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req-id.go
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req-id.go
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package middleware
// Ported from Goji's middleware, source:
// https://github.com/zenazn/goji/tree/master/web/middleware
import (
"context"
"net/http"
"github.com/unanet/go/pkg/log"
)
// A quick note on the statistics here: we're trying to calculate the chance that
// two randomly generated base62 prefixes will collide. We use the formula from
// http://en.wikipedia.org/wiki/Birthday_problem
//
// P[m, n] \approx 1 - e^{-m^2/2n}
//
// We ballpark an upper bound for $m$ by imagining (for whatever reason) a server
// that restarts every second over 10 years, for $m = 86400 * 365 * 10 = 315360000$
//
// For a $k$ character base-62 identifier, we have $n(k) = 62^k$
//
// Plugging this in, we find $P[m, n(10)] \approx 5.75%$, which is good enough for
// our purposes, and is surely more than anyone would ever need in practice -- a
// process that is rebooted a handful of times a day for a hundred years has less
// than a millionth of a percent chance of generating two colliding IDs.
// RequestID is a middleware that injects a request ID into the context of each
// request. A request ID is a string of the form "host.example.com/random-0001",
// where "random" is a base62 random string that uniquely identifies this go
// process, and where the last number is an atomically incremented request
// counter.
func RequestID(next http.Handler) http.Handler {
fn := func(w http.ResponseWriter, r *http.Request) {
ctx := r.Context()
requestID := r.Header.Get(log.RequestIDHeader)
if requestID == "" {
requestID = log.GetNextRequestID()
}
ctx = context.WithValue(ctx, log.RequestIDKey, requestID)
next.ServeHTTP(w, r.WithContext(ctx))
}
return http.HandlerFunc(fn)
}