# NOTIF-ADR-004 — FCM installation id is the primary target

## Status

Accepted.

## Context

Firebase now recommends the installation id (FID) and treats registration-token multicast paths as
legacy. A contact point model built on a single `token` string would encode the older model as the
only one, and a later migration would be a runtime interpretation problem: the same string field
would mean different things for different rows.

## Decision

`MobilePushTarget` is a sealed hierarchy of `FcmInstallationId`, `LegacyFcmRegistrationToken` and
`ApnsDeviceToken`. The kinds are separate types, never a discriminator on one string field, and each
carries its own `ContactPointType` so the uniqueness scope and the encryption associated data differ.

APNs tokens additionally carry their environment, because sandbox and production are separate
namespaces rather than a flag.

## Consequences

Migrating a target kind is a compile-time change with an exhaustive `switch`, not a runtime guess.
The adapter maps each kind to its own wire representation, so a provider changing one path cannot
silently change the other. The cost is one more type than a string field would need.