name, _ := caller(c.callerSkip)
+
+	c.With(tags...).
+		With(message(msg), level(LevelWarn)).
+		emit(&pb.Entry{
+			Kind:  pb.Kind_Log,
+			Scope: name,
+		})
 }
 
 // Error produces a message that communicates an error has occurred.
-func (o Client) Error(msg string, tags ...Tag) {
-	o.With(tags...).emit(&pb.Entry{
-		Kind:  pb.Kind_Log,
-		Scope: msg,
-		Value: &pb.Entry_String_{String_: "error"},
-	})
+func (c Client) Error(msg string, tags ...Tag) {
+	if c.level > LevelError {
+		return
+	}
+
+	name, _ := caller(c.callerSkip)
+
+	c.With(tags...).
+		With(message(msg), level(LevelError)).
+		emit(&pb.Entry{
+			Kind:  pb.Kind_Log,
+			Scope: name,
+		})
 }
 
-func (o Client) emit(entries ...*pb.Entry) {
-	now := pb.TimestampPB(o.now())
+func (c Client) emit(entries ...*pb.Entry) {
+	now := pb.TimestampPB(c.now())
 
-	tags := make([]*pb.Tag, 0, len(o.tags))
-	for _, tag := range o.tags {
+	tags := make([]*pb.Tag, 0, len(c.tags))
+	for _, tag := range c.tags {
 		tags = append(tags, tag.AsTagPB())
 	}
 
 	for _, entry := range entries {
 		entry.Timestamp = now
 		entry.Tags = tags
-
-		_ = DefaultFormat.Marshal(sink, entry)
 	}
 
-	_ = sink.Flush()
+	c.observer.Observe(entries)
 }
 
 // caller attempts to get method caller information. This information is used for tracing information across an
-// applications source code.
+// applications source code. Need to look into the performance of this call. Parts of it may be able to be cached.
 func caller(skip int) (name string, line int) {
 	pctr, _, line, ok := runtime.Caller(skip)
 	if !ok {

examples/json/main.go

@@ -25,11 +25,20 @@ import (
 
 	"go.pitz.tech/okit"
 	okithttp "go.pitz.tech/okit/http"
+	"go.pitz.tech/okit/observer"
 )
 
 func main() {
-	// Enable JSON output
-	okit.DefaultFormat = okit.JSONFormat{}
+	client := okit.NewClient().
+		WithObservers(
+			observer.Local(
+				observer.Format("json"),
+			),
+
+			// TODO: add remote observers
+		)
+
+	okit.Replace(client)
 
 	// Instrument HTTP Clients
 	okithttp.InstrumentClient(http.DefaultClient)

examples/overview/main.go

@@ -27,6 +27,9 @@ import (
 )
 
 func main() {
+	client := okit.NewClient().WithLevel(okit.LevelTrace)
+	okit.Replace(client)
+
 	var tags []okit.Tag
 
 	// Metric emission
@@ -39,9 +42,8 @@ func main() {
 	okit.Emit("user_signup", tags...)
 
 	// Tracing
-	ctx, done := okit.Trace(context.Background(), tags...)
-	defer done()
-	_ = ctx.Err() // not needed, removes unused error
+	ctx := context.Background()
+	defer okit.Trace(&ctx, tags...).Done()
 
 	// Logging
 	okit.Debug("a message used for debugging", tags...)

