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118 lines
3.4 KiB
Go
118 lines
3.4 KiB
Go
// Copyright 2016 Google Inc. All Rights Reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package trace
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import (
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crand "crypto/rand"
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"encoding/binary"
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"fmt"
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"math/rand"
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"sync"
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"time"
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"golang.org/x/time/rate"
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)
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type SamplingPolicy interface {
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// Sample returns a Decision.
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// If Trace is false in the returned Decision, then the Decision should be
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// the zero value.
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Sample(p Parameters) Decision
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}
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// Parameters contains the values passed to a SamplingPolicy's Sample method.
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type Parameters struct {
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HasTraceHeader bool // whether the incoming request has a valid X-Cloud-Trace-Context header.
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}
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// Decision is the value returned by a call to a SamplingPolicy's Sample method.
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type Decision struct {
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Trace bool // Whether to trace the request.
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Sample bool // Whether the trace is included in the random sample.
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Policy string // Name of the sampling policy.
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Weight float64 // Sample weight to be used in statistical calculations.
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}
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type sampler struct {
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fraction float64
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skipped float64
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*rate.Limiter
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*rand.Rand
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sync.Mutex
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}
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func (s *sampler) Sample(p Parameters) Decision {
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s.Lock()
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x := s.Float64()
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d := s.sample(p, time.Now(), x)
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s.Unlock()
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return d
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}
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// sample contains the a deterministic, time-independent logic of Sample.
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func (s *sampler) sample(p Parameters, now time.Time, x float64) (d Decision) {
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d.Sample = x < s.fraction
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d.Trace = p.HasTraceHeader || d.Sample
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if !d.Trace {
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// We have no reason to trace this request.
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return Decision{}
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}
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// We test separately that the rate limit is not tiny before calling AllowN,
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// because of overflow problems in x/time/rate.
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if s.Limit() < 1e-9 || !s.AllowN(now, 1) {
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// Rejected by the rate limit.
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if d.Sample {
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s.skipped++
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}
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return Decision{}
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}
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if d.Sample {
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d.Policy, d.Weight = "default", (1.0+s.skipped)/s.fraction
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s.skipped = 0.0
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}
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return
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}
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// NewLimitedSampler returns a sampling policy that randomly samples a given
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// fraction of requests. It also enforces a limit on the number of traces per
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// second. It tries to trace every request with a trace header, but will not
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// exceed the qps limit to do it.
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func NewLimitedSampler(fraction, maxqps float64) (SamplingPolicy, error) {
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if !(fraction >= 0) {
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return nil, fmt.Errorf("invalid fraction %f", fraction)
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}
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if !(maxqps >= 0) {
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return nil, fmt.Errorf("invalid maxqps %f", maxqps)
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}
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// Set a limit on the number of accumulated "tokens", to limit bursts of
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// traced requests. Use one more than a second's worth of tokens, or 100,
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// whichever is smaller.
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// See https://godoc.org/golang.org/x/time/rate#NewLimiter.
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maxTokens := 100
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if maxqps < 99.0 {
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maxTokens = 1 + int(maxqps)
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}
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var seed int64
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if err := binary.Read(crand.Reader, binary.LittleEndian, &seed); err != nil {
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seed = time.Now().UnixNano()
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}
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s := sampler{
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fraction: fraction,
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Limiter: rate.NewLimiter(rate.Limit(maxqps), maxTokens),
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Rand: rand.New(rand.NewSource(seed)),
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}
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return &s, nil
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}
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