hq/03-DESIGN/01-to-be/03-scenario-lifecycle.md

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/0032-a-scenario-is-an-isolated-address-space.md

Scenario lifecycle

The first thing the lab must do, and the only thing it must do before anything else can be written: materialise a mesh, return it to a known state, and destroy it (ADR 0029).

A declaration describes a scenario. This describes what happens to one.

The verbs

Verb	Does
raise	materialise a declaration into a running scenario instance
snapshot	name the current state of the whole scenario
restore	return the whole scenario to a named state
move	change a machine's position while the scenario runs
exec	run something on a machine, and get its output
destroy	tear the instance down

Six verbs, and raise plus destroy are enough to be useful. The rest are what make repetition cheap, and cheap repetition is what turns the bootstrap path into an inner development loop rather than a ceremony.

Raising, in order

The order is not arbitrary — each step needs the one before it to exist:

1. Segments. Isolated links, one per declared segment, belonging to this instance and joined to nothing outside it (ADR 0032).
2. Gateways. Derived, never declared as machines: a gateway is materialised for each distinct gateway: declaration, sitting on both its segment and its parent, carrying the translation, forwarding and mapping-expiry the declaration asked for.
3. Machines. Each on its segments, holding its addresses.
4. Policy. Rules between segments, applied on the gateways that route between them.
5. Placement. Artifacts onto machines.
6. Snapshot, if the declaration named one.

raise must wait for the machines to be usable, not for the call to return. Measured, those are 14 seconds apart for a single machine — and reporting the earlier number would be the mesh's own recurring failure, transport reported as effect.

Raising is convergent, not incremental: raising an instance that already exists brings it to the declared state rather than failing or duplicating. That is the same model the mesh itself uses, and a lab that behaved differently from the thing it tests would be teaching the wrong habit.

A failed raise leaves the wreckage

A step that fails stops the raise (ADR 0008) — and does not tear down.

Tearing down on failure destroys the only evidence of what went wrong, which is precisely backwards: a scenario that failed to raise is more interesting than one that succeeded. The instance stays, marked failed, with the step that failed named.

The caller decides what happens next, and the two callers want different things — the coordinator captures and destroys, a person opens a shell. That is the same one-runner-two-callers split the lab design already makes, applied to failure.

Snapshots are whole-scenario

A snapshot captures every machine and the state of the network between them, as one thing. Restoring returns all of it.

Per-machine snapshots would be cheaper and are wrong. The mesh keeps state that spans nodes — what is assigned where, which grants exist, what has been delivered — so restoring one machine to an earlier moment while its peers move on produces a mesh that has never existed and could not. The faults found there would be artefacts of the lab.

This is what makes fresh and upgrade both cheap and both default: one snapshot of a mesh that has never seen a change, another of a mesh running the previous version, and a restore between runs.

Moving a machine

move changes a machine's position while the scenario runs: to another segment, with different addresses, or to detached.

It is the roaming case, and it is a lifecycle operation rather than a declaration because the interesting part is the transition, not the destination. A mesh that forms correctly with a node at home and correctly with it away may still fail to notice it moved.

Moving does not invalidate a snapshot. A snapshot is a state to return to; a move is a change made after it, and returning undoes it like any other change.

Reaching in

Everything the lab does to a machine goes through the virtualisation layer, never over IP (ADR 0032). exec runs a command on a machine and returns its output.

This has one consequence worth stating plainly: a reachability question is asked from inside. Can this machine reach that one is exec on the first, testing the second — not a probe from the workstation. The workstation is not on the scenario's network and its opinion of reachability would be a different question with a misleadingly similar answer.

Anything a person wants to open in a browser needs a deliberate forward out of the instance. Nothing leaks by default.

The two callers

The lab design already establishes that the runner serves the coordinator and a person, and that anything only one of them can do will drift. Applied here:

	the coordinator	someone working on the mesh
wants	structured results, clean teardown, no prompts	readable output, the scenario left standing, one thing re-run without repeating the rest
on failure	capture, then destroy	leave it, open a shell

Both use the same verbs. The difference is what happens after the verdict, which is a caller's decision rather than a second implementation.

Open

• What a snapshot costs — measured, and the answer is a blocker. On the current host a snapshot is a full copy: 1.6 GB and ten seconds for one small machine, over two minutes when observed again, projecting to roughly a minute and a half for a four-machine reset-and-rerun cycle. The cause is the storage driver, not virtual machines. See research 010.
• Instance naming. A declaration is a kind and instances are many; how they are named decides whether a person can find the one they left standing yesterday.
• Does a scenario snapshot need the machines stopped? Answered by the integration test on its first run: no, but they must be flushed. A snapshot captures disk and not memory, so a write still in the guest's page cache is absent from it — not stale, absent. A file written seconds before a snapshot did not survive the restore. Flushing first buys write-durability; it does not buy application-consistency, and anything mid-transaction is still captured mid-transaction.
• What survives destroy. Logs and captures are the output of a failed run, so destroying the instance must not destroy them.
• Placement before the mesh is self-hosting. place: needs artifacts from somewhere, and before the mesh builds itself that somewhere is outside it — the open question from research 006.