Span Sampling & Filtering

Any APIs exposed to the user to sample or filter spans must adhere to the following design principles:

• The APIs are optimized for trace completeness
• The APIs are optimized for conclusive sampling decisions

Sample root spans with tracesSampleRate & tracesSampler

The SDK is automatically initialized with a tracesSampleRate of 0. When starting a root span, the configured rate is compared against a random number between 0 and 1 to decide if this root span will be sampled or not.

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 can 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.

Sentry.init({
  tracesSampler: ({ name, attributes, parentSampleRate }) => {
    // Inherit the trace parent's sample rate if there is one. Sampling is deterministic 
    // for one trace, e.g. if the parent was sampled, all children will be sampled at the same rate.
    if (typeof parentSampleRate === "number") {
      return parentSampleRate;
    }
    
    // Else, use a default sample rate (replacing tracesSampleRate).
    return 0.5;
  }
})

The parentSampleRate is a propagated value inside the baggage, using key sentry-sample_rand. The value stems from a truly random number between 0 and 1, generated when a new trace is started. If the SDK does not receive such a number in an incoming trace, a new, truly random number between 0 and 1 is generated.

In the following cases, the SDK must compare sample rates against this parentSampleRate instead of math.random():

• When a tracesSampler is configured, e.g. trace["sentry-sample_rand"] < tracesSampler()

• When the SDK is the head of trace, this applies to sample decisions based on tracesSampleRate, e.g. trace['sentry-sample_rand'] < config.tracesSampleRate

If the sentry-sample_rate (parentSampleRate) is not available for any reason for an inbound trace, but the trace has the sampled flag set to true, the SDK injects parentSampleRate: 1.0 into the tracesSampler.

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

Parent Sampling Origins

If the SDK can parse an org ID from the configured DSN, this value must be propagated as a baggage entry with the key sentry-org. Given a DSN of https://1234@o1.ingest.us.sentry.io/1, the org ID is 1, based on o1.

On incoming traces, the SDK must compare the sentry-org baggage value against its own parsed value from the DSN. Only if both match, the parent sampling decisions applies.

This behavior can be disabled by setting strictTracePropagation: false in the SDK init call.

The SDK must be configurable with an optional org: <org-id> setting that takes precedence over the parsed value from the DSN.

Filter spans with ignoreSpans & integration config

The SDK must implement a mechanism for users to filter out spans. The result must be binary (true/false). The ignoreSpans option accepts a glob pattern or string. The integrations option can perform in similar fashion or make explicit opt-out possible via a bool flag.

If both options are not feasible to be implemented in certain SDKs, other approaches must be explored that have the same outcome.

Sentry.init({
  ignoreSpans: [
		'GET /about',
		'events.signal *',
	],
	integrations: [
		fsIntegration: {
			ignoreSpans: [
				'fs.read',
			],
			readSpans: true,
			writeSpans: false,
		}
	]
})

Sanitize span attributes with beforeSendSpans

This callback must not allow the removal of any spans from the span tree. It receives a deep copy of all spans in the span tree and their attributes.

[
  {
    'name': 'GET /',
    'attributes': [
      'http.request.method': 'GET',
      'http.response.status_code': 200,
    ]
  },
]

Users can mutate any exposed properties to perform sanitation on sensitive data or Pii. The return value beforeSendSpans should be merged with the original span tree prior to emission.