Source: encode.go in package github.com/prometheus/common/expfmt

Copyright 2015 The Prometheus Authors Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


package expfmt

import (
	"fmt"
	"io"
	"net/http"

	"github.com/golang/protobuf/proto"
	"github.com/matttproud/golang_protobuf_extensions/pbutil"
	"github.com/prometheus/common/internal/bitbucket.org/ww/goautoneg"

	dto "github.com/prometheus/client_model/go"
)

Encoder types encode metric families into an underlying wire protocol.

type Encoder interface {
	Encode(*dto.MetricFamily) error
}

type encoder func(*dto.MetricFamily) error

func (e encoder) Encode(v *dto.MetricFamily) error {
	return e(v)
}

Negotiate returns the Content-Type based on the given Accept header. If no appropriate accepted type is found, FmtText is returned.

func Negotiate(h http.Header) Format {

Check for protocol buffer

		if ac.Type+"/"+ac.SubType == ProtoType && ac.Params["proto"] == ProtoProtocol {
			switch ac.Params["encoding"] {
			case "delimited":
				return FmtProtoDelim
			case "text":
				return FmtProtoText
			case "compact-text":
				return FmtProtoCompact
			}

Check for text format.

		ver := ac.Params["version"]
		if ac.Type == "text" && ac.SubType == "plain" && (ver == TextVersion || ver == "") {
			return FmtText
		}
	}
	return FmtText
}

NewEncoder returns a new encoder based on content type negotiation.

func NewEncoder(w io.Writer, format Format) Encoder {
	switch format {
	case FmtProtoDelim:
		return encoder(func(v *dto.MetricFamily) error {
			_, err := pbutil.WriteDelimited(w, v)
			return err
		})
	case FmtProtoCompact:
		return encoder(func(v *dto.MetricFamily) error {
			_, err := fmt.Fprintln(w, v.String())
			return err
		})
	case FmtProtoText:
		return encoder(func(v *dto.MetricFamily) error {
			_, err := fmt.Fprintln(w, proto.MarshalTextString(v))
			return err
		})
	case FmtText:
		return encoder(func(v *dto.MetricFamily) error {
			_, err := MetricFamilyToText(w, v)
			return err
		})
	}
	panic("expfmt.NewEncoder: unknown format")