Use Custom Metric Math when Metric.MeasureType is 8 (Custom). You create a CDP model that derives from CustomMetricMath, and place that model under the MetricMath child of a Metric block. MetricsManager stores the XML prototype and clones it into every runtime MetricNode that uses MeasureType 8.

Quick Steps

• Create a new CDP model deriving from CustomMetricMath.
• Implement the IMetricMath API (Initialize, StartWindow, Consume, FinishWindow).
• Add your custom model as the MetricMath child under a Metric with MeasureType set to 8.

IMetricMath API

MetricMathBase Helpers

MetricMathBase provides helper APIs and member variables that you can use directly in CustomMetricMath based models.

CustomMetricMath Extras

• Stores a serialized prototype of your model (GetPrototype) so every MetricNode can instantiate the same math with its own owner and name.
• Instantiate(proto, owner) builds the model from stored XML and returns an IMetricMath instance.

Example: PercentMath (two measures)

The following snippet shows a custom math that takes Measure[0] as a value and Measure[1] as the full-scale (100 %) reference. Enable/Block/Reset apply automatically through ProcessControlEvent and IsEnabled. Threshold is used only for control truth conversion.

class PercentMath : public CustomMetricMath
{
public:
  void Initialize(int measureType,
                  const std::vector<MetricChannelSpec>& channels,
                  const MetricOptions& opts,
                  IMetricPropertyFactory& factory,
                  const MetricCallbacks& callbacks) override
  {
    (void)measureType;
    MetricMathBase::Initialize(measureType, channels, opts, factory, callbacks);
    PropertyCreateFlags flags;
    flags |= CDP::System::Base::e_PropertySaveOnChange;
    flags |= CDP::System::Base::e_PropertyReparent;
    m_percent = MakeTypedProperty<double>("Percent", flags);
    m_threshold = std::isfinite(opts.threshold) ? opts.threshold : 0.5;
  }

  void InitDone() override
  {
    MetricMathBase::InitDone();
    // m_percent has now been restored to last saved value from previous run
    // same is true for any MakeTypedProperty created property that uses e_PropertySaveOnChange flag.
  }

  void StartWindow(uint64_t fromTs, uint64_t toTs) override
  {
    MetricMathBase::StartWindow(fromTs, toTs);
  }

  void Consume(const std::vector<MetricEvent>& events) override
  {
    if (!m_percent || m_measureIds.size() < 2)
      return;
    for (const auto& ev : events)
    {
      bool isMeasure = IsMeasureChannel(ev.channel);
      if (!isMeasure)
      {
        auto ctrl = ProcessControlEvent(ev, m_threshold, 1e-9);
        if (ctrl.resetTriggered)
          *m_percent = 0.0;
        continue;
      }

      if (!IsEnabled())
        continue;

      auto numeric = CDPUtils::TryGetDouble(ev.value);
      if (!numeric)
        continue;
      double value = *numeric;

      RecordHistogramSample(ev.channel, ev.value);

      if (ev.channel == m_measureIds.front())
        m_lastValue = value;
      else if (ev.channel == m_measureIds[1])
        m_lastFull = value;

      if (m_lastFull != 0.0)
      {
        double pct = (m_lastValue / m_lastFull) * 100.0;
        *m_percent = pct;
        if (m_callbacks.flushNow && (m_opts.loggingFlags & MetricLogging::EverySample))
          m_callbacks.flushNow(ev.value.GetTimeStamp());
      }
    }
  }

  void FinishWindow() override
  {
    MetricMathBase::FinishWindow();
  }

private:
  CDPProperty<double>* m_percent{nullptr};
  double m_lastValue{0.0};
  double m_lastFull{0.0};
  double m_threshold{0.5};
};