# ADR 042: Project-owned UI in the shared application

- HTML version: https://robbiepalmer.me/projects/personal-engineering-platform/adrs/042-project-owned-ui-in-the-shared-app
- Project: Personal Engineering Platform (https://robbiepalmer.me/projects/personal-engineering-platform.md)
- Status: Accepted
- Date: 2026-10-21
- Supersedes: personal-engineering-platform:041-project-owned-web-features-in-shared-frontend-hosts
- Initiatives: Semi-autonomous Software Development (https://robbiepalmer.me/initiatives/semi-autonomous-software-development.md)

## Context

[ADR 041](/projects/personal-engineering-platform/adrs/041-project-owned-web-features-in-shared-frontend-hosts)
established separate ownership for products hosted by the shared frontend.
Copier needs one stable subtree for each product while public routes and domains
can change independently. The subtree must remain part of the existing
application until a product needs its own runtime boundary.

## Decision

Call this component kind `project-ui` and place it under
`ui/app/_projects/<project>`. The `app` parent supplies the application context;
the private `_projects` directory groups implementation by stable project
identity without creating a URL segment.

The shared public route remains a thin adapter. For Personal Finance,
`ui/app/assettracker` mounts the public entry points owned by
`ui/app/_projects/personal-finance-app`. Its project profile records
`mount_path: /assettracker`, while its component identity and owned source path
remain independent of that address.

A project UI remains part of the existing `ui` mise and pnpm workspace. Its
standard tasks delegate to `//ui`. The directory marks ownership while build
and deployment boundaries remain unchanged.

## Alternatives

Putting product behavior directly in each public route would avoid an adapter,
but would couple project ownership to a route slug such as `assettracker` and
make a domain or route change a source-ownership migration.

Creating a separate Next.js application for each product would give stronger
runtime isolation. It would also duplicate build and deployment configuration
before the products need independent operation.

## Consequences

Products stay in one Next.js bundle without losing a Copier-owned boundary.
Giving a product its own domain does not require a source move.

The `_projects` convention is specific to the shared Next.js host. A project
that later needs independent dependencies, configuration, release cadence, or
failure isolation must still create a real application boundary and migrate
its component path deliberately.

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