@@ -27,6 +27,7 @@ type JSONFileLogger struct {

 	closed  bool

 	writer  *loggerutils.LogFile

 	readers map[*logger.LogWatcher]struct{} // stores the active log followers

+	tag     string                          // tag values requested by the user to log

 }

 func init() {

@@ -61,6 +62,12 @@ func New(info logger.Info) (logger.Logger, error) {

 		}

 	}

+	// no default template. only use a tag if the user asked for it

+	tag, err := loggerutils.ParseLogTag(info, "")

+	if err != nil {

+		return nil, err

+	}

+

 	var extra []byte

 	attrs, err := info.ExtraAttributes(nil)

 	if err != nil {

@@ -76,7 +83,7 @@ func New(info logger.Info) (logger.Logger, error) {

 	buf := bytes.NewBuffer(nil)

 	marshalFunc := func(msg *logger.Message) ([]byte, error) {

-		if err := marshalMessage(msg, extra, buf); err != nil {

+		if err := marshalMessage(msg, extra, buf, tag); err != nil {

 			return nil, err

 		}

 		b := buf.Bytes()

@@ -92,6 +99,7 @@ func New(info logger.Info) (logger.Logger, error) {

 	return &JSONFileLogger{

 		writer:  writer,

 		readers: make(map[*logger.LogWatcher]struct{}),

+		tag:     tag,

 	}, nil

 }

@@ -103,7 +111,7 @@ func (l *JSONFileLogger) Log(msg *logger.Message) error {

 	return err

 }

-func marshalMessage(msg *logger.Message, extra json.RawMessage, buf *bytes.Buffer) error {

+func marshalMessage(msg *logger.Message, extra json.RawMessage, buf *bytes.Buffer, tag string) error {

 	logLine := msg.Line

 	if !msg.Partial {

 		logLine = append(msg.Line, '\n')

@@ -113,6 +121,7 @@ func marshalMessage(msg *logger.Message, extra json.RawMessage, buf *bytes.Buffe

 		Stream:   msg.Source,

 		Created:  msg.Timestamp,

 		RawAttrs: extra,

+		Tag:      tag,

 	}).MarshalJSONBuf(buf)

 	if err != nil {

 		return errors.Wrap(err, "error writing log message to buffer")

@@ -130,6 +139,7 @@ func ValidateLogOpt(cfg map[string]string) error {

 		case "labels":

 		case "env":

 		case "env-regex":

+		case "tag":

 		default:

 			return fmt.Errorf("unknown log opt '%s' for json-file log driver", key)

 		}

@@ -57,6 +57,49 @@ func TestJSONFileLogger(t *testing.T) {

 	}

 }

+func TestJSONFileLoggerWithTags(t *testing.T) {

+	cid := "a7317399f3f857173c6179d44823594f8294678dea9999662e5c625b5a1c7657"

+	cname := "test-container"

+	tmp, err := ioutil.TempDir("", "docker-logger-")

+

+	require.NoError(t, err)

+

+	defer os.RemoveAll(tmp)

+	filename := filepath.Join(tmp, "container.log")

+	l, err := New(logger.Info{

+		Config: map[string]string{

+			"tag": "{{.ID}}/{{.Name}}", // first 12 characters of ContainerID and full ContainerName

+		},

+		ContainerID:   cid,

+		ContainerName: cname,

+		LogPath:       filename,

+	})

+

+	require.NoError(t, err)

+	defer l.Close()

+

+	err = l.Log(&logger.Message{Line: []byte("line1"), Source: "src1"})

+	require.NoError(t, err)

+

+	err = l.Log(&logger.Message{Line: []byte("line2"), Source: "src2"})

+	require.NoError(t, err)

+

+	err = l.Log(&logger.Message{Line: []byte("line3"), Source: "src3"})

+	require.NoError(t, err)

+

+	res, err := ioutil.ReadFile(filename)

+	require.NoError(t, err)

