sampling.md

Sampling

A cloud-scale web application generates a very large number of traces. You can set up one or more sampling strategies in your application to reduce the volume of trace data that it sends to Wavefront.

You set up a sampling strategy by configuring the WavefrontTracer with an instance of the ISampler interface. To do this, you add the ISampler interface to the WavefrontTracer builder.

For example, suppose you want to report approximately 1 out of every 5 traces. The following snippet shows how to configure a WavefrontTracer with a RateSampler:

// Create a WavefrontTracer builder
WavefrontTracer.Builder wfTracerBuilder = ... 

// Create a RateSampler with a rate of 20% and add it to the WavefrontTracer builder
wfTracerBuilder.WithSampler(new RateSampler(0.2));

... // (Optional) Further configure the WavefrontTracer builder

// Build the WavefrontTracer
ITracer tracer = wfTracerBuilder.Build();

Supported Sampling Strategies

The following table lists the supported sampling strategies. You create and configure a sampling strategy by configuring an ISampler implementation:

ISampler	Description
ConstantSampler	Allows either all traces or no traces. Specify true to sample all traces, or false to sample no traces.
DurationSampler	Allows a span if its duration exceeds a specified threshold. Specify the duration threshold as a number of milliseconds.
RateSampler	Allows a specified probabilistic rate of traces to be reported. Specify the rate of allowed traces as a number between 0.0 and 1.0.
CompositeSampler	Delegates the sampling decision to multiple other samplers and allows a span if any delegate decides to allows it. Specify a list of samplers to delegate to.

Note: Regardless of the sampling strategy, the WavefrontTracer:

• Allows all error spans (error=true span tag).
• Allows all spans that have a sampling priority greater than 0 (sampling.priority span tag).
• Includes all spans in the RED metrics that are automatically collected and reported.

Using Multiple Sampling Strategies

You can configure a WavefrontTracer with multiple sampling strategies. To do so, you use a CompositeSampler, which delegates the sampling decision to multiple other samplers and decides to allow a span if any of the delegate samplers decide to allow it.

For instance, suppose you want to report approximately 10% of traces but you also don't want to lose any spans that are over 60 seconds long. The following code snippet shows how to configure a WavefrontTracer with a RateSampler and a DurationSampler:

// Create a WavefrontTracer builder
WavefrontTracer.Builder wfTracerBuilder = ...  

// Create and configure the RateSampler and DurationSampler
ISampler rateSampler = new RateSampler(0.1);
ISampler durationSampler = new DurationSampler(60_000);

// Create and configure the CompositeSampler with a list of samplers
ISampler compositeSampler = new CompositeSampler(new List<ISampler>{ rateSampler, durationSampler });

// Add the CompositeSampler to the WavefrontTracer builder
wfTracerBuilder.WithSampler(compositeSampler);

... // (Optional) Further configure the WavefrontTracer builder

// Build the WavefrontTracer
ITracer tracer = wfTracerBuilder.Build();