Sampling

A sampler decides, for each export request, whether the handlers receive it. Use one to drop the telemetry you do not want to store, such as the requests of a noisy service or empty requests.

Write a sampler

A sampler derives from OpenTelemetrySampler, and overrides ShouldSampleLogsAsync, ShouldSampleTracesAsync or ShouldSampleMetricsAsync. The methods you do not override keep every request.

public sealed class KeepOnlyNonEmptyRequestsSampler : OpenTelemetrySampler
{
    public override ValueTask<bool> ShouldSampleLogsAsync(OpenTelemetryHandlerContext context, ExportLogsServiceRequest request, CancellationToken cancellationToken)
        => ValueTask.FromResult(request.ResourceLogs.Count > 0);

    public override ValueTask<bool> ShouldSampleTracesAsync(OpenTelemetryHandlerContext context, ExportTraceServiceRequest request, CancellationToken cancellationToken)
        => ValueTask.FromResult(request.ResourceSpans.Count > 0);

    public override ValueTask<bool> ShouldSampleMetricsAsync(OpenTelemetryHandlerContext context, ExportMetricsServiceRequest request, CancellationToken cancellationToken)
        => ValueTask.FromResult(request.ResourceMetrics.Count > 0);
}

Return true to keep the request, false to drop it. A dropped request is not passed to the handlers, and the client still gets a success response. To tell the client that records were rejected, call context.PartialSuccess.Reject, as a handler does: see Custom handlers.

Register a sampler

Add the sampler to Samplers, in the configure argument of AddOpenTelemetryReceiver or AddInMemoryOpenTelemetryReceiver:

var builder = WebApplication.CreateBuilder();
builder.Services.AddInMemoryOpenTelemetryReceiver(options =>
{
    options.Samplers.Add(new KeepOnlyNonEmptyRequestsSampler());
});

• The samplers run in the order of the list, before the handlers. The first one that returns false drops the request: the next ones are not called.
• The list is read once, when the receiver starts. Samplers added afterwards are ignored.
• A sampler is shared by all the requests, which can run in parallel: its state must be thread-safe.

Sample whole traces

A request sampler only sees the spans of one request. The spans of a trace often arrive in several requests, from several services, so a request sampler cannot decide on a trace as a whole. OpenTelemetryTailSampler buffers the spans of each trace until the trace is complete, then decides. See Tail sampling.