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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
package com.yahoo.vespa.streamingvisitors.tracing;
import java.util.Random;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.atomic.AtomicLong;
import java.util.function.Supplier;
/**
* Simple implementation of OpenCensus algorithm for probabilistic rate limiting as outlined in
* https://github.com/census-instrumentation/opencensus-specs/blob/master/trace/Sampling.md
*/
public class ProbabilisticSampleRate implements SamplingStrategy {
private final MonotonicNanoClock nanoClock;
private final Supplier<Random> randomSupplier;
private final double desiredSamplesPerSec;
private final AtomicLong lastSampledAtNanoTime = new AtomicLong(0);
public ProbabilisticSampleRate(MonotonicNanoClock nanoClock,
Supplier<Random> randomSupplier,
double desiredSamplesPerSec)
{
this.nanoClock = nanoClock;
this.randomSupplier = randomSupplier;
this.desiredSamplesPerSec = desiredSamplesPerSec;
}
public static ProbabilisticSampleRate withSystemDefaults(double desiredSamplesPerSec) {
return new ProbabilisticSampleRate(System::nanoTime, ThreadLocalRandom::current, desiredSamplesPerSec);
}
@Override
public boolean shouldSample() {
// This load might race with the store below, causing multiple threads to get a sample
// since the new timestamp has not been written yet, but it is extremely unlikely and
// the consequences are not severe since this is a probabilistic sampler that does not
// provide hard lower or upper bounds.
long lastSampledAt = lastSampledAtNanoTime.get(); // TODO getPlain? No transitive visibility requirements
long now = nanoClock.nanoTimeNow();
double secsSinceLastSample = (now - lastSampledAt) / 1_000_000_000.0;
// As the time elapsed since last sample increases, so does the probability of a new sample
// being selected.
double sampleProb = Math.min(secsSinceLastSample * desiredSamplesPerSec, 1.0);
if (randomSupplier.get().nextDouble() < sampleProb) {
lastSampledAtNanoTime.set(now); // TODO setPlain? No transitive visibility requirements
return true;
} else {
return false;
}
}
}
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