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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.
-
- // Package promhttp provides tooling around HTTP servers and clients.
- //
- // First, the package allows the creation of http.Handler instances to expose
- // Prometheus metrics via HTTP. 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 the creation of handlers that act differently on errors or allow to
- // log errors.
- //
- // Second, the package provides tooling to instrument instances of http.Handler
- // via middleware. Middleware wrappers follow the naming scheme
- // InstrumentHandlerX, where X describes the intended use of the middleware.
- // See each function's doc comment for specific details.
- //
- // Finally, the package allows for an http.RoundTripper to be instrumented via
- // middleware. Middleware wrappers follow the naming scheme
- // InstrumentRoundTripperX, where X describes the intended use of the
- // middleware. See each function's doc comment for specific details.
- package promhttp
-
- import (
- "compress/gzip"
- "fmt"
- "io"
- "net/http"
- "strings"
- "sync"
- "time"
-
- "github.com/prometheus/common/expfmt"
-
- "github.com/prometheus/client_golang/prometheus"
- )
-
- const (
- contentTypeHeader = "Content-Type"
- contentEncodingHeader = "Content-Encoding"
- acceptEncodingHeader = "Accept-Encoding"
- )
-
- var gzipPool = sync.Pool{
- New: func() interface{} {
- return gzip.NewWriter(nil)
- },
- }
-
- // Handler returns an http.Handler for the prometheus.DefaultGatherer, using
- // default HandlerOpts, i.e. it reports the first error as an HTTP error, it has
- // no error logging, and it applies compression if requested by the client.
- //
- // The returned http.Handler is already instrumented using the
- // InstrumentMetricHandler function and the prometheus.DefaultRegisterer. If you
- // create multiple http.Handlers by separate calls of the Handler function, the
- // metrics used for instrumentation will be shared between them, providing
- // global scrape counts.
- //
- // This function is meant to cover the bulk of basic use cases. If you are doing
- // anything that requires more customization (including using a non-default
- // Gatherer, different instrumentation, and non-default HandlerOpts), use the
- // HandlerFor function. See there for details.
- func Handler() http.Handler {
- return InstrumentMetricHandler(
- prometheus.DefaultRegisterer, HandlerFor(prometheus.DefaultGatherer, HandlerOpts{}),
- )
- }
-
- // HandlerFor returns an uninstrumented http.Handler for the provided
- // Gatherer. The behavior of the Handler is defined by the provided
- // HandlerOpts. Thus, HandlerFor is useful to create http.Handlers for custom
- // Gatherers, with non-default HandlerOpts, and/or with custom (or no)
- // instrumentation. Use the InstrumentMetricHandler function to apply the same
- // kind of instrumentation as it is used by the Handler function.
- func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
- var inFlightSem chan struct{}
- if opts.MaxRequestsInFlight > 0 {
- inFlightSem = make(chan struct{}, opts.MaxRequestsInFlight)
- }
-
- h := http.HandlerFunc(func(rsp http.ResponseWriter, req *http.Request) {
- if inFlightSem != nil {
- select {
- case inFlightSem <- struct{}{}: // All good, carry on.
- defer func() { <-inFlightSem }()
- default:
- http.Error(rsp, fmt.Sprintf(
- "Limit of concurrent requests reached (%d), try again later.", opts.MaxRequestsInFlight,
- ), http.StatusServiceUnavailable)
- return
- }
- }
- 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 {
- // Still report the error if no metrics have been gathered.
- httpError(rsp, err)
- return
- }
- case HTTPErrorOnError:
- httpError(rsp, err)
- return
- }
- }
-
- contentType := expfmt.Negotiate(req.Header)
- header := rsp.Header()
- header.Set(contentTypeHeader, string(contentType))
-
- w := io.Writer(rsp)
- if !opts.DisableCompression && gzipAccepted(req.Header) {
- header.Set(contentEncodingHeader, "gzip")
- gz := gzipPool.Get().(*gzip.Writer)
- defer gzipPool.Put(gz)
-
- gz.Reset(w)
- defer gz.Close()
-
- w = gz
- }
-
- enc := expfmt.NewEncoder(w, 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 and sending metric family:", err)
- }
- switch opts.ErrorHandling {
- case PanicOnError:
- panic(err)
- case ContinueOnError:
- // Handled later.
