How MeterProvider works between API and SDK

[Deniz]

Hi All,

I have been reading the source code of runtime (tag v7.0.2) System.Diagnostics.DiagnosticSource and opentelemetry dotnet client (tag 1.4.0-rc.1) in order to see how they work together. In the System.Diagnostics.DiagnosticSource while I was looking into the Instrument class I saw the use of the method which simply notifies the listener.

protected void RecordMeasurement(T measurement, ReadOnlySpan<KeyValuePair<string, object?>> tags)
        {
            DiagNode<ListenerSubscription>? current = _subscriptions.First;
            while (current is not null)
            {
                current.Value.Listener.NotifyMeasurement(this, measurement, tags, current.Value.State);
                current = current.Next;
            }
        }

In the listener, all callbacks are no-op which is expected I think (API should be separated from SDK, so no data should be passed)?

        // We initialize all measurement callback with no-op operations so we'll avoid the null checks during the execution;
        private MeasurementCallback<byte>    _byteMeasurementCallback    = (instrument, measurement, tags, state) => { /* no-op */ };
        private MeasurementCallback<short>   _shortMeasurementCallback   = (instrument, measurement, tags, state) => { /* no-op */ };
        private MeasurementCallback<int>     _intMeasurementCallback     = (instrument, measurement, tags, state) => { /* no-op */ };
        private MeasurementCallback<long>    _longMeasurementCallback    = (instrument, measurement, tags, state) => { /* no-op */ };
        private MeasurementCallback<float>   _floatMeasurementCallback   = (instrument, measurement, tags, state) => { /* no-op */ };
        private MeasurementCallback<double>  _doubleMeasurementCallback  = (instrument, measurement, tags, state) => { /* no-op */ };
        private MeasurementCallback<decimal> _decimalMeasurementCallback = (instrument, measurement, tags, state) => { /* no-op */ };

Correct me if I am wrong. Now, the real question here is - actually two questions -

1. There is a AggregationManager class which sets the callbacks in order to update the InstrumentState and again in that AggregationManager class depending on the type of the Instrument, an aggregator is being assigned in order to be called upon new measurement inside the InstrumentState. I wonder who is the consumer of that AggregationManager and how it's being used?

2. In the SDK code, also MeterProviderSdk uses a different aggregator (AggregatorStore) via Metric class. My question here is, how the code in System.Diagnostics.DiagnosticSource interacts with the MeterProvider in the SDK.
   When we include the SDK and configure it, do we replace the logic in the System.Diagnostics.DiagnosticSource with the one in the SDK by updating callbacks in MeterProviderSdk so MeterListener belongs to it will be responsible for collecting the data rather than the one in the API?

In summary, how the data pass between API and SDK parts, which gives us the Metric to be exported?

Thanks,

[cijothomas]

1. AggregationManager is not at all connected/related to OpenTelemetry. Its used by .NET runtime itself for tools like dotnet-counters.

2. OpenTelemetry uses MeterListener to subscribe to Metric updates from the API (DiagnosticSource): https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry/Metrics/MeterProviderSdk.cs#L152
   There is no logic to be replaced - DS gives call backs. OTel uses it to get raw measurements, and aggregates them following OTel spec.

[Deniz]

Thanks @cijothomas, so all aggregation happens in the SDK part? If AggregationManager is not connected to OTel, collecting data from Instrument defined in DiagnosticSource & aggregation happens in the dotnet-client (SDK) by using MeterListener and using AggregatorStore? You mean InstrumentState and other things I mentioned from DiagnosticSource is irrelevant to OTel (SDK)?

AggregationManager confused me, sorry if question is not clear.

[cijothomas]

All aggregation is done inside OTel SDK.