[SO Tagging] improve the browser-side cache mechanism

[pgayvallet]

At the moment, the SO tagging plugin fetches the tags at a fixed interval (15mins by default)

This was mostly driven by technical constraints, as the places where the feature was integrated in other applications (e.g dashboard) did not support async loading, so we had to keep an up-to-date cache with synchronous access.

https://github.com/elastic/kibana/blob/3730dd0779ed8efd74aee88f57422781ec1ac122/x-pack/plugins/saved_objects_tagging/public/plugin.ts#L71-L75

However, this approach creates a few problems:

• lot of traffic: any session kept open will continue requesting the tags at this fixed interval, even if no user is actively using the session / tab
• errors: it will continue trying to fetch the tags even if the server is down, generating errors in the proxies

Ideally, we would find a way to stop using a cache and load the list on-demand when an application needs it. However, this may be quite challenging, given the places SOT is integrated (e.g dash/vis edition popins)

If this can't be done, then we should at least look at solutions to limit the problems:

• Increasing the default cache_refresh_interval value?
• Detecting when a browser session is active/inactive and only fetch when the session is active?
• Others?

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)

[sebelga]

We don't have on our roadmaps Server-Sent events or any other pushing mechanism right ?

[pgayvallet]

AFAIK, there isn't any active discussions about SSE/WS or other pushing systems right now, at least from Core's side.