Indicators

Metadata about a user is not enough to know what they want. It is enough to suggest something. Metadata that is useful for suggesting an action is an indicator.

• the language the browser asks for
• the timezone the request came from
• the region, from the IP
• the device, or the connection — Save-Data, a 5G hint
• being inside a private network
• DNT, or Sec-GPC

Why these are only indicators

Locale detection is not accurate. A browser may still have the language it shipped with, and a traveller in a foreign country does not speak its language just because their IP says they are there. The detected language is useful for suggesting a switch, and useless for deciding one.

The same holds for time. A detected timezone is a fine reason to ask is this where you are? and a poor reason to reformat every date without asking.

Preferences

A preference is a choice the user made: they set a theme, they picked a currency, they confirmed a timezone. It is stored in a cookie and it is theirs to change.

Flags

A flag is a fact about the request. It comes from a header the client or the edge added, and the user did not choose it. It may be an environmental fact — the timezone the IP resolves to — or a flag in the other sense: a feature flag, which a cookie carries but which the app switches on, not the user.

Why keep them apart

Sometimes a user has no control over an indicator. The timezone the request came from is not the one they would pick; the language their browser sends is not the one they read. The app wants both: the preference to render with, and the flag to offer a change against.

const { locale, flags } = await indicators.read(params)

{flags.lang === 'fr' && locale !== 'fr' && (
  <p>Cette page existe en français. <Link href="/" locale="fr">Y aller</Link></p>
)}

Putting both in the path lets a static page do that. The banner is part of the lang~fr variant, rendered once and cached, and a visitor whose browser asks for French sees it without the page ever reading a header.

What goes where

| | Preference | Flag | | --- | --- | --- | | means | a choice | an observation | | default source | a cookie of the same name | a header of the same name | | who sets it | the user, through usePref | the client, the edge, or the app | | path segment | second | third |

The source is a default, not a rule. A feature flag reads from a cookie, and flags: { beta: { cookie: 'beta', values: ['on'] } } says so. What decides the kind is the meaning, because that is what a page does with it.