CONCEPT Cited by 1 source

sendBeacon telemetry transport¶

Definition¶

sendBeacon() (navigator.sendBeacon) is a browser API designed for sending small amounts of data to a server asynchronously and reliably, at a low priority, without blocking the page. It was added to the web platform specifically for telemetry and analytics use cases.

Key properties:

Low-priority network queue — doesn't compete with user-initiated requests (fetch, XHR) for bandwidth.
Page-unload safe — continues to transmit even if the user navigates away or closes the tab, which is the main reason telemetry libraries prefer it over fetch.
Fire-and-forget — no response callback; suited to data that doesn't need acknowledgement.
Size-limited — typically 64 KB per beacon (browser- specific).

Why observability SDKs use it¶

Telemetry data is by definition not on the user's critical path — the user doesn't care whether their page load trace arrived at the collector. Making the browser treat telemetry network calls as low-priority means:

Faster page loads (no contention for network slots).
Reliable delivery at page close (normal fetch would be cancelled).
No blocking of synchronous unload handlers.

Zalando's use¶

The browser SDK exports its spans and metrics via sendBeacon() "to push the network requests to be least critical" (Source: sources/2024-07-28-zalando-opentelemetry-for-javascript-observability-at-zalando). This pairs with concepts/cherry-picked-instrumentation-payload and the edge-proxy ingress pattern to keep telemetry off the critical path on both compute (small bundle, async-loaded) and network (low-priority transport).

Limitations¶

Body size cap makes it unsuitable for large payloads; for per-page-load metrics this is fine, but large OTel span batches may need chunking.
Not all browsers fully implement the feature (check compatibility), though coverage is broad now.
No response callback — errors / retries must be handled differently.

Seen in¶

sources/2024-07-28-zalando-opentelemetry-for-javascript-observability-at-zalando — "Additionally, we also pushed the network requests to be least critical by using sendBeacon()".