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88 lines
2.3 KiB
88 lines
2.3 KiB
package middleware
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import (
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"crypto/rand"
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"encoding/base64"
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"fmt"
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"net/http"
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"os"
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"strings"
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"sync/atomic"
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"github.com/zenazn/goji/web"
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)
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// Key to use when setting the request ID.
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const RequestIDKey = "reqID"
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var prefix string
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var reqid uint64
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/*
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A quick note on the statistics here: we're trying to calculate the chance that
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two randomly generated base62 prefixes will collide. We use the formula from
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http://en.wikipedia.org/wiki/Birthday_problem
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P[m, n] \approx 1 - e^{-m^2/2n}
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We ballpark an upper bound for $m$ by imagining (for whatever reason) a server
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that restarts every second over 10 years, for $m = 86400 * 365 * 10 = 315360000$
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For a $k$ character base-62 identifier, we have $n(k) = 62^k$
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Plugging this in, we find $P[m, n(10)] \approx 5.75%$, which is good enough for
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our purposes, and is surely more than anyone would ever need in practice -- a
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process that is rebooted a handful of times a day for a hundred years has less
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than a millionth of a percent chance of generating two colliding IDs.
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*/
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func init() {
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hostname, err := os.Hostname()
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if hostname == "" || err != nil {
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hostname = "localhost"
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}
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var buf [12]byte
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var b64 string
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for len(b64) < 10 {
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rand.Read(buf[:])
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b64 = base64.StdEncoding.EncodeToString(buf[:])
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b64 = strings.NewReplacer("+", "", "/", "").Replace(b64)
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}
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prefix = fmt.Sprintf("%s/%s", hostname, b64[0:10])
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}
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// RequestID is a middleware that injects a request ID into the context of each
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// request. A request ID is a string of the form "host.example.com/random-0001",
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// where "random" is a base62 random string that uniquely identifies this go
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// process, and where the last number is an atomically incremented request
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// counter.
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func RequestID(c *web.C, h http.Handler) http.Handler {
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fn := func(w http.ResponseWriter, r *http.Request) {
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if c.Env == nil {
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c.Env = make(map[interface{}]interface{})
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}
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myid := atomic.AddUint64(&reqid, 1)
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c.Env[RequestIDKey] = fmt.Sprintf("%s-%06d", prefix, myid)
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h.ServeHTTP(w, r)
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}
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return http.HandlerFunc(fn)
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}
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// GetReqID returns a request ID from the given context if one is present.
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// Returns the empty string if a request ID cannot be found.
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func GetReqID(c web.C) string {
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if c.Env == nil {
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return ""
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}
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v, ok := c.Env[RequestIDKey]
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if !ok {
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return ""
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}
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if reqID, ok := v.(string); ok {
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return reqID
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}
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return ""
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}
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