format.go

@@ -1,61 +0,0 @@
-package okit
-
-import (
-	"bufio"
-	"strconv"
-	"strings"
-	"time"
-
-	"github.com/golang/protobuf/jsonpb"
-	"go.pitz.tech/okit/pb"
-)
-
-// Format defines an abstraction for writing entries to stdout/stderr. This is predominantly used in report log, trace,
-// and event information from the process.
-type Format interface {
-	Marshal(writer *bufio.Writer, entry *pb.Entry) error
-}
-
-// JSONFormat writes entries using JSON encoded protocol buffers.
-type JSONFormat struct {
-	Marshaler jsonpb.Marshaler
-}
-
-func (f JSONFormat) Marshal(writer *bufio.Writer, entry *pb.Entry) error {
-	f.Marshaler.Marshal(writer, entry)
-	return writer.WriteByte('\n')
-}
-
-// TextFormat writes entries using a custom text format.
-type TextFormat struct{}
-
-func (f TextFormat) Marshal(writer *bufio.Writer, entry *pb.Entry) error {
-	sink.WriteString(entry.Timestamp.AsTime().Format(time.RFC3339))
-	sink.WriteByte('\t')
-	sink.WriteString(strings.ToUpper(entry.Kind.String()))
-	sink.WriteByte('\t')
-
-	switch v := entry.GetValue().(type) {
-	case *pb.Entry_String_:
-		sink.WriteString(v.String_)
-	case *pb.Entry_Double:
-		sink.WriteString(strconv.FormatFloat(v.Double, 'f', 5, 64))
-	case *pb.Entry_Duration:
-		sink.WriteString(v.Duration.AsDuration().String())
-	}
-	sink.WriteByte('\t')
-	sink.WriteString(entry.Scope)
-	sink.WriteByte('\t')
-
-	for _, tag := range entry.Tags {
-		qs := tag.AsQueryString()
-		if qs == "" {
-			continue
-		}
-
-		sink.WriteString(qs)
-		sink.WriteByte('&')
-	}
-
-	return sink.WriteByte('\n')
-}

format/format.go

@@ -0,0 +1,71 @@
+// Copyright (C) 2022 The OKit Authors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE
+// OR OTHER DEALINGS IN THE SOFTWARE.
+
+package format
+
+import (
+	"bufio"
+	"strings"
+	"time"
+
+	"github.com/golang/protobuf/jsonpb"
+	"go.pitz.tech/okit/pb"
+)
+
+// Marshaler defines an abstraction for writing entries to stdout/stderr. This is predominantly used in report log, trace,
+// and event information from the process.
+type Marshaler interface {
+	Marshal(writer *bufio.Writer, entry *pb.Entry) error
+}
+
+// JSON writes entries using JSON encoded protocol buffers.
+type JSON struct {
+	Marshaler jsonpb.Marshaler
+}
+
+func (f JSON) Marshal(writer *bufio.Writer, entry *pb.Entry) error {
+	f.Marshaler.Marshal(writer, entry)
+	return writer.WriteByte('\n')
+}
+
+// Text writes entries using a custom text format. Entries are written using a TSV format and tags are url-encoded
+// key-value pairs.
+type Text struct{}
+
+func (f Text) Marshal(writer *bufio.Writer, entry *pb.Entry) error {
+	writer.WriteString(entry.Timestamp.AsTime().Format(time.RFC3339))
+	writer.WriteByte('\t')
+	writer.WriteString(strings.ToUpper(entry.Kind.String()))
+	writer.WriteByte('\t')
+	writer.WriteString(entry.Scope)
+	writer.WriteByte('\t')
+
+	for _, tag := range entry.Tags {
+		qs := tag.AsQueryString()
+		if qs == "" {
+			continue
+		}
+
+		writer.WriteString(qs)
+		writer.WriteByte('&')
+	}
+
+	return writer.WriteByte('\n')
+}

http/endpoint.go

@@ -38,7 +38,7 @@ func (e *Endpoint) Health(w http.ResponseWriter, r *http.Request) {
 
 func (e *Endpoint) Metrics(w http.ResponseWriter, r *http.Request) {
 	// Render metrics endpoint
-	// Unlike prometheus metrics are pushed, not scraped. So there's no need for complicated protocol here.
+	// Unlike prometheus, metrics are pushed and not scraped. So there's no need for a complicated protocol here.
 }
 
 func (e *Endpoint) Trace(w http.ResponseWriter, r *http.Request) {

http/instrumentation.go

@@ -21,11 +21,19 @@
 package http
 
 import (
+	"context"
 	"net/http"
 
 	"go.pitz.tech/okit"
 )
 
