Merge pull request 'ADR 0107: persistent data is a directory bind, never a named volume' (#106) from decide/0107-persistent-data-is-a-directory-bind into main

2026-09-24 14:24:17 +00:00
parent a458f751c6 a93743708c
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+---
+topic: building it
+status: accepted
+date: 2026-09-24
+deciders: jochen
+reconstructed: false
+---
+
+# 107. Persistent data is a directory bind, never a named volume
+
+## Context
+
+Measured 2026-09-24, mid-migration: four modules mount a named Docker volume for real state —
+`mesh-store` (every database the mesh holds), `mesh-broker` (its data and TLS material),
+`mesh-registry` (every image), and `searxng`'s cache. Every other module in the catalogue —
+more than forty — mounts a host directory, `/var/lib/<module>/...`.
+
+The predecessor did not make this choice. HAL's own `postgres` bound `./db-data`, and its
+`lavinmq` bound `./data` — directories, both. The mesh's adoption of them
+([`a63ef3d`](https://git.novox.be/novox/mesh-catalog/commit/a63ef3d), "the postgres module
+adopts mesh-store instead of raising its own", 2026-09-16) introduced the named volume; three
+other modules followed the same shape since. Full account of what was found and fixed:
+[issue 115](../04-ISSUES/115-a-named-docker-volume-is-invisible-and-one-flag-from-gone/00-report.md).
+
+## The two guarantees are not the same guarantee
+
+A named volume and a host directory both survive **ordinary** container recreation — a rebuild,
+a `take`, the `docker rm -f` and push this migration already uses routinely. Neither loses data
+to that. That was never the question.
+
+What they do not both survive:
+
+- **`docker rm -fv`, `docker volume rm`, `docker system prune --volumes`** all target a named
+  volume specifically. The first is one character from the command this migration's own rules
+  already call for after every address change. A host directory has no equivalent command that
+  destroys it by accident — removing it is always a deliberate `rm -rf` on a path someone typed.
+- **Visibility.** Every tool this migration has used all night to find and verify data —
+  `ls`, `find`, `grep`, a backup job — reaches a host directory for free. A named volume requires
+  knowing to ask Docker (`docker volume inspect`) before its bytes, at
+  `/var/lib/docker/volumes/<name>/_data`, are reachable at all.
+
+## Decision
+
+**A container mount holding data that must survive is a host directory bind. A named volume is
+permitted only for data that is disposable if lost** — a cache, a scratch space, something the
+module rebuilds on next start without consequence. `searxng`'s `valkey` cache is close to this
+line and was converted anyway, for consistency and because it costs nothing to.
+
+Ownership is the one thing a host directory does not get for free that a named volume does:
+Docker initialises a fresh named volume's ownership to what the container's first process needs;
+a host directory is whatever created it. A directory made for this purpose must be given the
+image's expected UID before the container using it starts — read from the running instance being
+replaced when one exists, rather than guessed.
+
+## Consequences
+
+- The four modules were converted: `distribution`, `lavinmq`, `postgres`, `searxng`. Data copied
+  and verified byte-for-byte before each manifest changed; `mesh-store` stopped cleanly first, so
+  its copy is crash-consistent rather than a live read of a running postgres.
+- **The ownership gap above was not theoretical — it is what happened.** The new directories
+  were created by the operator's tooling running as root; `mesh-store` crash-looped on
+  `mkdir: ... Permission denied` until its directory's ownership was set to match what the
+  original volume already had. Worth a check at `module add` time — nothing catches this today
+  beyond the container failing to start.
+- Old named volumes were not deleted. They remain the rollback path until confidence in the new
+  mounts is established over time, not one clean start.
+
+## References
+
+- [Issue 115](../04-ISSUES/115-a-named-docker-volume-is-invisible-and-one-flag-from-gone/00-report.md)
+- `mesh-catalog` PR #54
@@ -172,6 +172,7 @@ python3 00-META/checks/index.py            fail if stale
 - **0086** — [A secret reaches a process as a file, and an exception is declared](0086-a-secret-reaches-a-process-as-a-file.md)
 - **0096** — [An upstream image is copied between registries, never through a machine's image store](0096-an-upstream-image-is-copied-between-registries.md)
 - **0097** — [A vendor image is a declared build input, and a recipe fetches nothing undeclared](0097-a-vendor-image-is-a-declared-build-input.md)
+- **0107** — [Persistent data is a directory bind, never a named volume](0107-persistent-data-is-a-directory-bind-never-a-named-volume.md)

