---
title: "Sampling"
url: https://develop.sentry.dev/sdk/telemetry/spans/sampling/
---

# Sampling

🚧 This document is work in progress.

This document uses key words such as "MUST", "SHOULD", and "MAY" as defined in [RFC 2119](https://www.ietf.org/rfc/rfc2119.txt) to indicate requirement levels.

Any APIs exposed to the user to sample spans MUST adhere to the following design principles:

* Sampling MUST only happen to a root span
* The APIs are optimized for trace completeness
* The APIs are optimized for conclusive sampling decisions

## [Sample with `tracesSampleRate`](https://develop.sentry.dev/sdk/telemetry/spans/sampling.md#sample-with-tracessamplerate)

The SDK is automatically initialized with a `tracesSampleRate` of `0.0`. When starting a root span, the configured rate is compared against a random number between `0.0` and `1.0` to decide if this root span will be sampled or not.

## [Sample with `tracesSampler`](https://develop.sentry.dev/sdk/telemetry/spans/sampling.md#sample-with-tracessampler)

If the SDK is configured with a `tracesSampler`, the `tracesSampleRate` no longer applies.

The `tracesSampler` callback MUST receive sufficient arguments from users to define their own sampling rules. This MAY include but is not limited to certain attributes from the root span, such as HTTP headers. The return value of the `tracesSampler` is a float between `0.0` and `1.0`.

If no `tracesSampler` is configured, a propagated sampling decision via the traceparent takes precedence over the `tracesSampleRate`. This behavior MAY be disabled by defining a `tracesSampler`.