Automatic Instrumentation

The Sentry SvelteKit SDK provides a BrowserTracing integration to add automatic instrumentation for monitoring the performance of browser applications, which is enabled by default. The SDK also automatically enables performance monitoring in your server-side code.

What Our Instrumentation Provides

The BrowserTracing integration creates a new transaction for each page load and navigation event, and creates a child span for every XMLHttpRequest or fetch request that occurs while those transactions are open.

Additionally, the SDK automatically creates transactions and spans for your universal and server-only load functions, as well as for all requests passing through the handle server hook of your application, such as page load or +server route requests.

Enable Instrumentation

To enable tracing, set either a tracesSampleRate or a tracesSampler in your SDK configuration options, as described in Set Up Performance.

Configuration Options

Supported options:

tracePropagationTargets

A list of strings and regular expressions. The JavaScript SDK will attach the sentry-trace and baggage headers to all outgoing XHR/fetch requests whose destination contains a string in the list or matches a regex in the list. If your frontend is making requests to a different domain, you'll need to add it there to propagate the sentry-trace and baggage headers to the backend services, which is required to link transactions together as part of a single trace.

The tracePropagationTargets option matches the entire request URL, not just the domain. Using stricter regex to match certain parts of the URL ensures that requests don't unnecessarily have additional headers attached.

The default value of tracePropagationTargets is ['localhost', /^\//]. This means that by default, tracing headers are only attached to requests that contain localhost in their URL or requests whose URL starts with a '/' (for example GET /api/v1/users).

For example:

• A frontend application is served from example.com.
• A backend service is served from api.example.com.
• During development, the backend service is served from localhost.
• The frontend application makes API calls to the backend.
• Set the tracePropagationTargets option to ["localhost", /^https:\/\/api\.example\.com/].
• Now outgoing XHR/fetch requests to your backend service will get the sentry-trace and baggage headers attached.

Sentry.init({
  // ...
  integrations: [Sentry.browserTracingIntegration()],

  // Set `tracePropagationTargets` to control for which URLs distributed tracing should be enabled
  tracePropagationTargets: ['localhost', /^https:\/\/yourserver\.io\/api/],
});

You will need to configure your web server CORS to allow the sentry-trace and baggage headers. The configuration might look like "Access-Control-Allow-Headers: sentry-trace" and "Access-Control-Allow-Headers: baggage", but it depends on your set up. If you do not allow the two headers, the request might be blocked.

beforeStartSpan

beforeStartSpan is called at the start of every pageload or navigation span, and is passed an object containing data about the span which will be started. With beforeStartSpan you can modify that data or drop the transaction entirely by returning undefined.

Sentry.init({
  // ...
  integrations: [
    Sentry.browserTracingIntegration({
      beforeStartSpan: context => {
        return {
          ...context,
          attributes: {
            ...context.attributes,
            myAwesomeTag: 'hi',
          },
        };
      },
    }),
  ],
});

shouldCreateSpanForRequest

This function can be used to filter out unwanted spans such as XHRs running health checks or something similar. If this function isn't specified, spans will be created for all requests.

Sentry.init({
  // ...
  integrations: [
    Sentry.browserTracingIntegration({
      shouldCreateSpanForRequest: url => {
        // Do not create spans for outgoing requests to a `/health/` endpoint
        return !url.match(/\/health\/?$/);
      },
    }),
  ],
});

idleTimeout

The idle time, measured in ms, to wait until the transaction will be finished, if there are no unfinished spans. The transaction will use the end timestamp of the last finished span as the endtime for the transaction.

The default is 1000.

finalTimeout

The maximum duration of the transaction, measured in ms. If the transaction duration hits the finalTimeout value, it will be finished.

The default is 30000.

heartbeatInterval

The time, measured in ms, one heartbeat takes. If no new spans were started or no open spans finished within three heartbeats, the transaction will be finished. The heartbeat count restarts whenever a new span is created or an open span is finished.

The default is 5000.

instrumentNavigation

This flag enables or disables creation of navigation transaction on history changes.

The default is true.

instrumentPageLoad

This flag enables or disables creation of pageload transaction on first pageload.

The default is true.

markBackgroundSpans

This option flags transactions when tabs are moved to the background with "cancelled". Because browser background tab timing is not suited for precise measurements of operations and can affect your statistics in nondeterministic ways, we recommend that this option be enabled.

The default is true.

enableLongTask

This option determines whether spans for long tasks automatically get created.

The default is true.

enableInp

This option determines whether interactions spans automatically get created when an Interaction to Next Paint (INP) event is detected. Interactions are scored and surfaced in the Web Vitals module.

The default is false.

Sentry.init({
  // ...
  integrations: [
    Sentry.browserTracingIntegration({
      enableInp: true,
    }),
  ],
});

interactionsSampleRate

This option determines the sample rate of INP spans. interactionsSampleRate is applied on top of tracesSampleRate, therefore if your interactionsSampleRate is set to 0.5 and your tracesSampleRate is set to 0.1, the outcome will be 0.05.

The default is 1.0.