+const (
+	// OkitTraceIDHeader is a string constant that defines a common key used to propagate an okit trace.
+	OkitTraceIDHeader = "x-okit-trace-id"
+	// OkitSpanIDHeader is a string constant that defines a common key used to propagate the parent span.
+	OkitSpanIDHeader = "x-okit-span-id"
+)
+
 // InstrumentClient updates the provided http.Client to use an instrumented http.RoundTripper. If the provided
 // client.Transport is nil, then http.DefaultTransport is used.
 func InstrumentClient(client *http.Client) {
@@ -46,16 +54,26 @@ type roundTripper struct {
 }
 
 func (r *roundTripper) RoundTrip(req *http.Request) (resp *http.Response, err error) {
+	ctx := req.Context()
 	statusCode := 0
-
-	ctx, done := okit.Trace(req.Context(),
+	tags := []okit.Tag{
 		okit.String("host", req.URL.Host),
 		okit.String("method", req.Method),
 		okit.String("path", req.URL.Path),
 		okit.Intp("status", &statusCode),
-		okit.Errp(&err),
-	)
-	defer done()
+	}
+
+	defer okit.Trace(&ctx, tags...).Done()
+
+	// propagate to server
+	// TODO: support spec compliant propagation
+
+	if v := ctx.Value(okit.SpanKey); v != nil {
+		if span, ok := v.(okit.Span); ok {
+			req.Header.Set(OkitTraceIDHeader, span.TraceID)
+			req.Header.Set(OkitSpanIDHeader, span.ID)
+		}
+	}
 
 	resp, err = r.delegate.RoundTrip(req.WithContext(ctx))
 	if err != nil {
@@ -79,15 +97,27 @@ type handler struct {
 }
 
 func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	statusCode := 200
+	ctx := r.Context()
 
-	ctx, done := okit.Trace(r.Context(),
+	// propagation from client
+	traceID := r.Header.Get(OkitTraceIDHeader)
+	spanID := r.Header.Get(OkitSpanIDHeader)
+	if traceID != "" && spanID != "" {
+		ctx = context.WithValue(ctx, okit.SpanKey, okit.Span{
+			TraceID: traceID,
+			ID:      spanID,
+		})
+	}
+
+	statusCode := 200
+	tags := []okit.Tag{
 		okit.String("host", r.URL.Host),
 		okit.String("method", r.Method),
 		okit.String("path", r.URL.Path),
 		okit.Intp("status", &statusCode),
-	)
-	defer done()
+	}
+
+	defer okit.Trace(&ctx, tags...).Done()
 
 	h.delegate.ServeHTTP(
 		&response{

observer/local.go

@@ -0,0 +1,81 @@
+// Copyright (C) 2022 The OKit Authors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE
+// OR OTHER DEALINGS IN THE SOFTWARE.
+
+package observer
+
+import (
+	"bufio"
+	"io"
+	"os"
+
+	"go.pitz.tech/okit/format"
+	"go.pitz.tech/okit/pb"
+)
+
+// LocalOption defines a mechanism that allows properties of the observer to be overridden or configured.
+type LocalOption func(l *local)
+
+// Output configures the output of the local observer to point to the provided writer.
+func Output(writer io.Writer) LocalOption {
+	return func(l *local) {
+		l.output = bufio.NewWriter(writer)
+	}
+}
+
+// Format configures how the information is written to the target writer.
+func Format(fmt string) LocalOption {
+	return func(l *local) {
+		switch fmt {
+		case "json":
+			l.format = format.JSON{}
+		default:
+			l.format = format.Text{}
+		}
+	}
+}
+
+// Local creates an Observer that writes logs to an output stream using a configured format.
+func Local(opts ...LocalOption) Observer {
+	l := &local{
+		output: bufio.NewWriter(os.Stdout),
+		format: format.Text{},
+	}
+
+	for _, o := range opts {
+		o(l)
+	}
+
+	return l
+}
+
+type local struct {
+	output *bufio.Writer
+	format format.Marshaler
+}
+
+func (l *local) Observe(entries []*pb.Entry) {
+	// TODO: how to handle internal errors
+
+	for _, entry := range entries {
+		_ = l.format.Marshal(l.output, entry)
+	}
+
+	_ = l.output.Flush()
+}

observer/observer.go

@@ -0,0 +1,39 @@
+// Copyright (C) 2022 The OKit Authors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE
+// OR OTHER DEALINGS IN THE SOFTWARE.
+
+package observer
+
+import (
+	"go.pitz.tech/okit/pb"
+)
+
+// Observer provides a mechanism to subscribe to entries within the okit system.
+type Observer interface {
+	Observe(entries []*pb.Entry)
+}
+
+// Composite provides a way for multiple observers to be plugged into the system.
+type Composite []Observer
+
+func (c Composite) Observe(entries []*pb.Entry) {
+	for _, observer := range c {
+		observer.Observe(entries)
+	}
+}

okit.go

@@ -21,19 +21,31 @@
 package okit
 
 import (
-	"bufio"
 	"context"
-	"os"
 )
 
