website/_content/blog/http-tracing.md

title	date	by	tags	summary
Introducing HTTP Tracing	2016-10-04	Jaana Burcu Dogan	http technical	How to use Go 1.7's HTTP tracing to understand your client requests.

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.

{{code "http-tracing/trace.go" /START/ /END/}}

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.

{{code "http-tracing/client.go"}}

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.