+

+	expected := `{"log":"line1\n","stream":"src1","tag":"a7317399f3f8/test-container","time":"0001-01-01T00:00:00Z"}

+{"log":"line2\n","stream":"src2","tag":"a7317399f3f8/test-container","time":"0001-01-01T00:00:00Z"}

+{"log":"line3\n","stream":"src3","tag":"a7317399f3f8/test-container","time":"0001-01-01T00:00:00Z"}

+`

+

+	if string(res) != expected {

+		t.Fatalf("Wrong log content: %q, expected %q", res, expected)

+	}

+}

+

 func BenchmarkJSONFileLoggerLog(b *testing.B) {

 	tmp := fs.NewDir(b, "bench-jsonfilelog")

 	defer tmp.Remove()

@@ -82,7 +125,7 @@ func BenchmarkJSONFileLoggerLog(b *testing.B) {

 	}

 	buf := bytes.NewBuffer(nil)

-	require.NoError(b, marshalMessage(msg, nil, buf))

+	require.NoError(b, marshalMessage(msg, nil, buf, ""))

 	b.SetBytes(int64(buf.Len()))

 	b.ResetTimer()

@@ -14,6 +14,8 @@ type JSONLog struct {

 	Created time.Time `json:"time"`

 	// Attrs is the list of extra attributes provided by the user

 	Attrs map[string]string `json:"attrs,omitempty"`

+	// Tags requested the operator

+	Tag string `json:"tag,omitempty"`

 }

 // Reset all fields to their zero value.

@@ -22,4 +24,5 @@ func (jl *JSONLog) Reset() {

 	jl.Stream = ""

 	jl.Created = time.Time{}

 	jl.Attrs = make(map[string]string)

+	jl.Tag = ""

 }

@@ -12,6 +12,7 @@ type JSONLogs struct {

 	Log     []byte    `json:"log,omitempty"`

 	Stream  string    `json:"stream,omitempty"`

 	Created time.Time `json:"time"`

+	Tag     string    `json:"tag,omitempty"`

 	// json-encoded bytes

 	RawAttrs json.RawMessage `json:"attrs,omitempty"`

@@ -37,6 +38,15 @@ func (mj *JSONLogs) MarshalJSONBuf(buf *bytes.Buffer) error {

 		buf.WriteString(`"stream":`)

 		ffjsonWriteJSONBytesAsString(buf, []byte(mj.Stream))

 	}

+	if len(mj.Tag) > 0 {

+		if first {

+			first = false

+		} else {

+			buf.WriteString(`,`)

+		}

+		buf.WriteString(`"tag":`)

+		ffjsonWriteJSONBytesAsString(buf, []byte(mj.Tag))

+	}

 	if len(mj.RawAttrs) > 0 {

 		if first {

 			first = false

@@ -29,6 +29,8 @@ func TestJSONLogsMarshalJSONBuf(t *testing.T) {

 		{Log: []byte{0x7F}}:            `^{\"log\":\"\x7f\",\"time\":`,

 		// with raw attributes

 		{Log: []byte("A log line"), RawAttrs: []byte(`{"hello":"world","value":1234}`)}: `^{\"log\":\"A log line\",\"attrs\":{\"hello\":\"world\",\"value\":1234},\"time\":`,

+		// with Tag set

+		{Log: []byte("A log line with tag"), Tag: "test-tag"}: `^{\"log\":\"A log line with tag\",\"tag\":\"test-tag\",\"time\":`,

 	}

 	for jsonLog, expression := range logs {

 		var buf bytes.Buffer

@@ -35,7 +35,7 @@ func BenchmarkJSONFileLoggerReadLogs(b *testing.B) {

 	}

 	buf := bytes.NewBuffer(nil)

-	require.NoError(b, marshalMessage(msg, nil, buf))

+	require.NoError(b, marshalMessage(msg, nil, buf, ""))

 	b.SetBytes(int64(buf.Len()))

 	b.ResetTimer()