@grafana/faro-web-tracing

This package provides tools for integrating OpenTelemetry based tracing with the Faro for the web.

See quick start document for instructions how to set up and use.

Multiple Faro instances on one page (micro-frontends)

OpenTelemetry allows only a single global tracer provider, propagator and context manager per browsing context, and the fetch/XHR instrumentations patch the global fetch/XMLHttpRequest only once. Because of this, when several Faro instances run on the same page (for example in a micro-frontend architecture), only the first instance to initialize tracing owns it: it registers the tracer provider and instruments fetch/XHR. Any Faro instance that adds a TracingInstrumentation afterwards detects the existing owner, logs a warning, and does not register a second provider or re-patch fetch/XHR (this also avoids OpenTelemetry "duplicate registration" errors).

As a result, all spans are exported by the owning instance — using its exporter, its resource (service.name), and its collector — no matter which instance created them, and only the owner produces automatic fetch/XHR spans. Later instances register no exporter of their own, so they never export spans directly; they can still create spans against the shared provider via faro.api.getOTEL() (for example, manual spans), but those spans are exported by the owning instance and attributed to it. This applies regardless of the isolate config option, since tracer-provider registration and fetch/XHR patching are inherently global.

Cross-origin trace-header propagation is preserved across instances: every instance's propagateTraceHeaderCorsUrls is merged into the owner, so the owning instance still injects W3C trace headers for URLs configured by any instance. Per-instance custom span attributes (applyCustomAttributesOnSpan) are not merged — only the owner's are applied.