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// Copyright 2016 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.
// Copyright (c) 2013, The Prometheus Authors
// All rights reserved.
//
// Use of this source code is governed by a BSD-style license that can be found
// in the LICENSE file.
// Package promhttp contains functions to create http.Handler instances to
// expose Prometheus metrics via HTTP. In later versions of this package, it
// will also contain tooling to instrument instances of http.Handler and
// http.RoundTripper.
//
// promhttp.Handler acts on the prometheus.DefaultGatherer. With HandlerFor,
// you can create a handler for a custom registry or anything that implements
// the Gatherer interface. It also allows to create handlers that act
// differently on errors or allow to log errors.
package promhttp
import (
"bytes"
"compress/gzip"
"fmt"
"io"
"net/http"
"strings"
"sync"
"github.com/prometheus/common/expfmt"
"github.com/prometheus/client_golang/prometheus"
)
const (
contentTypeHeader = "Content-Type"
contentLengthHeader = "Content-Length"
contentEncodingHeader = "Content-Encoding"
acceptEncodingHeader = "Accept-Encoding"
)
var bufPool sync.Pool
func getBuf() *bytes.Buffer {
buf := bufPool.Get()
if buf == nil {
return &bytes.Buffer{}
}
return buf.(*bytes.Buffer)
}
func giveBuf(buf *bytes.Buffer) {
buf.Reset()
bufPool.Put(buf)
}
// Handler returns an HTTP handler for the prometheus.DefaultGatherer. The
// Handler uses the default HandlerOpts, i.e. report the first error as an HTTP
// error, no error logging, and compression if requested by the client.
//
// If you want to create a Handler for the DefaultGatherer with different
// HandlerOpts, create it with HandlerFor with prometheus.DefaultGatherer and
// your desired HandlerOpts.
func Handler() http.Handler {
return HandlerFor(prometheus.DefaultGatherer, HandlerOpts{})
}
// HandlerFor returns an http.Handler for the provided Gatherer. The behavior
// of the Handler is defined by the provided HandlerOpts.
func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
mfs, err := reg.Gather()
if err != nil {
if opts.ErrorLog != nil {
opts.ErrorLog.Println("error gathering metrics:", err)
}
switch opts.ErrorHandling {
case PanicOnError:
panic(err)
case ContinueOnError:
if len(mfs) == 0 {
http.Error(w, "No metrics gathered, last error:\n\n"+err.Error(), http.StatusInternalServerError)
return
}
case HTTPErrorOnError:
http.Error(w, "An error has occurred during metrics gathering:\n\n"+err.Error(), http.StatusInternalServerError)
return
}
}
contentType := expfmt.Negotiate(req.Header)
buf := getBuf()
defer giveBuf(buf)
writer, encoding := decorateWriter(req, buf, opts.DisableCompression)
enc := expfmt.NewEncoder(writer, contentType)
var lastErr error
for _, mf := range mfs {
if err := enc.Encode(mf); err != nil {
lastErr = err
if opts.ErrorLog != nil {
opts.ErrorLog.Println("error encoding metric family:", err)
}
switch opts.ErrorHandling {
case PanicOnError:
panic(err)
case ContinueOnError:
// Handled later.
case HTTPErrorOnError:
http.Error(w, "An error has occurred during metrics encoding:\n\n"+err.Error(), http.StatusInternalServerError)
return
}
}
}
if closer, ok := writer.(io.Closer); ok {
closer.Close()
}
if lastErr != nil && buf.Len() == 0 {
http.Error(w, "No metrics encoded, last error:\n\n"+err.Error(), http.StatusInternalServerError)
return
}
header := w.Header()
header.Set(contentTypeHeader, string(contentType))
header.Set(contentLengthHeader, fmt.Sprint(buf.Len()))
if encoding != "" {
header.Set(contentEncodingHeader, encoding)
}
w.Write(buf.Bytes())
// TODO(beorn7): Consider streaming serving of metrics.
})
}
// HandlerErrorHandling defines how a Handler serving metrics will handle
// errors.
type HandlerErrorHandling int
// These constants cause handlers serving metrics to behave as described if
// errors are encountered.
const (
// Serve an HTTP status code 500 upon the first error
// encountered. Report the error message in the body.
HTTPErrorOnError HandlerErrorHandling = iota
// Ignore errors and try to serve as many metrics as possible. However,
// if no metrics can be served, serve an HTTP status code 500 and the
// last error message in the body. Only use this in deliberate "best
// effort" metrics collection scenarios. It is recommended to at least
// log errors (by providing an ErrorLog in HandlerOpts) to not mask
// errors completely.
ContinueOnError
// Panic upon the first error encountered (useful for "crash only" apps).
PanicOnError
)
// Logger is the minimal interface HandlerOpts needs for logging. Note that
// log.Logger from the standard library implements this interface, and it is
// easy to implement by custom loggers, if they don't do so already anyway.
type Logger interface {
Println(v ...interface{})
}
// HandlerOpts specifies options how to serve metrics via an http.Handler. The
// zero value of HandlerOpts is a reasonable default.
type HandlerOpts struct {
// ErrorLog specifies an optional logger for errors collecting and
// serving metrics. If nil, errors are not logged at all.
ErrorLog Logger
// ErrorHandling defines how errors are handled. Note that errors are
// logged regardless of the configured ErrorHandling provided ErrorLog
// is not nil.
ErrorHandling HandlerErrorHandling
// If DisableCompression is true, the handler will never compress the
// response, even if requested by the client.
DisableCompression bool
}
// decorateWriter wraps a writer to handle gzip compression if requested. It
// returns the decorated writer and the appropriate "Content-Encoding" header
// (which is empty if no compression is enabled).
func decorateWriter(request *http.Request, writer io.Writer, compressionDisabled bool) (io.Writer, string) {
if compressionDisabled {
return writer, ""
}
header := request.Header.Get(acceptEncodingHeader)
parts := strings.Split(header, ",")
for _, part := range parts {
part := strings.TrimSpace(part)
if part == "gzip" || strings.HasPrefix(part, "gzip;") {
return gzip.NewWriter(writer), "gzip"
}
}
return writer, ""
}
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