-// DefaultClient provides a default client implementation.
-var DefaultClient = NewClient()
+var (
+	global chan Client
+)
 
-// DefaultFormat is used to format how data is written to stdout.
-var DefaultFormat Format = TextFormat{}
+func init() {
+	global = make(chan Client, 1)
+	global <- NewClient()
+}
 
-// TODO: make this configurable
-var sink = bufio.NewWriter(os.Stdout)
+// Replace updates the default Client to be the provided one.
+func Replace(c Client) {
+	<-global
+	global <- c
+}
+
+// C returns a copy of the current default Client.
+func C() Client {
+	c := <-global
+	global <- c
+
+	return c
+}
 
 // ContextKey is a holder structure that allows data to be attached to contexts.
 type ContextKey string
@@ -43,36 +55,40 @@ var SpanKey = ContextKey("okit.span")
 
 // With returns a client containing additional tags.
 func With(tags ...Tag) Client {
-	return DefaultClient.WithCallerSkip(3).With(tags...)
+	return C().With(tags...)
 }
 
 // Emit emits an event with the provided tags.
 func Emit(event string, tags ...Tag) {
-	DefaultClient.WithCallerSkip(3).Emit(event, tags...)
+	C().WithCallerSkip(3).Emit(event, tags...)
 }
 
 // Observe reports a metric value with the provided tags.
 func Observe(metric string, value float64, tags ...Tag) {
-	DefaultClient.WithCallerSkip(3).Observe(metric, value, tags...)
+	C().WithCallerSkip(3).Observe(metric, value, tags...)
 }
 
 // Trace traces a function call. It implements distributed tracing and bookend events.
-func Trace(ctx context.Context, tags ...Tag) (context.Context, DoneFunc) {
-	return DefaultClient.WithCallerSkip(3).Trace(ctx, tags...)
+func Trace(ctxp *context.Context, tags ...Tag) ActiveTrace {
+	return C().WithCallerSkip(3).Trace(ctxp, tags...)
 }
 
+// Debug writes a message at a debug level.
 func Debug(msg string, tags ...Tag) {
-	DefaultClient.WithCallerSkip(3).Debug(msg, tags...)
+	C().WithCallerSkip(3).Debug(msg, tags...)
 }
 
+// Info writes a message at a informational level.
 func Info(msg string, tags ...Tag) {
-	DefaultClient.WithCallerSkip(3).Info(msg, tags...)
+	C().WithCallerSkip(3).Info(msg, tags...)
 }
 
+// Warn writes a message at a warning level.
 func Warn(msg string, tags ...Tag) {
-	DefaultClient.WithCallerSkip(3).Warn(msg, tags...)
+	C().WithCallerSkip(3).Warn(msg, tags...)
 }
 
+// Error writes a message at a error level.
 func Error(msg string, tags ...Tag) {
-	DefaultClient.WithCallerSkip(3).Error(msg, tags...)
+	C().WithCallerSkip(3).Error(msg, tags...)
 }

pb/obkit.pb.go

@@ -385,13 +385,6 @@ type Entry struct {
 	Kind Kind `protobuf:"varint,3,opt,name=kind,proto3,enum=obkit.Kind" json:"kind,omitempty"`
 	// Tags is an optional field that contains a list of metadata associated with the entry.
 	Tags []*Tag `protobuf:"bytes,4,rep,name=tags,proto3" json:"tags,omitempty"`
-	// Value is an optional field that defines a value associated with the entry (e.g. in the case of a log, metric, or trace).
-	//
-	// Types that are assignable to Value:
-	//	*Entry_String_
-	//	*Entry_Double
-	//	*Entry_Duration
-	Value isEntry_Value `protobuf_oneof:"value"`
 }
 
 func (x *Entry) Reset() {
@@ -454,56 +447,6 @@ func (x *Entry) GetTags() []*Tag {
 	return nil
 }
 
