The Go Blog

Introducing HTTP Tracing

4 October 2016

Introduction

In Go 1.7 we introduced HTTP tracing, a facility to gather fine-grained information throughout the lifecycle of an HTTP client request. Support for HTTP tracing is provided by the net/http/httptrace package. The collected information can be used for debugging latency issues, service monitoring, writing adaptive systems, and more.

HTTP events

The httptrace package provides a number of hooks to gather information during an HTTP round trip about a variety of events. These events include:

  • Connection creation
  • Connection reuse
  • DNS lookups
  • Writing the request to the wire
  • Reading the response

Tracing events

You can enable HTTP tracing by putting an *httptrace.ClientTrace containing hook functions into a request's context.Context. Various http.RoundTripper implementations report the internal events by looking for context's *httptrace.ClientTrace and calling the relevant hook functions.

The tracing is scoped to the request's context and users should put a *httptrace.ClientTrace to the request context before they start a request.

    req, _ := http.NewRequest("GET", "http://example.com", nil)
    trace := &httptrace.ClientTrace{
        DNSDone: func(dnsInfo httptrace.DNSDoneInfo) {
            fmt.Printf("DNS Info: %+v\n", dnsInfo)
        },
        GotConn: func(connInfo httptrace.GotConnInfo) {
            fmt.Printf("Got Conn: %+v\n", connInfo)
        },
    }
    req = req.WithContext(httptrace.WithClientTrace(req.Context(), trace))
    if _, err := http.DefaultTransport.RoundTrip(req); err != nil {
        log.Fatal(err)
    }

During a round trip, http.DefaultTransport will invoke each hook as an event happens. The program above will print the DNS information as soon as the DNS lookup is complete. It will similarly print connection information when a connection is established to the request's host.

Tracing with http.Client

The tracing mechanism is designed to trace the events in the lifecycle of a single http.Transport.RoundTrip. However, a client may make multiple round trips to complete an HTTP request. For example, in the case of a URL redirection, the registered hooks will be called as many times as the client follows HTTP redirects, making multiple requests. Users are responsible for recognizing such events at the http.Client level. The program below identifies the current request by using an http.RoundTripper wrapper.

package main

import (
    "fmt"
    "log"
    "net/http"
    "net/http/httptrace"
)

// transport is an http.RoundTripper that keeps track of the in-flight
// request and implements hooks to report HTTP tracing events.
type transport struct {
    current *http.Request
}

// RoundTrip wraps http.DefaultTransport.RoundTrip to keep track
// of the current request.
func (t *transport) RoundTrip(req *http.Request) (*http.Response, error) {
    t.current = req
    return http.DefaultTransport.RoundTrip(req)
}

// GotConn prints whether the connection has been used previously
// for the current request.
func (t *transport) GotConn(info httptrace.GotConnInfo) {
    fmt.Printf("Connection reused for %v? %v\n", t.current.URL, info.Reused)
}

func main() {
    t := &transport{}

    req, _ := http.NewRequest("GET", "https://google.com", nil)
    trace := &httptrace.ClientTrace{
        GotConn: t.GotConn,
    }
    req = req.WithContext(httptrace.WithClientTrace(req.Context(), trace))

    client := &http.Client{Transport: t}
    if _, err := client.Do(req); err != nil {
        log.Fatal(err)
    }
}

The program will follow the redirect of google.com to www.google.com and will output:

Connection reused for https://google.com? false
Connection reused for https://www.google.com/? false

The Transport in the net/http package supports tracing of both HTTP/1 and HTTP/2 requests.

If you are an author of a custom http.RoundTripper implementation, you can support tracing by checking the request context for an *httptest.ClientTrace and invoking the relevant hooks as the events occur.

Conclusion

HTTP tracing is a valuable addition to Go for those who are interested in debugging HTTP request latency and writing tools for network debugging for outbound traffic. By enabling this new facility, we hope to see HTTP debugging, benchmarking and visualization tools from the community — such as httpstat.

By Jaana Burcu Dogan

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