Design Notes on Barn

Why Barn Has No Projects

Thu, 27 Aug 2026 19:00:00 +0800

Note

Renamed 2026-09-29: names now follow the unreleased Barn 0.9.0 candidate. The prior source review was on 2026-09-26: the original publication date is retained; the text below reflects the implementation reviewed on this date. See Status for released versus candidate behavior and dated acceptance evidence.

Barn began with a familiar VM-manager abstraction: a working directory was a project, a hidden marker gave it identity, a registry found projects again, and a host-global lease kept their private networks from colliding. That model can support many independent VM sets. It was also the wrong model for the product Barn was actually becoming.

Why Every Barn Node Has Two NICs

Thu, 27 Aug 2026 20:00:00 +0800

Note

A Pigsty Inventory names machines by stable addresses. PostgreSQL replication, etcd membership, HAProxy backends, VIPs, monitoring targets, and Ansible all assume that 10.10.10.11 continues to mean the same node. A loopback port forward can expose SSH or PostgreSQL to the host, but it cannot provide that network identity to peers.

Declarative Does Not Mean Destructive

Thu, 27 Aug 2026 21:00:00 +0800

Note

“Declarative” is often shortened to “make reality equal the file.” That is a useful slogan until the file is incomplete, the wrong branch is checked out, or one YAML group is temporarily removed. If absence is treated as deletion, an ordinary editing mistake becomes a destructive operation.

A PID Is Not a Virtual Machine

Fri, 28 Aug 2026 10:00:00 +0800

Note

A pidfile answers one question: which integer did a process have when the file was written? It does not prove that the process is still alive, that the PID was not reused, that the executable is QEMU, or that this particular QEMU owns the node an operator wants to stop.

repo.yaml Is Intent; catalog.json Is Evidence

Sat, 29 Aug 2026 19:00:00 +0800

Note

A static image repository sounds like a directory of qcow2 files plus a JSON index. The difficult part is deciding which facts a maintainer may write by hand and which facts must be derived from the bytes being published.