96 lines
3.9 KiB
Markdown
96 lines
3.9 KiB
Markdown
|
# httpsnoop
|
||
|
|
|
||
|
|
Package httpsnoop provides an easy way to capture http related metrics (i.e.
|
||
|
|
response time, bytes written, and http status code) from your application's
|
||
|
|
http.Handlers.
|
||
|
|
|
||
|
|
Doing this requires non-trivial wrapping of the http.ResponseWriter interface,
|
||
|
|
which is also exposed for users interested in a more low-level API.
|
||
|
|
|
||
|
|
[](https://godoc.org/github.com/felixge/httpsnoop)
|
||
|
|
[](https://travis-ci.org/felixge/httpsnoop)
|
||
|
|
|
||
|
|
## Usage Example
|
||
|
|
|
||
|
|
```go
|
||
|
|
// myH is your app's http handler, perhaps a http.ServeMux or similar.
|
||
|
|
var myH http.Handler
|
||
|
|
// wrappedH wraps myH in order to log every request.
|
||
|
|
wrappedH := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
|
||
|
|
m := httpsnoop.CaptureMetrics(myH, w, r)
|
||
|
|
log.Printf(
|
||
|
|
"%s %s (code=%d dt=%s written=%d)",
|
||
|
|
r.Method,
|
||
|
|
r.URL,
|
||
|
|
m.Code,
|
||
|
|
m.Duration,
|
||
|
|
m.Written,
|
||
|
|
)
|
||
|
|
})
|
||
|
|
http.ListenAndServe(":8080", wrappedH)
|
||
|
|
```
|
||
|
|
|
||
|
|
## Why this package exists
|
||
|
|
|
||
|
|
Instrumenting an application's http.Handler is surprisingly difficult.
|
||
|
|
|
||
|
|
However if you google for e.g. "capture ResponseWriter status code" you'll find
|
||
|
|
lots of advise and code examples that suggest it to be a fairly trivial
|
||
|
|
undertaking. Unfortunately everything I've seen so far has a high chance of
|
||
|
|
breaking your application.
|
||
|
|
|
||
|
|
The main problem is that a `http.ResponseWriter` often implements additional
|
||
|
|
interfaces such as `http.Flusher`, `http.CloseNotifier`, `http.Hijacker`, `http.Pusher`, and
|
||
|
|
`io.ReaderFrom`. So the naive approach of just wrapping `http.ResponseWriter`
|
||
|
|
in your own struct that also implements the `http.ResponseWriter` interface
|
||
|
|
will hide the additional interfaces mentioned above. This has a high change of
|
||
|
|
introducing subtle bugs into any non-trivial application.
|
||
|
|
|
||
|
|
Another approach I've seen people take is to return a struct that implements
|
||
|
|
all of the interfaces above. However, that's also problematic, because it's
|
||
|
|
difficult to fake some of these interfaces behaviors when the underlying
|
||
|
|
`http.ResponseWriter` doesn't have an implementation. It's also dangerous,
|
||
|
|
because an application may choose to operate differently, merely because it
|
||
|
|
detects the presence of these additional interfaces.
|
||
|
|
|
||
|
|
This package solves this problem by checking which additional interfaces a
|
||
|
|
`http.ResponseWriter` implements, returning a wrapped version implementing the
|
||
|
|
exact same set of interfaces.
|
||
|
|
|
||
|
|
Additionally this package properly handles edge cases such as `WriteHeader` not
|
||
|
|
being called, or called more than once, as well as concurrent calls to
|
||
|
|
`http.ResponseWriter` methods, and even calls happening after the wrapped
|
||
|
|
`ServeHTTP` has already returned.
|
||
|
|
|
||
|
|
Unfortunately this package is not perfect either. It's possible that it is
|
||
|
|
still missing some interfaces provided by the go core (let me know if you find
|
||
|
|
one), and it won't work for applications adding their own interfaces into the
|
||
|
|
mix. You can however use `httpsnoop.Unwrap(w)` to access the underlying
|
||
|
|
`http.ResponseWriter` and type-assert the result to its other interfaces.
|
||
|
|
|
||
|
|
However, hopefully the explanation above has sufficiently scared you of rolling
|
||
|
|
your own solution to this problem. httpsnoop may still break your application,
|
||
|
|
but at least it tries to avoid it as much as possible.
|
||
|
|
|
||
|
|
Anyway, the real problem here is that smuggling additional interfaces inside
|
||
|
|
`http.ResponseWriter` is a problematic design choice, but it probably goes as
|
||
|
|
deep as the Go language specification itself. But that's okay, I still prefer
|
||
|
|
Go over the alternatives ;).
|
||
|
|
|
||
|
|
## Performance
|
||
|
|
|
||
|
|
```
|
||
|
|
BenchmarkBaseline-8 20000 94912 ns/op
|
||
|
|
BenchmarkCaptureMetrics-8 20000 95461 ns/op
|
||
|
|
```
|
||
|
|
|
||
|
|
As you can see, using `CaptureMetrics` on a vanilla http.Handler introduces an
|
||
|
|
overhead of ~500 ns per http request on my machine. However, the margin of
|
||
|
|
error appears to be larger than that, therefor it should be reasonable to
|
||
|
|
assume that the overhead introduced by `CaptureMetrics` is absolutely
|
||
|
|
negligible.
|
||
|
|
|
||
|
|
## License
|
||
|
|
|
||
|
|
MIT
|