Sampling

// reservoir sampling: a uniform sample of k items from a stream of unknown length, in O(k) memory
function reservoir<T>(stream: Iterable<T>, k: number): T[] {
  const out: T[] = []; let i = 0;
  for (const x of stream) { if (i < k) out.push(x); else { const j = Math.floor(Math.random() * (i + 1)); if (j < k) out[j] = x; } i++; }
  return out;
}

# OpenTelemetry Collector: tail sampling keeps every error and slow trace, plus 5% of the rest
processors:
  tail_sampling:
    decision_wait: 10s
    policies:
      - { name: errors, type: status_code, status_code: { status_codes: [ERROR] } }
      - { name: slow, type: latency, latency: { threshold_ms: 500 } }
      - { name: baseline, type: probabilistic, probabilistic: { sampling_percentage: 5 } }