Custom Sampler always has no/empty context when combined with ParentBaseSampler

[derN3rd]

My setup

An express api written in typescript. With user authentication middleware and ~200 routes.

Info	Value
node version	20.19.0
TS version	5.5.4
framework	express js
otel versions	2.0.0 (latest for all used packages)
provider	NodeTracerProvider
spanProcessors	BatchSpanProcessor
contextManager	AsyncHooksContextManager
idGenerator	AWSXRayIdGenerator
exporter	OTLPTraceExporter
propagator	AWSXRayPropagator

Hey there,

I'm currently trying to build a custom sampler that will just always sample when a specific Header value is being send or when a parent trace context was passed.

For this I build these two functions (sampler and middleware to fill the context):

type CustomSamplerOptions = {
  sampleRate?: number // 0.05 = 5%
}

export const TRACE_ENABLED_CONTEXT_KEY = createContextKey('traceEnabled')

export class CustomRequestSampler implements Sampler {
  private readonly sampleRate: number

  constructor(options: CustomSamplerOptions) {
    this.sampleRate = options.sampleRate ?? 0.01
  }

  shouldSample(context: Context, traceId: string): SDKSamplingResult {
    const enabledKey = context.getValue(TRACE_ENABLED_CONTEXT_KEY) as boolean

    console.log('sampler tracing context', (context as any)._currentContext)
    if (enabledKey === true) {
      return {
        decision: SamplingDecision.RECORD_AND_SAMPLED,
      }
    }

    const hash = parseInt(traceId.slice(-8), 16) // use last part of traceId for uniformity
    const ratio = hash / 0xffffffff

    if (ratio < this.sampleRate) {
      return {
        decision: SamplingDecision.RECORD_AND_SAMPLED,
      }
    }

    return {
      decision: SamplingDecision.NOT_RECORD,
    }
  }

  toString(): string {
    return `CustomRequestSampler{sampleRate=${this.sampleRate}}`
  }
}

const baseSampler = new CustomRequestSampler({ sampleRate: 0.1 })

// here is the ParentBasedSampler that wraps the custom sampler. When I use the baseSampler directly, it works
export const sampler = new ParentBasedSampler({ root: baseSampler })


export function traceEnabledHeaderMiddleware(
  req: Request,
  res: Response,
  next: NextFunction
): void {
  const traceEnabled = req.get('X-Trace-Enabled')

  const activeCtx = context.active()
  const ctxWithValues = activeCtx.setValue(
    TRACE_ENABLED_CONTEXT_KEY,
    traceEnabled === 'true'
  )

  // try 1
  context.bind(ctxWithValues, req)

  // try 2
  context.with(ctxWithValues, () => {
    next()
  })
}

Does anyone have an idea on how I can configure the outer ParentBasedSampler to not 'remove' the context that is being passed to the CustomRequestSampler?

Thanks in advance

[derN3rd]

I think I found the solution:

Instead of doing this

new ParentBasedSampler({ root: baseSampler })

I have to do this:

new ParentBasedSampler({
  root: baseSampler,
  localParentNotSampled: baseSampler,
  localParentSampled: baseSampler,
})

So all traces with a local parent that are going through are passed to the baseSampler.

The remote spans should not be affected by this, because they should always be sampled based on their parent status