feat(otel): export traces to PostHog in production

src/instrumentation.test.ts

@@ -7,6 +7,7 @@ import { PeriodicExportingMetricReader } from "@opentelemetry/sdk-metrics";
 import { NodeSDK } from "@opentelemetry/sdk-node";
 import { BatchSpanProcessor } from "@opentelemetry/sdk-trace-node";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { POSTHOG_API_KEY, POSTHOG_TRACES_URL } from "./lib/posthog-config.ts";
 
 const mockStart = vi.fn();
 const mockShutdown = vi.fn().mockResolvedValue(undefined);
@@ -166,7 +167,7 @@ describe("instrumentation", () => {
     startInstrumentation({ OTEL_EXPORTER_OTLP_ENDPOINT: "http://localhost:4318" });
 
     expect(NodeSDK).toHaveBeenCalledOnce();
-    const config = vi.mocked(NodeSDK).mock.calls[0]?.[0];
+    const config = vi.mocked(NodeSDK).mock.calls.at(-1)?.[0];
     expect(config?.spanProcessors).toHaveLength(1);
     expect(config?.logRecordProcessors).toHaveLength(1);
     expect(config?.metricReader).toBeDefined();
@@ -190,7 +191,7 @@ describe("instrumentation", () => {
       DEPLOY_ENVIRONMENT: "production",
     });
 
-    const config = vi.mocked(NodeSDK).mock.calls[0]?.[0];
+    const config = vi.mocked(NodeSDK).mock.calls.at(-1)?.[0];
     expect(config?.logRecordProcessors).toHaveLength(1);
     expect(BatchLogRecordProcessor).toHaveBeenCalledWith({
       exporter: expect.any(OTLPLogExporter),
@@ -205,14 +206,74 @@ describe("instrumentation", () => {
     );
   });
 
+  it.each([
+    "prod",
+    "production",
+  ])("adds a PostHog trace exporter in production even without Axiom endpoints (DEPLOY_ENVIRONMENT=%s)", async (deploymentEnvironment) => {
+    const { startInstrumentation } = await import("./instrumentation.ts");
+
+    startInstrumentation({
+      DEPLOY_ENVIRONMENT: deploymentEnvironment,
+    });
+
+    const config = vi.mocked(NodeSDK).mock.calls.at(-1)?.[0];
+    expect(config?.spanProcessors).toHaveLength(1);
+    expect(config?.instrumentations).toEqual([mockAutoInstrumentations]);
+    expect(BatchSpanProcessor).toHaveBeenCalledWith(expect.any(OTLPTraceExporter));
+    expect(OTLPTraceExporter).toHaveBeenCalledWith(
+      expect.objectContaining({
+        url: POSTHOG_TRACES_URL,
+        headers: expect.objectContaining({
+          Authorization: `Bearer ${POSTHOG_API_KEY}`,
+        }),
+      }),
+    );
+  });
+
+  it("keeps Axiom and PostHog trace exporters alongside each other in production with an Axiom endpoint", async () => {
+    const { startInstrumentation } = await import("./instrumentation.ts");
+
+    startInstrumentation({
+      DEPLOY_ENVIRONMENT: "production",
+      OTEL_EXPORTER_OTLP_ENDPOINT: "http://localhost:4318",
+    });
+
+    const config = vi.mocked(NodeSDK).mock.calls.at(-1)?.[0];
+    expect(config?.spanProcessors).toHaveLength(2);
+    expect(BatchSpanProcessor).toHaveBeenCalledTimes(2);
+  });
+
+  it("adds PostHog traces without PostHog logs for production services configured with explicit OTLP endpoints", async () => {
+    const { startInstrumentation } = await import("./instrumentation.ts");
+
+    startInstrumentation({
+      NODE_ENV: "production",
+      OTEL_EXPORTER_OTLP_TRACES_ENDPOINT: "http://collector:4318/v1/traces",
+      OTEL_EXPORTER_OTLP_LOGS_ENDPOINT: "http://collector:4318/v1/logs",
+    });
+
+    const config = vi.mocked(NodeSDK).mock.calls.at(-1)?.[0];
+    expect(config?.spanProcessors).toHaveLength(2);
+    expect(config?.logRecordProcessors).toHaveLength(1);
+    expect(OTLPTraceExporter).toHaveBeenCalledTimes(2);
+    expect(OTLPTraceExporter).toHaveBeenLastCalledWith({
+      url: POSTHOG_TRACES_URL,
+      headers: {
+        Authorization: `Bearer ${POSTHOG_API_KEY}`,
+      },
+    });
+    expect(OTLPLogExporter).toHaveBeenCalledOnce();
+    expect(OTLPLogExporter).toHaveBeenCalledWith();
+  });
+
   it("only configures trace processors and auto instrumentations when only traces endpoint exists", async () => {
     const { startInstrumentation } = await import("./instrumentation.ts");
 
     startInstrumentation({
       OTEL_EXPORTER_OTLP_TRACES_ENDPOINT: "http://localhost:4318/v1/traces",
     });
 
-    const config = vi.mocked(NodeSDK).mock.calls[0]?.[0];
+    const config = vi.mocked(NodeSDK).mock.calls.at(-1)?.[0];
     expect(config?.spanProcessors).toHaveLength(1);
     expect(config?.logRecordProcessors).toHaveLength(0);
     expect(config?.metricReader).toBeUndefined();
@@ -227,7 +288,7 @@ describe("instrumentation", () => {
       OTEL_EXPORTER_OTLP_LOGS_ENDPOINT: "http://localhost:4318/v1/logs",
     });
 
