How to manually extract context or span from incoming http request in NodeJS?

Question

I trying to migrate my Node.js application from jaeger-client to @opentelemetry/* packages

In my Node.js application i have a simple http server and i want to create span on each response.

With jaeger-client i did it in the following way:

import { FORMAT_HTTP_HEADERS } from 'opentracing';
import { initTracerFromEnv } from 'jaeger-client';

const tracer = initTracerFromEnv(/* some options here */);

// app is expressjs app
app.get('/home', req => {
  const rootSpan = tracer.startSpan('response', {
    childOf: tracer.extract(FORMAT_HTTP_HEADERS, req.headers),
  });

  // ...make child spans of rootSpan
});

I want to connect my root span of response with spans from other application, that performs request to my Node.js application server. About the same as I would do it with jaeger-client.

How can i make it with OpenTelemetry instead of jeager-client and opentracing? Is it possible to create child spans manually, without auto instrumentations?

Answer 1

You can do the Context Propagation by extracting the parent context from the request headers and creating the child span with it, as follows:

const opentelemetry = require('@opentelemetry/api');
const {SpanKind, ROOT_CONTEXT} = require("@opentelemetry/api");


app.get('/home', req => {
  // Get incoming context from headers
  const remoteCtx = opentelemetry.propagation.extract(ROOT_CONTEXT, req.headers);

  // ...make child spans of remoteSpan

  // Create child span passing parent context 
  const childSpan = tracer.startSpan(
    'childSpan',
    remoteCtx
  );

  // ... Do important stuff

  
  // End the span
  childSpan.end();
});