-func (m *Entry) GetValue() isEntry_Value {
-	if m != nil {
-		return m.Value
-	}
-	return nil
-}
-
-func (x *Entry) GetString_() string {
-	if x, ok := x.GetValue().(*Entry_String_); ok {
-		return x.String_
-	}
-	return ""
-}
-
-func (x *Entry) GetDouble() float64 {
-	if x, ok := x.GetValue().(*Entry_Double); ok {
-		return x.Double
-	}
-	return 0
-}
-
-func (x *Entry) GetDuration() *Duration {
-	if x, ok := x.GetValue().(*Entry_Duration); ok {
-		return x.Duration
-	}
-	return nil
-}
-
-type isEntry_Value interface {
-	isEntry_Value()
-}
-
-type Entry_String_ struct {
-	String_ string `protobuf:"bytes,5,opt,name=string,proto3,oneof"` // used for logs
-}
-
-type Entry_Double struct {
-	Double float64 `protobuf:"fixed64,6,opt,name=double,proto3,oneof"` // used for metrics
-}
-
-type Entry_Duration struct {
-	Duration *Duration `protobuf:"bytes,7,opt,name=duration,proto3,oneof"` // used for traces
-}
-
-func (*Entry_String_) isEntry_Value() {}
-
-func (*Entry_Double) isEntry_Value() {}
-
-func (*Entry_Duration) isEntry_Value() {}
-
 // ClientInfo is used to capture specific information about the client application that is emitting information.
 type ClientInfo struct {
 	state         protoimpl.MessageState
@@ -647,7 +590,7 @@ var file_obkit_proto_rawDesc = []byte{
 	0x61, 0x6d, 0x70, 0x18, 0x0b, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x10, 0x2e, 0x6f, 0x62, 0x6b, 0x69,
 	0x74, 0x2e, 0x54, 0x69, 0x6d, 0x65, 0x73, 0x74, 0x61, 0x6d, 0x70, 0x48, 0x00, 0x52, 0x09, 0x74,
 	0x69, 0x6d, 0x65, 0x73, 0x74, 0x61, 0x6d, 0x70, 0x42, 0x07, 0x0a, 0x05, 0x76, 0x61, 0x6c, 0x75,
-	0x65, 0x22, 0xfa, 0x01, 0x0a, 0x05, 0x45, 0x6e, 0x74, 0x72, 0x79, 0x12, 0x2e, 0x0a, 0x09, 0x74,
+	0x65, 0x22, 0x8e, 0x01, 0x0a, 0x05, 0x45, 0x6e, 0x74, 0x72, 0x79, 0x12, 0x2e, 0x0a, 0x09, 0x74,
 	0x69, 0x6d, 0x65, 0x73, 0x74, 0x61, 0x6d, 0x70, 0x18, 0x01, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x10,
 	0x2e, 0x6f, 0x62, 0x6b, 0x69, 0x74, 0x2e, 0x54, 0x69, 0x6d, 0x65, 0x73, 0x74, 0x61, 0x6d, 0x70,
 	0x52, 0x09, 0x74, 0x69, 0x6d, 0x65, 0x73, 0x74, 0x61, 0x6d, 0x70, 0x12, 0x14, 0x0a, 0x05, 0x73,
@@ -656,29 +599,23 @@ var file_obkit_proto_rawDesc = []byte{
 	0x0b, 0x2e, 0x6f, 0x62, 0x6b, 0x69, 0x74, 0x2e, 0x4b, 0x69, 0x6e, 0x64, 0x52, 0x04, 0x6b, 0x69,
 	0x6e, 0x64, 0x12, 0x1e, 0x0a, 0x04, 0x74, 0x61, 0x67, 0x73, 0x18, 0x04, 0x20, 0x03, 0x28, 0x0b,
 	0x32, 0x0a, 0x2e, 0x6f, 0x62, 0x6b, 0x69, 0x74, 0x2e, 0x54, 0x61, 0x67, 0x52, 0x04, 0x74, 0x61,
-	0x67, 0x73, 0x12, 0x18, 0x0a, 0x06, 0x73, 0x74, 0x72, 0x69, 0x6e, 0x67, 0x18, 0x05, 0x20, 0x01,
-	0x28, 0x09, 0x48, 0x00, 0x52, 0x06, 0x73, 0x74, 0x72, 0x69, 0x6e, 0x67, 0x12, 0x18, 0x0a, 0x06,
-	0x64, 0x6f, 0x75, 0x62, 0x6c, 0x65, 0x18, 0x06, 0x20, 0x01, 0x28, 0x01, 0x48, 0x00, 0x52, 0x06,
-	0x64, 0x6f, 0x75, 0x62, 0x6c, 0x65, 0x12, 0x2d, 0x0a, 0x08, 0x64, 0x75, 0x72, 0x61, 0x74, 0x69,
-	0x6f, 0x6e, 0x18, 0x07, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x0f, 0x2e, 0x6f, 0x62, 0x6b, 0x69, 0x74,
-	0x2e, 0x44, 0x75, 0x72, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x48, 0x00, 0x52, 0x08, 0x64, 0x75, 0x72,
-	0x61, 0x74, 0x69, 0x6f, 0x6e, 0x42, 0x07, 0x0a, 0x05, 0x76, 0x61, 0x6c, 0x75, 0x65, 0x22, 0x3c,