 ### How it is checked

@@ -0,0 +1,75 @@
+---
+status: resolved
+opened: 2026-09-24
+located-in: [mesh-catalog]
+fixed-by: mesh-catalog PR #54 — distribution, lavinmq, postgres and searxng converted to host directory binds; data copied and verified (mesh-store stopped cleanly first for a crash-consistent copy), old named volumes kept as the rollback path
+amended-design:
+---
+
+# 115 — A named Docker volume is invisible to the operator, and one flag from gone
+
+## What was observed
+
+On the control-node, 2026-09-24, mid-migration, checking every module in the catalogue for how it
+mounts its data. Five container mounts across four modules use a **named Docker volume** rather
+than a host directory:
+
+```
+distribution   mesh-registry  →  mesh-registry-data:/var/lib/registry
+lavinmq        mesh-broker    →  mesh-broker-data:/var/lib/lavinmq  (+ mesh-broker-tls)
+postgres       mesh-store     →  mesh-store-data:/var/lib/postgresql/data
+searxng        valkey         →  searxng-valkey-data:/data
+```
+
+Every other module in the catalogue — more than forty of them — mounts a host directory,
+`/var/lib/<module>/...`, matching what `DATA-CUTOVER.md` and every rehearsed recipe tonight
+assumes. These four are the exception, not a second convention.
+
+`mesh-store` is the one that matters most: it holds every database migrated tonight, including a
+live `keycloak` restore verified minutes before this was written.
+
+## Why it matters beyond this instance
+
+[`03-DESIGN/01-to-be/22-the-work-ahead.md`](../../03-DESIGN/01-to-be/22-the-work-ahead.md) shows
+this was a deliberate choice, not an oversight — *"Data survives on the named volumes"* — but that
+sentence answers a narrower question than the one this issue raises. It says a named volume
+survives **ordinary container recreation** (a rebuild, a `take`, a routine `docker rm -f` and
+push), which is true and which every module already gets from either a named volume or a host
+directory equally.
+
+What it does not address: a named volume is **one flag away from deleted**, in a way a host
+directory structurally cannot be.
+
+- `docker rm -f` alone does not remove a named volume — it persists, unreferenced, until something
+  targets it by name.
+- `docker rm -fv`, `docker volume rm`, and `docker system prune --volumes` all do target it, and
+  the difference from the command this migration already uses routinely (`docker rm -f` — see
+  `HANDOFF.md`'s own "a restart does not re-read anything" rule) is one character.
+- A named volume is invisible to an operator working the way this migration has worked all
+  night: `ls`, `find`, `grep` across `/services/*` and `/var/lib/*`. Finding it requires knowing to
+  ask Docker (`docker volume inspect`), and its actual bytes sit under
+  `/var/lib/docker/volumes/<name>/_data`, a path nothing points at.
+- Nothing external can back it up, snapshot it, or notice it growing without going through
+  Docker's own volume machinery — a host directory is a directory; a filesystem-level backup job
+  already reaches it for free.
+
+`mesh-store` carries the sharpest version of this: every module's database, the mesh's own
+inventory, identity and licence stores — the single foundation piece the rest of the mesh depends
+on — sits somewhere the operator's ordinary tools do not look.
+
+## Decided
+
+**Persistent data is a host directory bind, never a named volume. A named volume may hold only
+data that is disposable if lost.** All four hold real state and all four are in scope — including
+`mesh-registry` and `searxng`'s cache, not only `mesh-store`. Decided 2026-09-24; the record is
+[ADR 0107](../../02-DECISIONS/0107-persistent-data-is-a-directory-bind-never-a-named-volume.md).
+
+## Open questions
+
+- The safe migration path for each, in order of stakes — `mesh-store` live and holding every
+  database migrated tonight, `mesh-broker`, `mesh-registry`, then `searxng`'s cache, which is
+  genuinely disposable and may not need migrating at all if it is rebuilt rather than moved.
+- Should the catalogue refuse a module declaring a named volume for anything but disposable data
+  at `module add`, the way [issue 091](../091-a-module-definition-carries-a-machine-port/00-report.md)
+  asks the same of a hardcoded machine port — so a convention violation is caught at registration
+  rather than found by reading the whole catalogue?