Custom Instrumentation

On this page you will learn how to manually propagate trace information into and out of your JavaScript application.

Distributed tracing will be set up automatically if:

• You've Set Up Tracing, or
• You're using one of the SDKs that include tracing propagation out of the box:
  • @sentry/nextjs
  • @sentry/sveltekit
  • @sentry/remix
  • @sentry/astro

If you are using a different package, and have not enabled tracing, you can manually set up your application for distributed tracing to work.

Enabling Distributed Tracing

To enable distributed tracing for your frontend, add browserTracingIntegration to your Sentry.init() options as described in the Automatic Instrumentation docs.

If you want to use distributed tracing but not tracing, set the tracesSampleRate option to 0.

Sentry.init({
  dsn: "https://examplePublicKey@o0.ingest.sentry.io/0",
  integrations: [Sentry.browserTracingIntegration()],

  // If this is 0, tracing is disabled
  // but distributed tracing is not
  tracesSampleRate: 0,
});

Continuing a Trace Manually

By default, the browserTracingIntegration will automatically continue a trace found in a <meta> tags - see Automatic Trace Propagation for details. If you want to continue a different trace, for example because you cannot propagate the trace through meta tags but through some different mechanism, you can do this as follows:

const client = Sentry.init({
  dsn: "https://examplePublicKey@o0.ingest.sentry.io/0",
  integrations: [
    Sentry.browserTracingIntegration({
      // Disable default pageload instrumentation
      instrumentPageLoad: false,
    }),
  ],
});

// Start the pageload span manually
const sentryTrace = getSentryTraceStringFromSomewhere();
const baggage = getBaggageStringFromSomewhere();

Sentry.startBrowserTracingPageLoadSpan(
  client,
  {
    name: window.location.pathname,
  },
  { sentryTrace, baggage },
);

Enabling Distributed Tracing without browserTracingIntegration

If you don't want to use the browserTracingIntegration integration, you can manually extract and inject tracing data in your application to connect multiple systems. For this, you must:

• Extract and store incoming tracing information from HTML <meta> tags when loading the page.
• Inject tracing information to any outgoing requests.

To learn more about distributed tracing, see our Distributed Tracing docs.

Extract Tracing Information From HTML meta Tags

If you have a server that renders your application's HTML (SSR) and is also running a Sentry SDK, you can connect your backend to your backend via tracing.

To do this, have your server render HTML <meta> tags with the Sentry trace information. In your frontend, extract that tracing information when the page is loading and use it to create new transactions connected to that incoming backend trace.

Some Sentry backend SDKs provide a built-in way to inject these <meta> tags into rendered HTML. For example:

• Python
• Ruby
• PHP

Then, on pageload you can do the following:

import { propagationContextFromHeaders } from "@sentry/utils";
import * as Sentry from "@sentry/browser";

// Read meta tag values
const sentryTrace = document.querySelector(
  "meta[name=sentry-trace]",
)?.content;
const baggage = document.querySelector("meta[name=baggage]")?.content;

// Generate a propagation context from the meta tags
const propagationContext = propagationContextFromHeaders(
  sentryTrace,
  baggage,
);
Sentry.getCurrentScope().setPropagationContext(propagationContext);

Sentry.startSpan(
  {
    name: `Pageload: ${window.location.pathname}`,
    op: "pageload",
  },
  () => {
    // do something
  },
);

In this example, we create a new root span that is attached to the trace specified in the sentry-trace and baggage HTML <meta> tags.

Inject Tracing Information into Outgoing Requests

For distributed tracing to work, the two headers that you extracted and stored in the active root span, sentry-trace and baggage, must be added to outgoing HTTP requests.

Here's an example of how to collect and inject this tracing information to outgoing requests:

const activeSpan = Sentry.getActiveSpan();
const rootSpan = activeSpan ? Sentry.getRootSpan(activeSpan) : undefined;

// Create `sentry-trace` header
const sentryTraceHeader = rootSpan
  ? Sentry.spanToTraceHeader(rootSpan)
  : undefined;

// Create `baggage` header
const sentryBaggageHeader = rootSpan
  ? Sentry.spanToBaggageHeader(rootSpan)
  : undefined;

// Make outgoing request
fetch("https://example.com", {
  method: "GET",
  headers: {
    baggage: sentryBaggageHeader,
    "sentry-trace": sentryTraceHeader,
  },
}).then((response) => {
  // ...
});

In this example, tracing information is propagated to the project running at https://example.com. If this project uses a Sentry SDK, it will extract and save the tracing information for later use.

The two services are now connected with your custom distributed tracing implementation.

Starting a New Trace

Available since SDK version 8.5.0.

In case the SDK's default behavior for the trace duration does not fit your needs, you can manually start a new trace that will no longer be connected to the current (distributed) trace. This means that spans or errors collected by the SDK during this new trace will not be connected to spans or errors collected before or after this new trace.

To start a new trace that remains valid throughout the duration of a callback, use startNewTrace:

myButton.addEventListener("click", async () => {
  Sentry.startNewTrace(() => {
    Sentry.startSpan(
      { op: "ui.interaction.click", name: "fetch click" },
      async () => {
        await fetch("http://example.com");
      },
    );
  });
});

Once the callback ends, the SDK will continue the previous trace (if available).

Verification

If you make outgoing requests from your project to other services, check if the headers sentry-trace and baggage are present in the request. If so, distributed tracing is working.