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

Status: ACCEPTED
Date: 2026-05-08
Deciders: cproudlock

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

PROPOSED: 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

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.
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 a single core Alembic chain: flask db upgrade. Bundled plugins do NOT carry their own migration chains - their tables are folded into the core chain (migration 7c04_fold_plugin_schema). This was a deliberate resolution of the Phase 7B footgun where bundled-plugin baselines and the core baseline both created the same tables, so flask plugin upgrade-all would conflict. A fresh flask db upgrade reproduces the live schema exactly (verified on a scratch DB).

External (out-of-tree) plugins per ADR-003 may still ship their own migrations; the framework supports per-plugin chains for them. Only the in-tree bundled plugins are consolidated into core.

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)

Docs

Install and operate

Data import

Plugins

Integrations

Project

ROADMAP

ADRs

Proposals

PROPOSAL-ge-enforce-plugin