Opentelemetry Instrumentation

OpenTelemetry Instrumentation

OpenTelemetry is the vendor-neutral standard for traces, metrics, and logs. The SDK is split between what gets recorded (Tracer/Meter/Logger), how it's sampled, how it's exported, and what context propagates. Most pain comes from misaligning those four.

When to use

• New service that needs distributed tracing.
• Existing service emits spans, but trace assembly breaks across the network boundary.
• Vendor backend shows "unknown_service" or trace IDs that don't link.
• You're paying too much for traces and need head/tail sampling.
• Async work (worker_threads, queues) drops the parent context.
• ESM-on-Node startup runs auto-instrumentation too late, missing http/express patches.

Core capabilities

Node SDK setup (auto-instrumentation)

// instrumentation.ts — must be loaded BEFORE any instrumented module.
import { NodeSDK } from '@opentelemetry/sdk-node';
import { OTLPTraceExporter } from '@opentelemetry/exporter-trace-otlp-http';
import { OTLPMetricExporter } from '@opentelemetry/exporter-metrics-otlp-http';
import { PeriodicExportingMetricReader } from '@opentelemetry/sdk-metrics';
import { getNodeAutoInstrumentations } from '@opentelemetry/auto-instrumentations-node';
import { resourceFromAttributes } from '@opentelemetry/resources';
import { ATTR_SERVICE_NAME, ATTR_SERVICE_VERSION } from '@opentelemetry/semantic-conventions';

const sdk = new NodeSDK({
  resource: resourceFromAttributes({
    [ATTR_SERVICE_NAME]: 'orders-api',
    [ATTR_SERVICE_VERSION]: process.env.GIT_SHA ?? 'dev',
    'deployment.environment': process.env.NODE_ENV ?? 'dev',
  }),
  traceExporter: new OTLPTraceExporter({ url: process.env.OTEL_EXPORTER_OTLP_ENDPOINT }),
  metricReader: new PeriodicExportingMetricReader({
    exporter: new OTLPMetricExporter(),
    exportIntervalMillis: 60_000,
  }),
  instrumentations: [getNodeAutoInstrumentations({
    // Disable auto-instrumentation for things you'll wrap manually.
    '@opentelemetry/instrumentation-fs': { enabled: false },
  })],
});
sdk.start();

Loading order:

# CJS — preload via -r
node -r ./instrumentation.js src/index.js

# ESM — use --import (Node 20.6+) or the dedicated register file
node --import @opentelemetry/auto-instrumentations-node/register src/index.mjs

Auto-instrumentation patches modules at require/import time. If your app imports http before the SDK starts, it's too late — those requests get no spans.

Span lifecycle, manually

import { trace, context, SpanStatusCode } from '@opentelemetry/api';

const tracer = trace.getTracer('orders');

async function processOrder(order: Order) {
  return tracer.startActiveSpan('order.process', async (span) => {
    try {
      span.setAttribute('order.id', order.id);
      span.setAttribute('order.total_cents', order.totalCents);

      const result = await charge(order);
      span.setAttribute('order.charge_id', result.chargeId);

      span.setStatus({ code: SpanStatusCode.OK });
      return result;
    } catch (err) {
      span.recordException(err);
      span.setStatus({ code: SpanStatusCode.ERROR, message: err.message });
      throw err;
    } finally {
      span.end();
    }
  });
}

startActiveSpan sets the span as the current context for the duration of the callback. Anything started inside (DB calls, HTTP requests via instrumentation) becomes a child.

Resource attributes — the ones that matter

Vendors use these for service grouping, environment filtering, deploy markers:

service.name              orders-api          # required
service.version           1.4.7-abc123        # for deploy markers
service.namespace         payments            # multi-tenant grouping
deployment.environment    production          # filter by env
host.name                 ${HOSTNAME}         # k8s pod name in DD/HC
process.runtime.name      nodejs              # auto-set
telemetry.sdk.language    nodejs              # auto-set

Skipping service.name is the #1 newbie mistake. Backend UIs collapse all your services into "unknown_service".

Sampling

Three layers, head sampling first:

import { ParentBasedSampler, TraceIdRatioBasedSampler } from '@opentelemetry/sdk-trace-base';

const sdk = new NodeSDK({
  sampler: new ParentBasedSampler({
    root: new TraceIdRatioBasedSampler(0.1), // sample 10% of root spans
    // Children inherit the parent's decision (default).
  }),
});

• Head sampling — decision made when the trace starts; cheap; can't see outcomes.
• Tail sampling — decision made by the OTel Collector after seeing the full trace; expensive (Collector buffers); can keep all errors.

Most teams: head-sample heavily (1-10%) but force keep error traces by passing a sampling-decision attribute up.

Exporters

Wrap with BatchSpanProcessor (default in NodeSDK) so spans are batched and sent on a schedule. SimpleSpanProcessor sends synchronously — only for tests.

