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jschoubben a72fea5342 ADR 0031 and the scenario declaration

The lab provides the underlay; the mesh builds the overlay. This is the
boundary that decides whether the lab is worth having: a scenario that
assigns overlay addresses, elects the hub and writes peer configuration
certifies its own work — if the mesh's peering is broken, that scenario
still comes up green. The most valuable thing the lab can test is exactly
the part pre-building would replace.

So a scenario declares what a hosting provider and a home router would
provide: segments, which machine sits where at which address, what NAT is
between them, which ports are forwarded, which machines are detached. It
declares nothing about overlay addresses, hubs, peering, names or
certificates, all of which become outcomes to observe.

The declaration has four parts — segments, machines, place, snapshot — and
the two scenario classes differ only in place. That is what makes one a
strict subset of the other rather than a fork.

Research 004's most important finding becomes a format constraint rather
than a footnote: the routable segment must use RFC 5737 documentation
space, because the mesh decides public versus private by matching the
address, and a private range there makes the hub test as unreachable while
the mesh silently never forms. A segment without behind: is routable, and a
non-documentation address in it should be refused before anything is
raised — ADR 0008 applied to a configuration file, since the failure it
prevents has no error at all.

Four things left open, including the one that matters most: a lab machine
is always privileged, so the user and edge profiles have no scenario that
exercises them.

2026-08-23 22:33:52 +02:00

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layer, status, code, updated, decisions

layer

status

code

updated

decisions

to-be

designed

mesh-lab

2026-08-23

02-DECISIONS/0029-the-labs-first-scenario-has-no-pipeline.md

02-DECISIONS/0031-the-lab-provides-the-underlay.md

02-DECISIONS/0016-a-lab-node-is-a-virtual-machine.md

The scenario declaration

A scenario is a declaration of an underlay, plus what to put on it. It is the interface everything in the lab hangs off, so it is worth getting small.

It states what a hosting provider and a home router would provide, and nothing the mesh is responsible for (ADR 0031).

The shape

scenario: published-behind-nat

segments:
  wan:
    cidr: 203.0.113.0/24        # RFC 5737 — never routes on the real internet
  lan:
    cidr: 192.168.1.0/24
    behind: wan                 # NAT; the lab materialises a router

machines:
  anchor:
    segment: wan
    address: 203.0.113.10
  home-server:
    segment: lan
    address: 192.168.1.135
    forwarded: [443]            # reachable from wan through the router
  workstation:
    segment: lan
    address: 192.168.1.250
  laptop:
    segment: detached           # reachable by nothing until attached

place:
  all: [host]
  anchor: [substrate]

snapshot: raised

That is a complete bootstrap scenario. Nothing in it mentions the overlay, a hub, peering, names or certificates — all of which are outcomes to be observed.

The four parts

segments — the networks that exist. behind: declares NAT, and is the only place a router comes from: the lab materialises one without being asked, because NAT has to run somewhere. This is the one implicit machine in an otherwise explicit declaration.

machines — what sits where. A machine has a segment and an address, and that is nearly all. forwarded: opens a port through the router, which is what makes published but behind NAT reproducible — the case that exists only in production today. segment: detached is a machine on no network, which is how a roaming node is expressed at rest.

place — what goes inside. all: applies to every machine; a machine name overrides for that machine. This is the only part that differs between the two scenario classes.

snapshot — names the state once placement finishes, so a run can return to it without raising everything again. Snapshots are what make repetition cheap, and cheap repetition is what makes the bootstrap path the inner development loop rather than a ceremony.

Why the addresses are load-bearing

The routable segment uses RFC 5737 documentation space, and this is not a stylistic choice.

The mesh decides public versus private by matching the address. A private range on the segment meant to be routable makes the hub test as unreachable, and the mesh silently never forms — no error, no failed step, just a mesh that does not exist. Research 004 calls this the single most important fact in its analysis.

So the format should make this hard to get wrong rather than merely documented: a segment without behind: is a routable segment, and an address in it that is not documentation space is a declaration error, refused before anything is raised. That is ADR 0008 applied to a configuration file — the failure it prevents is silent, so the check has to be loud.

The same declaration serves both classes

The bootstrap and full scenarios differ only in place: (ADR 0029). Everything about the underlay is identical, which is what makes one a strict subset of the other rather than a fork.

# bootstrap — tiers 0 and 1
place:
  all: [host]
  anchor: [substrate]

# full — adds a control plane, a forge, and a module under test
place:
  all: [host]
  anchor: [substrate, control, forge]
module: a-web-service
assert:
  - the service answers on its published name
  - the certificate presented is valid for that name

module: and assert: are meaningless in a bootstrap scenario and absent from one. A bootstrap scenario's verdict comes from what the host reports about the state it reconciled, not from an assertion runner — which is why assertion execution is second in the build order, not first.

What a scenario deliberately cannot say

Overlay addresses, the hub, peer configuration. Outcomes, not inputs (ADR 0031).
What a machine is in mesh terms — server or workstation, its site, its names. Mesh configuration, established by the mesh.
A host's capability profile. Detected, never declared.
Steps. A scenario is a desired state. Anything expressed as an ordered list of actions belongs in the lifecycle, not the declaration.

Open

user and edge profiles have no scenario. A lab machine is always privileged, so the two profiles that exist for unprivileged and phone-like participation cannot currently be exercised. Either the lab grows a way to run the host unprivileged, or those profiles are developed against something that is not a virtual machine.
Attaching and detaching during a run. segment: detached covers a machine at rest; moving one between segments while a scenario is live is what makes a roaming node interesting, and that is lifecycle rather than declaration.
Where place: gets its artifacts from. Before the mesh is self-hosting these come from outside; afterwards from the mesh itself. The declaration should not have to care, which suggests a named source rather than a path.
Multiple scenarios at once. Each needs its own segments and addresses. Whether the declaration carries absolute addresses, as above, or a template the lab allocates from, decides whether two scenarios can run side by side.

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