show the difference of storing a sampling decision on the client for subsequent trace propagations vs. not doing that

sampling.js

// Assumption for this script:
// All backend requests happen without an active client-side span. Meaning, depending on the values stored
// PropagationContext, server side transactions are either sampled with a server-side sample rate or 
// force-sampled by continuing the trace from the client.

const serverSampleRate = 0.05;
const clientSampleRate = 0.2;

const serverTxnsPerPage = 10;

const repetitions = 10_000;

let longTraceSampledTxns = 0;
let shortTracesSampledTxns = 0;

for (let i = 0; i < repetitions; i++) {
  const clientSideSampled = sampleFrontendSpan();
  if (clientSideSampled) {
    longTraceSampledTxns++;
    for (let j = 0; j < serverTxnsPerPage; j++) {
      const backendSampled = sampleBackendSpan(true);
      if (backendSampled) {
        longTraceSampledTxns++;
      }
    }
  }
}

for (let i = 0; i < repetitions; i++) {
  const clientSideSampled = sampleFrontendSpan();
  if (clientSideSampled) {
    shortTracesSampledTxns++;
  }
  for (let j = 0; j < serverTxnsPerPage; j++) {
    const backendSampled = sampleBackendSpan(false);
    if (backendSampled) {
      shortTracesSampledTxns++;
    }
  }
}

function sampleFrontendSpan() {
  return Math.random() < clientSampleRate;
}

function sampleBackendSpan(force) {
  return force || Math.random() < serverSampleRate;
}

console.log('Long traces - txns sampled:', longTraceSampledTxns);
console.log('Short traces - txns sampled:', shortTracesSampledTxns);