2026-08-23 22:40:55 +00:00
4 changed files with 206 additions and 4 deletions
@@ -0,0 +1,71 @@
 ---
 status: accepted
 date: 2026-08-23
 deciders: jochen
 reconstructed: false
 ---
 # 32. A scenario is an isolated address space, and the lab never reaches into it over IP
 ## Context
 A scenario declares literal addresses —
 [the declaration](../03-DESIGN/01-to-be/02-scenario-declaration.md) is full of them, and it has
 to be, because reproducing *published but behind NAT* means saying which address the world sees.
 That raises a question the declaration left open: **two scenarios at once.** Several agents
 working means several scenarios, and the lab design already calls that a requirement. But two
 scenarios built from the same declaration want the same addresses, and there are only three
 documentation ranges in existence.
 ## Considered options
 1. **Allocate addresses from a pool at raise time**, rewriting the declaration's literals.
   Rejected. It makes the addresses in a declaration a fiction, so a scenario reproducing a
   specific topology no longer reproduces it; it breaks the RFC-range validation, since
   allocated addresses would have to come from somewhere real; and the numbers a person reads
   in the file stop being the numbers they will see in a capture.
 2. **One scenario at a time.** Rejected — it is the requirement, not an inconvenience. A gate
   an agent has to queue for is a gate that gets bypassed.
 3. **Give each scenario its own network stack, so the addresses do not collide.** Chosen.
 ## Decision
 **A scenario is a closed address space.** Every segment materialises as its own isolated link,
 belonging to one scenario instance. Two scenarios raised from the same declaration hold the same
 addresses and never meet, because nothing joins their links.
 The declaration therefore keeps its literal addresses, and they mean exactly what they say.
 **The consequence that constrains everything else: the lab never reaches into a scenario over
 IP.** It talks to a machine through the virtualisation layer's own channel — the same way one
 executes a command in a container without the container being routable.
 That is not a preference. If the lab reached machines by address, the workstation running it
 would need a route into each scenario, and two scenarios carrying the same prefix would give it
 two routes to the same destination. Concurrency would be impossible, and it would fail in the
 worst available way: not with an error, but by one scenario's traffic arriving in another.
 ## Consequences
 - Scenarios are concurrent by construction, with no allocation, no bookkeeping and no limit
  beyond the machine's capacity.
 - The three documentation ranges stop being a scarce resource. Every scenario may use all of
  them, because no two scenarios share a link.
 - **The lab cannot use IP to check anything**, which is more of a constraint than it first
  appears: *"can this machine reach that one"* has to be asked **from inside the scenario**, by
  executing on a machine, rather than probed from outside. That is the honest way to ask it
  anyway — reachability from the workstation is not the question.
 - A scenario is a unit that can be paused, snapshotted and destroyed whole, because nothing
  outside holds a reference into it.
 - The lab needs a scenario **instance** identity distinct from the scenario name in the
  declaration: the declaration is a kind, and several instances of one kind may exist.
 - **The workstation is not on the scenario's network, so it is not a node in it.** Anything a
  developer wants to reach — a web interface, a database — needs an explicit, deliberate
  forward out of the scenario, which is a feature rather than a gap: nothing leaks by default.
 ## References
 - [ADR 0031](0031-the-lab-provides-the-underlay.md) — the declaration whose literal addresses
  this preserves.
 - [ADR 0029](0029-the-labs-first-scenario-has-no-pipeline.md) — the lifecycle jobs this shapes.
@@ -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.
@@ -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).
@@ -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