Set queue limits or risk a memory leak if the collector is down:

new BatchSpanProcessor(exporter, {
  maxQueueSize: 2048,
  maxExportBatchSize: 512,
  scheduledDelayMillis: 5000,
});

Propagating context across async boundaries

Most cases auto-propagate via context.with(). Two cases that don't:

// 1. worker_threads — context doesn't cross.
const ctx = context.active();
worker.postMessage({ traceparent: serializeContext(ctx), payload });
// In the worker: context.with(deserializeContext(msg.traceparent), () => …)

// 2. Detached promises (fire-and-forget).
const detached = context.bind(context.active(), async () => {
  await sendNotification();   // gets the right parent span
});
detached();                   // not awaited

Browser/client traces

import { WebTracerProvider } from '@opentelemetry/sdk-trace-web';
import { ZoneContextManager } from '@opentelemetry/context-zone';
import { registerInstrumentations } from '@opentelemetry/instrumentation';
import { FetchInstrumentation } from '@opentelemetry/instrumentation-fetch';

const provider = new WebTracerProvider();
provider.register({ contextManager: new ZoneContextManager() });

registerInstrumentations({
  instrumentations: [new FetchInstrumentation({
    propagateTraceHeaderCorsUrls: [/^https:\/\/api\.your-domain\.com/],
  })],
});

The CORS allowlist matters: traceparent is a custom header; without it, your backend allowlists, the browser blocks the request preflight.

Metrics — pick the right instrument

import { metrics } from '@opentelemetry/api';
const meter = metrics.getMeter('orders');

const ordersTotal = meter.createCounter('orders.total');
const orderValue  = meter.createHistogram('order.value_cents', { unit: 'cent' });
const queueDepth  = meter.createObservableGauge('queue.depth');

ordersTotal.add(1, { region: 'us-west' });
orderValue.record(order.totalCents, { region: 'us-west' });
queueDepth.addCallback((obs) => obs.observe(getQueueDepth()));

Histograms beat averages — averages hide tail latency. Counters are monotonic; never use them for "current value" (use a Gauge).

Anti-patterns

Span context dropping in worker_threads

Symptom: Worker spans appear as new traces with no parent. Diagnosis: OTel context is per-async-context; worker_threads is a fresh context. Fix: Serialize the W3C traceparent across the message channel and re-establish on the other side using the W3C Trace Context propagator.

Auto-instrumentation imported AFTER target modules

Symptom: http/express requests have no spans even though the SDK is loaded. Diagnosis: Auto-instrumentation patches modules at require time. If the app imported express before SDK init, it's already monkey-patched-or-not for the rest of the process. Fix: Use --require (CJS) or --import (ESM) to load instrumentation BEFORE the entry. Never import './instrumentation.ts' from index.ts.

Missing service.name

Symptom: All traces show unknown_service:node in the vendor UI. Diagnosis: Resource attributes weren't set; default fallback kicked in. Fix: Always set service.name in resourceFromAttributes. Verify with the Console exporter locally before deploying.

Sampling drift between services

Symptom: Traces are missing middle hops; you see only the entry and the database. Diagnosis: Service A samples at 10% with no parent context; Service B receives the trace with sampled=0 but its own sampler ignores parent and samples at 100%. Fix: Use ParentBasedSampler on every service so they honor upstream decisions.

Logging span IDs without joining via OTLP logs

Symptom: Logs say "trace_id=xxx" but the vendor UI doesn't link logs to traces. Diagnosis: Most vendors auto-link only when logs come through the OTLP log pipeline (or via an SDK-native logger). Fix: Switch to @opentelemetry/api-logs or use a logger integration that injects trace context AND ships via OTLP.

Memory leak when collector unreachable

Symptom: Heap grows steadily; eventually OOM. Diagnosis: BatchSpanProcessor buffer has no cap, or the cap is too high, and the collector has been down for hours. Fix: Set maxQueueSize. Drop on overflow is the right behavior — better than crashing your service.

Quality gates

• Every service sets service.name, service.version, deployment.environment.
• SDK init runs before any instrumented module imports (verified by a missing-span test).
• ParentBasedSampler configured on every service.
• Sampling rate documented in the runbook with per-environment values.
• OTLP endpoint health-checked at boot; clear error on failure.
• BatchSpanProcessor.maxQueueSize set to a finite value.
• Error paths call span.recordException and set SpanStatusCode.ERROR.
• Histograms used for latency/value; counters never used for current-state metrics.
• Browser fetch instrumentation has a propagateTraceHeaderCorsUrls allowlist.
• Trace context propagated across async boundaries (worker_threads, message queues).

NOT for

• Datadog APM, New Relic, AppDynamics agents — vendor-native; different setup.
• Structured-logging-only observability — different layer. → structured-logging-design for the schema/redaction side; grafana-dashboard-builder for the visualization side.
• Pre-OTel tracers (Jaeger client, Zipkin Brave) — migration target should be OTel.
• eBPF-based service maps — different layer entirely.