-	0x0a, 0x0a, 0x43, 0x6c, 0x69, 0x65, 0x6e, 0x74, 0x49, 0x6e, 0x66, 0x6f, 0x12, 0x12, 0x0a, 0x04,
-	0x6e, 0x61, 0x6d, 0x65, 0x18, 0x01, 0x20, 0x01, 0x28, 0x09, 0x52, 0x04, 0x6e, 0x61, 0x6d, 0x65,
-	0x12, 0x1a, 0x0a, 0x08, 0x69, 0x6e, 0x73, 0x74, 0x61, 0x6e, 0x63, 0x65, 0x18, 0x02, 0x20, 0x01,
-	0x28, 0x09, 0x52, 0x08, 0x69, 0x6e, 0x73, 0x74, 0x61, 0x6e, 0x63, 0x65, 0x22, 0x5b, 0x0a, 0x06,
-	0x50, 0x61, 0x63, 0x6b, 0x65, 0x74, 0x12, 0x29, 0x0a, 0x06, 0x63, 0x6c, 0x69, 0x65, 0x6e, 0x74,
-	0x18, 0x01, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x11, 0x2e, 0x6f, 0x62, 0x6b, 0x69, 0x74, 0x2e, 0x43,
-	0x6c, 0x69, 0x65, 0x6e, 0x74, 0x49, 0x6e, 0x66, 0x6f, 0x52, 0x06, 0x63, 0x6c, 0x69, 0x65, 0x6e,
-	0x74, 0x12, 0x26, 0x0a, 0x07, 0x65, 0x6e, 0x74, 0x72, 0x69, 0x65, 0x73, 0x18, 0x02, 0x20, 0x03,
-	0x28, 0x0b, 0x32, 0x0c, 0x2e, 0x6f, 0x62, 0x6b, 0x69, 0x74, 0x2e, 0x45, 0x6e, 0x74, 0x72, 0x79,
-	0x52, 0x07, 0x65, 0x6e, 0x74, 0x72, 0x69, 0x65, 0x73, 0x2a, 0x3e, 0x0a, 0x04, 0x4b, 0x69, 0x6e,
-	0x64, 0x12, 0x0b, 0x0a, 0x07, 0x55, 0x6e, 0x6b, 0x6e, 0x6f, 0x77, 0x6e, 0x10, 0x00, 0x12, 0x09,
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-	0x72, 0x69, 0x63, 0x10, 0x02, 0x12, 0x09, 0x0a, 0x05, 0x54, 0x72, 0x61, 0x63, 0x65, 0x10, 0x03,
-	0x12, 0x07, 0x0a, 0x03, 0x4c, 0x6f, 0x67, 0x10, 0x04, 0x42, 0x1c, 0x5a, 0x1a, 0x63, 0x6f, 0x64,
-	0x65, 0x2e, 0x70, 0x69, 0x74, 0x7a, 0x2e, 0x74, 0x65, 0x63, 0x68, 0x2f, 0x6d, 0x79, 0x61, 0x2f,
-	0x6f, 0x6b, 0x69, 0x74, 0x2f, 0x70, 0x62, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
+	0x67, 0x73, 0x22, 0x3c, 0x0a, 0x0a, 0x43, 0x6c, 0x69, 0x65, 0x6e, 0x74, 0x49, 0x6e, 0x66, 0x6f,
+	0x12, 0x12, 0x0a, 0x04, 0x6e, 0x61, 0x6d, 0x65, 0x18, 0x01, 0x20, 0x01, 0x28, 0x09, 0x52, 0x04,
+	0x6e, 0x61, 0x6d, 0x65, 0x12, 0x1a, 0x0a, 0x08, 0x69, 0x6e, 0x73, 0x74, 0x61, 0x6e, 0x63, 0x65,
+	0x18, 0x02, 0x20, 0x01, 0x28, 0x09, 0x52, 0x08, 0x69, 0x6e, 0x73, 0x74, 0x61, 0x6e, 0x63, 0x65,
+	0x22, 0x5b, 0x0a, 0x06, 0x50, 0x61, 0x63, 0x6b, 0x65, 0x74, 0x12, 0x29, 0x0a, 0x06, 0x63, 0x6c,
+	0x69, 0x65, 0x6e, 0x74, 0x18, 0x01, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x11, 0x2e, 0x6f, 0x62, 0x6b,
+	0x69, 0x74, 0x2e, 0x43, 0x6c, 0x69, 0x65, 0x6e, 0x74, 0x49, 0x6e, 0x66, 0x6f, 0x52, 0x06, 0x63,
+	0x6c, 0x69, 0x65, 0x6e, 0x74, 0x12, 0x26, 0x0a, 0x07, 0x65, 0x6e, 0x74, 0x72, 0x69, 0x65, 0x73,
+	0x18, 0x02, 0x20, 0x03, 0x28, 0x0b, 0x32, 0x0c, 0x2e, 0x6f, 0x62, 0x6b, 0x69, 0x74, 0x2e, 0x45,
+	0x6e, 0x74, 0x72, 0x79, 0x52, 0x07, 0x65, 0x6e, 0x74, 0x72, 0x69, 0x65, 0x73, 0x2a, 0x3e, 0x0a,
+	0x04, 0x4b, 0x69, 0x6e, 0x64, 0x12, 0x0b, 0x0a, 0x07, 0x55, 0x6e, 0x6b, 0x6e, 0x6f, 0x77, 0x6e,
+	0x10, 0x00, 0x12, 0x09, 0x0a, 0x05, 0x45, 0x76, 0x65, 0x6e, 0x74, 0x10, 0x01, 0x12, 0x0a, 0x0a,
+	0x06, 0x4d, 0x65, 0x74, 0x72, 0x69, 0x63, 0x10, 0x02, 0x12, 0x09, 0x0a, 0x05, 0x54, 0x72, 0x61,
+	0x63, 0x65, 0x10, 0x03, 0x12, 0x07, 0x0a, 0x03, 0x4c, 0x6f, 0x67, 0x10, 0x04, 0x42, 0x1c, 0x5a,
+	0x1a, 0x63, 0x6f, 0x64, 0x65, 0x2e, 0x70, 0x69, 0x74, 0x7a, 0x2e, 0x74, 0x65, 0x63, 0x68, 0x2f,
+	0x6d, 0x79, 0x61, 0x2f, 0x6f, 0x6b, 0x69, 0x74, 0x2f, 0x70, 0x62, 0x62, 0x06, 0x70, 0x72, 0x6f,
+	0x74, 0x6f, 0x33,
 }
 
