1package middleware 2 3// Ported from Goji's middleware, source: 4// https://github.com/zenazn/goji/tree/master/web/middleware 5 6import ( 7 "context" 8 "crypto/rand" 9 "encoding/base64" 10 "fmt" 11 "net/http" 12 "os" 13 "strings" 14 "sync/atomic" 15) 16 17// Key to use when setting the request ID. 18type ctxKeyRequestID int 19 20// RequestIDKey is the key that holds the unique request ID in a request context. 21const RequestIDKey ctxKeyRequestID = 0 22 23// RequestIDHeader is the name of the HTTP Header which contains the request id. 24// Exported so that it can be changed by developers 25var RequestIDHeader = "X-Request-Id" 26 27var prefix string 28var reqid uint64 29 30// A quick note on the statistics here: we're trying to calculate the chance that 31// two randomly generated base62 prefixes will collide. We use the formula from 32// http://en.wikipedia.org/wiki/Birthday_problem 33// 34// P[m, n] \approx 1 - e^{-m^2/2n} 35// 36// We ballpark an upper bound for $m$ by imagining (for whatever reason) a server 37// that restarts every second over 10 years, for $m = 86400 * 365 * 10 = 315360000$ 38// 39// For a $k$ character base-62 identifier, we have $n(k) = 62^k$ 40// 41// Plugging this in, we find $P[m, n(10)] \approx 5.75%$, which is good enough for 42// our purposes, and is surely more than anyone would ever need in practice -- a 43// process that is rebooted a handful of times a day for a hundred years has less 44// than a millionth of a percent chance of generating two colliding IDs. 45 46func init() { 47 hostname, err := os.Hostname() 48 if hostname == "" || err != nil { 49 hostname = "localhost" 50 } 51 var buf [12]byte 52 var b64 string 53 for len(b64) < 10 { 54 rand.Read(buf[:]) 55 b64 = base64.StdEncoding.EncodeToString(buf[:]) 56 b64 = strings.NewReplacer("+", "", "/", "").Replace(b64) 57 } 58 59 prefix = fmt.Sprintf("%s/%s", hostname, b64[0:10]) 60} 61 62// RequestID is a middleware that injects a request ID into the context of each 63// request. A request ID is a string of the form "host.example.com/random-0001", 64// where "random" is a base62 random string that uniquely identifies this go 65// process, and where the last number is an atomically incremented request 66// counter. 67func RequestID(next http.Handler) http.Handler { 68 fn := func(w http.ResponseWriter, r *http.Request) { 69 ctx := r.Context() 70 requestID := r.Header.Get(RequestIDHeader) 71 if requestID == "" { 72 myid := atomic.AddUint64(&reqid, 1) 73 requestID = fmt.Sprintf("%s-%06d", prefix, myid) 74 } 75 ctx = context.WithValue(ctx, RequestIDKey, requestID) 76 next.ServeHTTP(w, r.WithContext(ctx)) 77 } 78 return http.HandlerFunc(fn) 79} 80 81// GetReqID returns a request ID from the given context if one is present. 82// Returns the empty string if a request ID cannot be found. 83func GetReqID(ctx context.Context) string { 84 if ctx == nil { 85 return "" 86 } 87 if reqID, ok := ctx.Value(RequestIDKey).(string); ok { 88 return reqID 89 } 90 return "" 91} 92 93// NextRequestID generates the next request ID in the sequence. 94func NextRequestID() uint64 { 95 return atomic.AddUint64(&reqid, 1) 96} 97