# ADR-004: Deployment topology (per-site instances)

- **Status:** ACCEPTED
- **Date:** 2026-05-08
- **Deciders:** ShopDB maintainers

## Context

shopdb-flask manages shop-floor inventory. If multiple GE Aerospace sites adopt it, the deployment can take one of two shapes:

| Model | How it works |
|---|---|
| **Per-site instances** | Each site runs its own Flask + MySQL + Vue stack. Each site has its own DB, its own users, its own enabled-plugin list, its own deploy. Sites are isolated. |
| **Multi-tenant single instance** | One central Flask + MySQL + Vue stack serves all sites. A `siteid` foreign key on every asset partitions data. Auth distinguishes which site a user belongs to. |

The codebase today is single-tenant per deployment. There is no `siteid` column, no tenant filter, no cross-site auth model. Plugins can be enabled / disabled but only globally for the running instance.

## Decision

**Per-site instances.** Each adopting site runs its own dedicated stack. The framework does not support multi-tenancy.

Each site:

- Owns its database (own credentials, own backup policy, own retention). The
  database charset is part of the contract: it must be **utf8mb4**
  (`utf8mb4_unicode_ci`). The migration chain creates every table utf8mb4, and
  the connection pins `?charset=utf8mb4`. A site that creates the database with
  a different default charset (older MySQL defaults to latin1) gets a schema
  that silently diverges from every other site. See `docs/DEPLOY.md`.
- Picks its own enabled plugins
- Configures its own JWT secret, CORS allowlist, Zabbix integration, Active Directory binding
- Deploys at its own cadence

The framework provides:

- A `Dockerfile` and `docker-compose.yml` template suitable for a single-site deploy
- A `.env.example` listing all required environment variables
- A `docs/DEPLOY.md` walking through a fresh-site install

## Consequences

### Positive

- Simpler code: no tenant filter on every query, no cross-tenant auth, no shared-state partitioning bugs.
- Sites are independent. A schema change at one site does not affect another. A plugin crash at one site does not blast radius to other sites.
- Clear ownership: each site's IT team owns their own stack and data. Compliance and audit boundaries match operational boundaries.
- Aligns with how GE Aerospace sites already operate (independent IT, independent shop floors).

### Negative / cost

- No cross-site reporting out of the box. If GE corporate ever wants a fleet-wide view, it has to be built on top (e.g., a roll-up dashboard that queries each site's API). That layer is out of scope for the framework.
- Each site administers its own stack. Higher operational overhead than a single central instance, but each site already runs its own infrastructure.
- Updates require visiting each site's deploy. Fine for the current adoption model; revisit if dozens of sites adopt.

### Neutral

- No `siteid` column needed. The existence of one DB per site is the partition.

## Alternatives considered

1. **Multi-tenant single instance.** Lower operational overhead at scale, easier cross-site reporting, but adds significant code complexity and risk: every query needs a tenant filter, auth gets complex, schema migrations affect every site at once, and a bug at one site can leak data across sites. Rejected for v1; revisit if and only if more than five sites adopt and operational overhead becomes painful.
2. **Hybrid: per-site DB but central app server.** Adds the operational complexity of multi-tenancy without isolating the failure domain (one app crash = all sites down). Rejected.

## Migration strategy (resolved)

Deploys run two commands: `flask db upgrade` then `flask plugin upgrade-all`
(lean/ADR-014 sites add an optional `flask plugin prune-schema` at initial
provisioning). The core Alembic chain applied by `flask db upgrade` creates the
full core AND bundled-plugin schema through the chain head (this includes
migration `7c04_fold_plugin_schema`). But every bundled plugin still carries its
own Alembic chain per ADR-008: `flask plugin upgrade-all` stamps each plugin's
own chain (the `alembic_version_<plugin>` tables) and applies any plugin-specific
migrations added after the ownership cutover. The earlier Phase 7B state that
folded everything into core with no per-plugin chains was superseded by ADR-008's
per-plugin ownership. A fresh `flask db upgrade` reproduces the live core schema
exactly (verified on a scratch DB).

External (out-of-tree) plugins per ADR-003 ship their own migrations too; the
framework runs the same per-plugin chain mechanism (ADR-008) for both bundled and
external plugins.

## Open questions

- Should the framework provide an optional **read-only fleet roll-up** mode where a "central" instance can pull aggregate metrics from each site's API? Defer. Out of scope for v1.
- Backup strategy per site: framework recommendation, or each site decides? Framework should publish a recommended backup runbook (mysqldump + offsite copy) but not enforce.
- Auth federation: each site has its own user table, or sites can share an LDAP / SSO? Recommend documenting the LDAP config knob in `.env.example` so sites can plug in their own auth without code change.

## References

- `shopdb/config.py` (currently single-tenant, no `siteid`)
- ADR-001 (asset model is per-site, not cross-site)
- ADR-003 (plugin distribution per site)