 var (
@@ -710,14 +647,13 @@ var file_obkit_proto_depIdxs = []int32{
 	2, // 2: obkit.Entry.timestamp:type_name -> obkit.Timestamp
 	0, // 3: obkit.Entry.kind:type_name -> obkit.Kind
 	3, // 4: obkit.Entry.tags:type_name -> obkit.Tag
-	1, // 5: obkit.Entry.duration:type_name -> obkit.Duration
-	5, // 6: obkit.Packet.client:type_name -> obkit.ClientInfo
-	4, // 7: obkit.Packet.entries:type_name -> obkit.Entry
-	8, // [8:8] is the sub-list for method output_type
-	8, // [8:8] is the sub-list for method input_type
-	8, // [8:8] is the sub-list for extension type_name
-	8, // [8:8] is the sub-list for extension extendee
-	0, // [0:8] is the sub-list for field type_name
+	5, // 5: obkit.Packet.client:type_name -> obkit.ClientInfo
+	4, // 6: obkit.Packet.entries:type_name -> obkit.Entry
+	7, // [7:7] is the sub-list for method output_type
+	7, // [7:7] is the sub-list for method input_type
+	7, // [7:7] is the sub-list for extension type_name
+	7, // [7:7] is the sub-list for extension extendee
+	0, // [0:7] is the sub-list for field type_name
 }
 
