novox/hq
3
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity

Scenario lifecycle, and how two scenarios coexist

Browse Source 
ADR 0032: a scenario is a closed address space. Every segment materialises
as its own isolated link belonging to one instance, so two scenarios raised
from the same declaration hold the same addresses and never meet. The
declaration keeps its literal addresses and they mean what they say —
allocating from a pool would have made them a fiction, so a scenario
reproducing a specific topology would stop reproducing it.

The constraint that follows shapes everything: the lab never reaches into a
scenario over IP. It talks to machines through the virtualisation layer's
own channel. If it reached them by address, the workstation would need a
route into each scenario, and two carrying the same prefix would give it
two routes to one destination — failing not with an error but by one
scenario's traffic arriving in another.

That also makes reachability an honest question. Can this machine reach
that one is asked from INSIDE, by executing on the first, rather than
probed from a workstation that is not on the network and whose opinion
would be a different question with a misleadingly similar answer.

The lifecycle itself: six verbs, of which raise and destroy are enough to
be useful and the rest are what make repetition cheap. Raising is
convergent rather than incremental, because a lab behaving differently
from the thing it tests teaches the wrong habit.

A failed raise leaves the wreckage standing. Tearing down on failure
destroys the only evidence, which is backwards — a scenario that failed to
raise is more interesting than one that succeeded.

Snapshots are whole-scenario. Per-machine would be cheaper and wrong: the
mesh keeps state spanning nodes, so restoring one machine while its peers
move on produces a mesh that has never existed, and faults found there
would be artefacts of the lab.

Closes the declaration's open question about running several scenarios at
once.
This commit is contained in:
jschoubben
2026-08-23 23:53:00 +02:00
parent e88a6df924
commit a253afe020
4 changed files with 206 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files
03-DESIGN/01-to-be/02-scenario-declaration.md
-4
View File
@@ -634,10 +634,6 @@ is the one real absence, and it is exactly the double-NAT case.
- **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, and the shape above
writes addresses absolutely. Whether a scenario carries literal addresses or a template the
lab allocates from decides whether two can run side by side — and there are only three
documentation ranges to go round.
- **Nested forwarding** — `published:` names one gateway, so a machine behind two cannot be
published through both.
- **An address changing in place**, as a DHCP lease expiring under a machine that has not moved.
03-DESIGN/01-to-be/03-scenario-lifecycle.md
+134
View File
@@ -0,0 +1,134 @@
---
layer: to-be
status: designed
code: [mesh-lab]
updated: 2026-08-23
decisions:
- 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](../../02-DECISIONS/0029-the-labs-first-scenario-has-no-pipeline.md)).
A [declaration](02-scenario-declaration.md) 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](../../02-DECISIONS/0032-a-scenario-is-an-isolated-address-space.md)).
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.
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](../../02-DECISIONS/0008-a-failed-step-fails-the-job.md)) — 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](../../02-DECISIONS/0032-a-scenario-is-an-isolated-address-space.md)). `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.** Whole-scenario snapshots of several machines are the operation the
inner loop repeats most, so their cost sets the loop's speed. If restoring is slow, the loop
is slow, and everything above is theory.
- **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.
- **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](../../01-RESEARCH/006-mesh-from-scratch/code-skeleton.md).
03-DESIGN/01-to-be/README.md
+1
View File
@@ -12,6 +12,7 @@ document is written and this one's status becomes `implemented`.
| [`00-work-breakdown.md`](00-work-breakdown.md) | How the decomposition gets built, in what order, and where a human must look | [ADR 0015](../../02-DECISIONS/0015-mesh-brokers-nodes-host-agents-think.md) |
| [`01-end-to-end-testing.md`](01-end-to-end-testing.md) | The lab: a real mesh a change can be run against before it reaches nodes | [ADR 0016](../../02-DECISIONS/0016-a-lab-node-is-a-virtual-machine.md), [0029](../../02-DECISIONS/0029-the-labs-first-scenario-has-no-pipeline.md) |
| [`02-scenario-declaration.md`](02-scenario-declaration.md) | What a scenario declares — the underlay, and what to place on it | [ADR 0031](../../02-DECISIONS/0031-the-lab-provides-the-underlay.md) |
| [`03-scenario-lifecycle.md`](03-scenario-lifecycle.md) | What happens to a scenario — raise, snapshot, restore, move, destroy | [ADR 0032](../../02-DECISIONS/0032-a-scenario-is-an-isolated-address-space.md) |
## Not yet written