ADR 0107: persistent data is a directory bind, never a named volume

Records the rule the operator gave directly, mid-session, after checking that HAL's own postgres and lavinmq both used a directory bind and the mesh's adoption of them three weeks ago switched to a named volume without a reason recorded anywhere. Already built and rolled out on novox (mesh-catalog PR #54) before this record -- urgent enough to fix first and write down after. Includes the incident: the new host directories needed the container's own UID, which a named volume gets for free and a directory bind does not; mesh-store crash-looped on Permission denied until ownership was matched to what the original volume already had. Closes issue 115. Checks pass.
2026-09-24 16:21:51 +02:00
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+---
+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?