docs(adr): record PostgreSQL 18 sole canonical DB decision as ADR-005

docs/adr/ADR-005-postgresql-18-sole-canonical-db.md

@@ -0,0 +1,131 @@
+# ADR-005: PostgreSQL 18 sole canonical DB
+
+- Status: Accepted
+- Date: 2026-08-31
+- Deciders: platform stream
+- References: ADR index (section 70), Architecture wiki (sections 1, 4, 9),
+  Technology-Stack wiki (sections 5.2, 5.4, 6, 7), Engineering-Standards wiki
+  (sections 24, 25, 58)
+
+## Context
+
+EPPP is a greenfield personal blogging platform owned by a single small
+stream. The first visible release (v0.1) is intentionally small, but the
+codebase ships explicit, versioned boundaries from day one — content types,
+content blocks, page sections, themes, extensions, extension settings,
+navigation, visitor preferences, database migrations, media storage,
+background jobs, events and rendering — and all of these need durable,
+transactional storage.
+
+ADR-001 already records that the platform operates a single canonical
+PostgreSQL database within a modular monolith; this ADR is the focused record
+of the database choice itself. There is no v1 requirement for distributed
+transactions, service discovery, message brokers or independent data stores,
+so the database can be one engine, one instance, one consistency boundary.
+
+The choice is already operationalised in the workspace:
+
+- `compose.yaml` pins `postgres:18.6-bookworm` as the documented runtime
+  target (Technology-Stack §5.2/§5.4, golden tuple §7; E00-S03-T01), with a
+  `pg_isready` health gate and a named `db-data` volume for persistence.
+- `packages/database-postgres` is the single workspace package allowed to
+  import `pg` and Kysely (dependency direction `database-postgres → core
+  ports → Kysely/pg`, Engineering-Standards §24); domain and extension
+  packages never import the driver.
+- The migration runner coordinates on a PostgreSQL advisory lock
+  (`pg_advisory_lock`, E00-S03-T04) and gates app readiness on migration
+  completion (E00-S03-T06).
+- Extensions are the growth mechanism; extension-owned tables and migrations
+  live in independent chains (`eppp_extension_migrations`) sharing the same
+  canonical database (ADR-001, Engineering-Standards §25).
+
+PostgreSQL 18 is Class A in the runtime stack (fixed upstream EOL date,
+Technology-Stack §5.1) with a documented support window to 2030-11-14; the
+LTS strategy mandates always running the current minor, with a major upgrade
+as a separate operator procedure (Technology-Stack §6).
+
+## Decision
+
+EPPP uses **PostgreSQL 18 as the sole canonical database**. One PostgreSQL 18
+instance is the only database engine in the platform; it holds all core data,
+and extension-owned tables live in independent migration chains inside the
+same instance. `pg`/Kysely imports are isolated to `packages/database-postgres`
+— the adapter boundary every database access passes through — and no other
+database engine is introduced at v1.
+The decision text — **PostgreSQL 18 sole canonical DB** — matches the ADR
+index entry (ADR-005, section 70).
+
+## Alternatives
+
+- **SQLite** — rejected: the embedded, file-backed, single-writer model does
+  not fit the long-lived connection-backed server plus optional worker and
+  background jobs, and it offers no advisory-lock coordination for concurrent
+  migration runners; operational tooling for the Compose deployment model is
+  weaker than PostgreSQL's.
+- **MySQL / MariaDB** — rejected: no v1 requirement needs their
+  differentiators; PostgreSQL provides the standards compliance, constraints,
+  JSONB and advisory locks that the migration runner and the versioned
+  contract model rely on, and a second SQL dialect would add cost without
+  benefit.
+- **MongoDB / document store** — rejected: no v1 need for schemaless
+  documents; the platform's explicit versioned contracts (content types,
+  blocks, schemas validated by TypeBox/Ajv) benefit from a relational,
+  constraint-enforcing store.
+- **Polyglot persistence (multiple engines)** — rejected: no v1 requirement
+  justifies the operational and cognitive cost; a single canonical database
+  keeps one transaction and consistency boundary (ADR-001).
+- **Cloud-managed database (e.g. RDS/Aurora)** — rejected for v1: the primary
+  runtime model is Docker Compose with local parity; a managed service can be
+  revisited on evidence if hosting needs change.
+- **Older PostgreSQL major / unpinned minor** — rejected: 18 is the
+  documented golden tuple; pinning the exact 18.6 minor makes the database
+  container reproducible (E00-S03-T01).
+
+## Consequences
+
+- Positive: one transaction and consistency boundary across the whole
+  platform; mature operational tooling (standard backups, `pg_isready` health
+  gates already in `compose.yaml`); advisory locks coordinate concurrent
+  migration runners; constraints and JSONB support the versioned contract
+  model; the adapter boundary keeps engine specifics contained in one package.
+- Negative: a heavier operational footprint than embedded options — a
+  dedicated service with credentials, volume persistence and health/readiness
+  gating; schema changes demand migration discipline (expand/contract, never
+  edit a released migration — Engineering-Standards §25); the single canonical
+  database is a single point of failure (mitigated by volume persistence,
+  health gates and standard backup tooling).
+- Neutral: the engine is encapsulated behind core ports, so moving to a
+  different engine later is a deliberate, evidence-based change rather than a
+  default; extensions share the canonical database with their own migration
+  chains.
+
+## Operational impact
+
+- A dedicated `db` service in `compose.yaml`, pinned to
+  `postgres:18.6-bookworm`, with a `pg_isready` healthcheck that gates app
+  start and a named `db-data` volume that persists across restart/recreate
+  (`docker compose down -v` resets it).
+- Credentials come from the environment (`DATABASE_URL`, Compose
+  `POSTGRES_*` defaults), never embedded in the image (E00-S02-T08).
+- One core migration chain plus extension-owned chains
+  (`eppp_extension_migrations`) run behind the advisory migration lock at
+  startup; readiness is reported only after migrations complete (E00-S03-T06).
+- Backups use standard PostgreSQL tooling against the volume; version policy
+  follows the LTS strategy — always run the current minor, treat a major
+  upgrade as a separate operator procedure (Technology-Stack §6).
+- At v1 there is no replication, multi-instance or sharding; scaling stays
+  vertical and the database remains the single shared store.
+
+## Revisit trigger
+
+- Revisit this ADR when a module or extension needs a genuinely different
+  data model (graph, search index, time-series) and there is evidence that
+  PostgreSQL features are insufficient — per the architecture review gates
+  (ADR index section 68); the adapter boundary keeps such a change contained.
+- Revisit if the platform grows to multiple independent products or teams
+  requiring independent data stores or distributed transactions (shared with
+  the ADR-001 revisit trigger).
+- Revisit if the golden tuple or Technology-Stack changes the database choice
+  or its support window — for example when PostgreSQL 18 EOL (2030-11-14)
+  approaches and a major upgrade becomes a scheduled operator procedure, or a
+  new major becomes the documented runtime target.