 func init() { file_obkit_proto_init() }
@@ -808,11 +744,6 @@ func file_obkit_proto_init() {
 		(*Tag_Duration)(nil),
 		(*Tag_Timestamp)(nil),
 	}
-	file_obkit_proto_msgTypes[3].OneofWrappers = []interface{}{
-		(*Entry_String_)(nil),
-		(*Entry_Double)(nil),
-		(*Entry_Duration)(nil),
-	}
 	type x struct{}
 	out := protoimpl.TypeBuilder{
 		File: protoimpl.DescBuilder{

pb/obkit.pbext.go

@@ -29,20 +29,24 @@ import (
 
 //go:generate protoc --go_out=paths=source_relative:. -I=. obkit.proto
 
+// AsDuration converts a protocol level Duration to a Golang time.Duration.
 func (x *Duration) AsDuration() time.Duration {
 	return time.Duration(x.Nanos)
 }
 
+// DurationPB converts a time.Duration to a protocol level Duration.
 func DurationPB(d time.Duration) *Duration {
 	return &Duration{
 		Nanos: int64(d),
 	}
 }
 
+// AsTime converts a protocol level Timestamp to a Golang time.Time.
 func (x *Timestamp) AsTime() time.Time {
 	return time.Unix(x.Seconds, int64(x.Nanos))
 }
 
+// TimestampPB converts a time.Time to a protocol level Timestamp.
 func TimestampPB(t time.Time) *Timestamp {
 	return &Timestamp{
 		Seconds: t.Unix(),
@@ -50,7 +54,7 @@ func TimestampPB(t time.Time) *Timestamp {
 	}
 }
 
-// AsQueryString converts the associated tag to a URI query string
+// AsQueryString converts the associated tag to a URI query string.
 func (x *Tag) AsQueryString() string {
 	val := ""
 

pb/obkit.proto

@@ -79,13 +79,6 @@ message Entry {
 
   // Tags is an optional field that contains a list of metadata associated with the entry.
   repeated Tag tags = 4;
-
-  // Value is an optional field that defines a value associated with the entry (e.g. in the case of a log, metric, or trace).
-  oneof value {
-    string string = 5;      // used for logs
-    double double = 6;      // used for metrics
-    Duration duration = 7;  // used for traces
-  }
 }
 
 // ClientInfo is used to capture specific information about the client application that is emitting information.
@@ -100,9 +93,12 @@ message ClientInfo {
 
 // Packet defines the set of data that is contained within each message sent to the server.
 message Packet {
+  // Id provides an id for the packet so the server can deduplicate packets of information being sent to them.
+  bytes id = 1;
+
   // Client contains information that identifies the emitting client.
-  ClientInfo client = 1;
+  ClientInfo client = 2;
 
   // Entries are processed in batches, allowing local clients to buffer if necessary.
-  repeated Entry entries = 2;
+  repeated Entry entries = 3;
 }

tags.ext.go

@@ -25,3 +25,9 @@ func Err(err error) Tag { return Tag{"err", err} }
 
 // Errp returns a Tag whose value is read after submission.
 func Errp(err *error) Tag { return Tag{"err", err} }
+
+// level returns a tag that contains the string representation of a level.
+func level(level Level) Tag { return Tag{"level", (&level).String()} }
+
+// message returns a tag containing message string to index.
+func message(msg string) Tag { return Tag{"msg", msg} }