Explain the execution flow of a RUM session being recorded and ingested?

When a page loads (or a mobile app launches), the RUM SDK initializes and starts a new session, generating a unique session ID that will tag every event produced during that session's lifetime.

As the user interacts with the app - navigating pages, clicking, waiting on network requests - the SDK captures each of these as structured RUM events (view, action, resource, error) locally, buffering them rather than sending each one individually as it happens.

If the backend is also instrumented with Datadog APM (or OpenTelemetry ingested by Datadog), the SDK injects trace context into outgoing network requests, allowing the resulting backend trace to be linked back to the specific RUM session and view that triggered it.

The SDK periodically flushes its buffered events to Datadog's RUM intake over HTTPS, batching them to reduce request overhead rather than making a network call per individual event; if Session Replay is enabled, DOM snapshots and mutations are captured and sent through a related but separate replay pipeline.

Once ingested, RUM events are subject to Datadog's own retention/sampling rules for RUM (which can differ from APM's), and a default cross-product retention filter can additionally guarantee that traces for a small percentage of sessions remain indexed on the APM side, so a given session's user-facing view and its backend trace can both actually be inspected together.

sequenceDiagram
  participant SDK as RUM SDK (browser/mobile)
  participant App as Backend service (APM)
  participant DD as Datadog RUM intake
  SDK->>SDK: Init session, generate session ID
  SDK->>SDK: Capture view/action/resource/error events
  SDK->>App: Network request (trace context injected)
  App-->>SDK: Response
  SDK->>DD: Periodic batched flush of buffered events
  DD->>DD: Apply RUM sampling/retention, cross-product APM link

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