-    const config = vi.mocked(NodeSDK).mock.calls[0]?.[0];
+    const config = vi.mocked(NodeSDK).mock.calls.at(-1)?.[0];
     expect(config?.spanProcessors).toHaveLength(0);
     expect(config?.logRecordProcessors).toHaveLength(1);
     expect(config?.instrumentations).toEqual([]);
@@ -241,7 +302,7 @@ describe("instrumentation", () => {
       OTEL_EXPORTER_OTLP_METRICS_ENDPOINT: "http://localhost:4318/v1/metrics",
     });
 
-    const config = vi.mocked(NodeSDK).mock.calls[0]?.[0];
+    const config = vi.mocked(NodeSDK).mock.calls.at(-1)?.[0];
     expect(config?.spanProcessors).toHaveLength(0);
     expect(config?.logRecordProcessors).toHaveLength(0);
     expect(config?.metricReader).toBeDefined();

src/instrumentation.ts

@@ -6,14 +6,44 @@ import { resourceFromAttributes } from "@opentelemetry/resources";
 import { BatchLogRecordProcessor, type LogRecordProcessor } from "@opentelemetry/sdk-logs";
 import { PeriodicExportingMetricReader } from "@opentelemetry/sdk-metrics";
 import { NodeSDK } from "@opentelemetry/sdk-node";
-import { BatchSpanProcessor } from "@opentelemetry/sdk-trace-node";
+import { BatchSpanProcessor, type SpanProcessor } from "@opentelemetry/sdk-trace-node";
 import { ATTR_SERVICE_NAME } from "@opentelemetry/semantic-conventions";
-import { POSTHOG_API_KEY, POSTHOG_LOGS_URL } from "./lib/posthog-config.ts";
+import { POSTHOG_API_KEY, POSTHOG_LOGS_URL, POSTHOG_TRACES_URL } from "./lib/posthog-config.ts";
 
 function isProductionDeployment(environment: string | undefined): boolean {
   return environment === "prod" || environment === "production";
 }
 
+function createSpanProcessors(env: Record<string, string | undefined>): SpanProcessor[] {
+  const endpoint = env.OTEL_EXPORTER_OTLP_ENDPOINT;
+  const tracesEndpoint = env.OTEL_EXPORTER_OTLP_TRACES_ENDPOINT;
+  const hasAxiomTraceExport = Boolean(endpoint || tracesEndpoint);
+  const hasPostHogTraceExport =
+    isProductionDeployment(env.DEPLOY_ENVIRONMENT) ||
+    (env.NODE_ENV === "production" && hasAxiomTraceExport);
+
+  const processors: SpanProcessor[] = [];
+
+  if (hasAxiomTraceExport) {
+    processors.push(new BatchSpanProcessor(new OTLPTraceExporter()));
+  }
+
+  if (hasPostHogTraceExport) {
+    processors.push(
+      new BatchSpanProcessor(
+        new OTLPTraceExporter({
+          url: POSTHOG_TRACES_URL,
+          headers: {
+            Authorization: `Bearer ${POSTHOG_API_KEY}`,
+          },
+        }),
+      ),
+    );
+  }
+
+  return processors;
+}
+
 function createLogRecordProcessors(env: Record<string, string | undefined>): LogRecordProcessor[] {
   const endpoint = env.OTEL_EXPORTER_OTLP_ENDPOINT;
   const logsEndpoint = env.OTEL_EXPORTER_OTLP_LOGS_ENDPOINT;
@@ -47,17 +77,18 @@ function createLogRecordProcessors(env: Record<string, string | undefined>): Log
 }
 
 /**
- * Starts OpenTelemetry instrumentation when OTLP export env vars are set.
- * Returns the SDK instance for shutdown, or undefined if OTel is disabled.
+ * Starts OpenTelemetry instrumentation when a telemetry export is configured
+ * (OTLP env vars or a production PostHog export). Returns the SDK instance for
+ * shutdown, or undefined if OTel is disabled.
  */
 export function startInstrumentation(
   env: Record<string, string | undefined> = process.env,
 ): NodeSDK | undefined {
   const endpoint = env.OTEL_EXPORTER_OTLP_ENDPOINT;
-  const tracesEndpoint = env.OTEL_EXPORTER_OTLP_TRACES_ENDPOINT;
   const metricsEndpoint = env.OTEL_EXPORTER_OTLP_METRICS_ENDPOINT;
 
-  const hasTraceExport = Boolean(endpoint || tracesEndpoint);
+  const spanProcessors = createSpanProcessors(env);
+  const hasTraceExport = spanProcessors.length > 0;
   const logRecordProcessors = createLogRecordProcessors(env);
   const hasLogExport = logRecordProcessors.length > 0;
   const hasMetricExport = Boolean(endpoint || metricsEndpoint);
@@ -71,7 +102,7 @@ export function startInstrumentation(
     resource: resourceFromAttributes({
       [ATTR_SERVICE_NAME]: serviceName,
     }),
-    spanProcessors: hasTraceExport ? [new BatchSpanProcessor(new OTLPTraceExporter())] : [],
+    spanProcessors,
     logRecordProcessors,
     metricReader: hasMetricExport
       ? new PeriodicExportingMetricReader({

src/lib/posthog-config.ts

@@ -4,3 +4,5 @@ export const POSTHOG_API_KEY = "phc_GsvyihTLSXrWGKYYGz84m44nuT59kYEwEXNnI0JICtg"
 export const POSTHOG_HOST = "https://us.i.posthog.com";
 
 export const POSTHOG_LOGS_URL = `${POSTHOG_HOST}/i/v1/logs`;
+
+export const POSTHOG_TRACES_URL = `${POSTHOG_HOST}/i/v1/traces`;