- case HTTPErrorOnError:
- httpError(rsp, err)
- return
- }
- }
- }
-
- if lastErr != nil {
- httpError(rsp, lastErr)
- }
- })
-
- if opts.Timeout <= 0 {
- return h
- }
- return http.TimeoutHandler(h, opts.Timeout, fmt.Sprintf(
- "Exceeded configured timeout of %v.\n",
- opts.Timeout,
- ))
- }
-
- // InstrumentMetricHandler is usually used with an http.Handler returned by the
- // HandlerFor function. It instruments the provided http.Handler with two
- // metrics: A counter vector "promhttp_metric_handler_requests_total" to count
- // scrapes partitioned by HTTP status code, and a gauge
- // "promhttp_metric_handler_requests_in_flight" to track the number of
- // simultaneous scrapes. This function idempotently registers collectors for
- // both metrics with the provided Registerer. It panics if the registration
- // fails. The provided metrics are useful to see how many scrapes hit the
- // monitored target (which could be from different Prometheus servers or other
- // scrapers), and how often they overlap (which would result in more than one
- // scrape in flight at the same time). Note that the scrapes-in-flight gauge
- // will contain the scrape by which it is exposed, while the scrape counter will
- // only get incremented after the scrape is complete (as only then the status
- // code is known). For tracking scrape durations, use the
- // "scrape_duration_seconds" gauge created by the Prometheus server upon each
- // scrape.
- func InstrumentMetricHandler(reg prometheus.Registerer, handler http.Handler) http.Handler {
- cnt := prometheus.NewCounterVec(
- prometheus.CounterOpts{
- Name: "promhttp_metric_handler_requests_total",
- Help: "Total number of scrapes by HTTP status code.",
- },
- []string{"code"},
- )
- // Initialize the most likely HTTP status codes.
- cnt.WithLabelValues("200")
- cnt.WithLabelValues("500")
- cnt.WithLabelValues("503")
- if err := reg.Register(cnt); err != nil {
- if are, ok := err.(prometheus.AlreadyRegisteredError); ok {
- cnt = are.ExistingCollector.(*prometheus.CounterVec)
- } else {
- panic(err)
- }
- }
-
- gge := prometheus.NewGauge(prometheus.GaugeOpts{
- Name: "promhttp_metric_handler_requests_in_flight",
- Help: "Current number of scrapes being served.",
- })
- if err := reg.Register(gge); err != nil {
- if are, ok := err.(prometheus.AlreadyRegisteredError); ok {
- gge = are.ExistingCollector.(prometheus.Gauge)
- } else {
- panic(err)
- }
- }
-
- return InstrumentHandlerCounter(cnt, InstrumentHandlerInFlight(gge, handler))
- }
-
- // 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
- // The number of concurrent HTTP requests is limited to
- // MaxRequestsInFlight. Additional requests are responded to with 503
- // Service Unavailable and a suitable message in the body. If
- // MaxRequestsInFlight is 0 or negative, no limit is applied.
- MaxRequestsInFlight int
- // If handling a request takes longer than Timeout, it is responded to
- // with 503 ServiceUnavailable and a suitable Message. No timeout is
- // applied if Timeout is 0 or negative. Note that with the current
- // implementation, reaching the timeout simply ends the HTTP requests as
- // described above (and even that only if sending of the body hasn't
- // started yet), while the bulk work of gathering all the metrics keeps
- // running in the background (with the eventual result to be thrown
- // away). Until the implementation is improved, it is recommended to
- // implement a separate timeout in potentially slow Collectors.
- Timeout time.Duration
- }
-
- // gzipAccepted returns whether the client will accept gzip-encoded content.
- func gzipAccepted(header http.Header) bool {
- a := header.Get(acceptEncodingHeader)
- parts := strings.Split(a, ",")
- for _, part := range parts {
- part = strings.TrimSpace(part)
- if part == "gzip" || strings.HasPrefix(part, "gzip;") {
- return true
- }
- }
- return false
- }
-
- // httpError removes any content-encoding header and then calls http.Error with
- // the provided error and http.StatusInternalServerErrer. Error contents is
- // supposed to be uncompressed plain text. However, same as with a plain
- // http.Error, any header settings will be void if the header has already been
- // sent. The error message will still be written to the writer, but it will
- // probably be of limited use.
- func httpError(rsp http.ResponseWriter, err error) {
- rsp.Header().Del(contentEncodingHeader)
- http.Error(
- rsp,
- "An error has occurred while serving metrics:\n\n"+err.Error(),
- http.StatusInternalServerError,
- )
- }
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