feat: added new option propagateToReply

README.md

@@ -134,6 +134,8 @@ The plugin accepts the the following configuration properties:
 
   - The ignored routes will still have access to `request.openTelemetry`, but `activeSpan` will be `undefined`.
 
+- **`propagateToReply` : `boolean`** - When `true`, the current span context will be injected into the reply headers using your registered propagators. Defaults to `false`.
+
 ### Request Decorator
 
 This plugin decorates the request with an `openTelemetry` function that returns an object with the following properties:
@@ -164,6 +166,8 @@ This plugin registers the following Fastify hooks:
 
 - `onRoute`: Added only if `wrapRoutes` is enabled.
 
+- `onSend`: Added only if `propagateToReply` is enabled.
+
 ### OpenTelemetry Compatibility
 
 As the [OpenTelemetry API](https://github.com/open-telemetry/opentelemetry-js-api#version-compatibility) uses a

fastify-opentelemetry.d.ts

@@ -35,6 +35,7 @@ declare namespace fastifyOpenTelemetry {
     },
     wrapRoutes?: boolean | string[],
     ignoreRoutes?: string[] | ((path: string, method: string) => boolean),
+    propagateToReply?: boolean,
   }
 
   export const fastifyOpenTelemetry: FastifyPluginCallback<OpenTelemetryPluginOptions>

index.js

@@ -47,7 +47,8 @@ async function openTelemetryPlugin (fastify, opts = {}) {
     wrapRoutes,
     exposeApi = true,
     formatSpanName = defaultFormatSpanName,
-    ignoreRoutes = []
+    ignoreRoutes = [],
+    propagateToReply = false
   } = opts
 
   const shouldIgnoreRoute = typeof ignoreRoutes === 'function'
@@ -146,10 +147,19 @@ async function openTelemetryPlugin (fastify, opts = {}) {
     span.setAttributes(formatSpanAttributes.error(error))
   }
 
+  async function onSend (request, reply, payload) {
+    const { inject } = request.openTelemetry()
+    const propagationHeaders = {}
+    inject(propagationHeaders)
+    reply.headers(propagationHeaders)
+    return payload
+  };
+
   fastify.addHook('onRequest', onRequest)
   if (wrapRoutes) fastify.addHook('onRequest', onRequestWrapRoutes)
   fastify.addHook('onResponse', onResponse)
   fastify.addHook('onError', onError)
+  if (propagateToReply) fastify.addHook('onSend', onSend)
 }
 
 module.exports = fp(openTelemetryPlugin, {

test/fixtures/openTelemetryApi.js

@@ -5,6 +5,17 @@ const {
   propagation,
   trace
 } = require('@opentelemetry/api')
+const {
+  BasicTracerProvider,
+  ConsoleSpanExporter,
+  SimpleSpanProcessor
+} = require('@opentelemetry/sdk-trace-base')
+const { W3CTraceContextPropagator } = require('@opentelemetry/core')
+
+const provider = new BasicTracerProvider()
+const exporter = new ConsoleSpanExporter()
+provider.addSpanProcessor(new SimpleSpanProcessor(exporter))
+provider.register({ propagator: new W3CTraceContextPropagator() })
 
 const { name: moduleName, version: moduleVersion } = require('../../package.json')
 

test/index.test.js

@@ -463,3 +463,29 @@ test('should use request.routerPath if request.routeOptions does not exist', asy
     'should not contain router path when no matching routes found'
   )
 })
+
+test('add propagation headers to reply when propagateToReply is true', async ({ same, teardown }) => {
+  const fastify = require('fastify')()
+
+  await fastify.register(openTelemetryPlugin, { wrapRoutes: true, propagateToReply: true })
+
+  async function testHandler (request, reply) {
+    const { activeSpan } = request.openTelemetry()
+    reply.headers({ one: 'ok' })
+    return { body: activeSpan.spanContext() }
+  }
+
+  fastify.get('/test', testHandler)
+
+  await fastify.ready()
+
+  teardown(() => {
+    resetHistory()
+    fastify.close()
+  })
+
+  const res = await fastify.inject({ ...injectArgs, url: '/test' })
+  const spanContext = res.json().body
+  same(res.statusCode, 200)
+  same(res.headers.traceparent, `00-${spanContext.traceId}-${spanContext.spanId}-0${spanContext.traceFlags}`)
+})

test/types/fastify-opentelemetry.test-d.ts

@@ -46,5 +46,6 @@ expectType(<OpenTelemetryPluginOptions>({
   },
   ignoreRoutes: (path: string, method: string) => method === 'OPTIONS',
   formatSpanName: (request: FastifyRequest) => `${request.method} constant-part`,
-  wrapRoutes: true
+  wrapRoutes: true,
+  propagateToReply: true
 }))