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Ratelimiter #1211
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Ratelimiter #1211
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,67 @@ | ||
| package httputil | ||
|
|
||
| import ( | ||
| "net/http" | ||
| "sync" | ||
|
|
||
| "golang.org/x/time/rate" | ||
| ) | ||
|
|
||
| // RateLimiter wraps the provided RoundTripper with a limiter allowing 10 | ||
| // requests/second/host. | ||
| // | ||
| // It responds to HTTP 429 responses by automatically decreasing the rate. | ||
| func RateLimiter(next http.RoundTripper) http.RoundTripper { | ||
| return &ratelimiter{ | ||
| rt: next, | ||
| lm: sync.Map{}, | ||
| } | ||
| } | ||
|
|
||
| // Ratelimiter implements the limiting by using a concurrent map and Limiter | ||
| // structs. | ||
| type ratelimiter struct { | ||
| rt http.RoundTripper | ||
| lm sync.Map | ||
| } | ||
|
|
||
| const rateCap = 10 | ||
|
|
||
| // RoundTrip implements http.RoundTripper. | ||
| func (r *ratelimiter) RoundTrip(req *http.Request) (*http.Response, error) { | ||
| key := req.URL.Host | ||
| li, ok := r.lm.Load(key) | ||
| if !ok { | ||
| // Limiter allows "rateCap" per sec, one at a time. | ||
| l := rate.NewLimiter(rate.Limit(rateCap), 1) | ||
| li, _ = r.lm.LoadOrStore(key, l) | ||
| } | ||
| l := li.(*rate.Limiter) | ||
| if err := l.Wait(req.Context()); err != nil { | ||
| return nil, err | ||
| } | ||
| res, err := r.rt.RoundTrip(req) | ||
| // This seems to be the contract that http.Transport implements. | ||
| if err != nil { | ||
| return nil, err | ||
| } | ||
| switch res.StatusCode { | ||
| case http.StatusOK: | ||
| // Try increasing on OK. | ||
| if lim := l.Limit(); lim < rateCap { | ||
| l.SetLimit(lim + 1) | ||
| } | ||
| case http.StatusTooManyRequests: | ||
| // Try to allow some requests, eventually. | ||
| l.SetLimit(detune(l.Limit())) | ||
| } | ||
| return res, nil | ||
| } | ||
|
|
||
| // Detune reduces the rate. | ||
| func detune(in rate.Limit) rate.Limit { | ||
| if in <= 1 { | ||
| return in / 2 | ||
| } | ||
| return in - 1 | ||
| } | ||
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,56 @@ | ||
| package httputil | ||
|
|
||
| import ( | ||
| "net/http" | ||
| "net/http/httptest" | ||
| "sync" | ||
| "testing" | ||
| "time" | ||
| ) | ||
|
|
||
| func TestRate(t *testing.T) { | ||
| const nReq = 20 | ||
|
|
||
| var wg sync.WaitGroup | ||
| wg.Add(nReq) | ||
| begin := make(chan struct{}) | ||
| var last struct { | ||
| sync.Mutex | ||
| t time.Time | ||
| } | ||
| srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) { | ||
| last.Lock() | ||
| last.t = time.Now() | ||
| last.Unlock() | ||
| w.WriteHeader(http.StatusOK) | ||
| })) | ||
| defer srv.Close() | ||
| cl := srv.Client() | ||
| cl.Transport = RateLimiter(cl.Transport) | ||
|
|
||
| for i := 0; i < nReq; i++ { | ||
| go func() { | ||
| defer wg.Done() | ||
| <-begin | ||
| res, err := cl.Get(srv.URL) | ||
| if err != nil { | ||
| t.Error(err) | ||
| return | ||
| } | ||
| res.Body.Close() | ||
| }() | ||
| } | ||
|
|
||
| first := time.Now() | ||
| close(begin) | ||
| wg.Wait() | ||
|
|
||
| t.Logf("begin: %v", first) | ||
| t.Logf("end: %v", last.t) | ||
| rate := nReq / last.t.Sub(first).Seconds() | ||
| t.Logf("rate: %v", rate) | ||
|
|
||
| if rate < (rateCap-1) || rate > (rateCap+1) { | ||
| t.Error("rate outside acceptable bounds") | ||
| } | ||
| } |
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I may be reading this wrong, but when/if the downstream recovers, would the limit be "stuck" at a lower than optimal rate?
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Yes.
I'm not sure what to do to re-ramp the limit. Using
SetLimitAtwould result in the limit getting similarly stuck, by my thinking. Perhaps a second Limiter with token representing an increase to the limit?There was a problem hiding this comment.
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I am not so familiar with the request patterns or how a certain node can become overwhelmed, but would it make sense to have a second case (or an else case) that increases the Limit (retune) when
status == <good status>and then taking theMin(newLimit, lim)to ensure you don't go over the max? (In general, detune should reduce faster than retune increases it).In the long run it seems like using a client to mitigate against server errors would lead to a different loadbalancing strategy to the
RoundTrip, but that seems like a discussion for a different time.