# Sampling

[Sampling](/docs/concepts/sampling/) is a process that restricts the amount of
traces that are generated by a system. The Ruby SDK offers several
[head samplers](/docs/concepts/sampling#head-sampling).

## Default behavior

By default, all spans are sampled, and thus, 100% of traces are sampled. If you
do not need to manage data volume, don't bother setting a sampler.

Specifically, the default sampler is a composite of [ParentBased][] and
[ALWAYS_ON][] that ensures the root span in a trace is always sampled, and that
all child spans respect use their parent's sampling flag to make a sampling
decision. This guarantees that all spans in a trace are sampled by default.

[ParentBased]:
  https://www.rubydoc.info/gems/opentelemetry-sdk/OpenTelemetry/SDK/Trace/Samplers/ParentBased
[ALWAYS_ON]:
  https://www.rubydoc.info/gems/opentelemetry-sdk/OpenTelemetry/SDK/Trace/Samplers

## TraceIDRatioBased Sampler

The most common head sampler to use is the [TraceIdRatioBased][] sampler. It
deterministically samples a percentage of traces that you pass in as a
parameter.

[TraceIdRatioBased]:
  https://www.rubydoc.info/gems/opentelemetry-sdk/OpenTelemetry/SDK/Trace/Samplers/TraceIdRatioBased

### Environment Variables

You can configure a `TraceIdRatioBased` sampler with environment variables:

```shell
export OTEL_TRACES_SAMPLER="traceidratio"
export OTEL_TRACES_SAMPLER_ARG="0.1"
```

This tells the SDK to sample spans such that only 10% of traces get exported.

### Configuration in Code

Although it is possible to configure a `TraceIdRatioBased` sampler in code, it's
not recommended. Doing so requires you to manually set up a Tracer Provider with
all the right configuration options, which is hard to get right compared to just
using `OpenTelemetry::SDK.configure`.