Context

Most of the data in the Core API is exposed as Observables. Many of these observables aren't a sequence of events, but instead represent the latest value/state. These value observables are implemented as shared/multicast replay subjects. As such, consumers of these observables are usually interested in the first emitted value (and may subscribe to future values). In contrast, in consuming an event observable consumers typically care about the whole sequence of values or the latest.

const status$ = from(['init', 'setup', 'start', 'stop']); // hypothetical event stream

const currentStatus = await status$.pipe(last()).toPromise();
const currentAdminClient = await core.elasticsearch.adminClient$.pipe(first()).toPromise();

Problem

When consuming Core API's it's not clear whether you're consuming a replay/value stream or an event stream and the only way to be sure is to read through the internal code.

See: #35429 (comment)