`seat <name> --to <node>/<module>` makes one assignment the holder of a seat in
the same write that removes the previous one. The row is new (migration 0039);
without one, the resolver derives the holder as it always did — the sole eligible
assignment, two refused — so nothing changes for a mesh that never hands a seat
over. With one, the recorded assignment holds and any other whose module could
hold the seat is eligible and silent: not refused, not holding. That is what lets
the next holder run beside the current one until the switch (hq design 26, design
28 task 5.3, ADR 0131).
Why: the controller finds its own bus through a seat, and the day that seat was
left with nobody in it — because two eligible holders could not coexist and the
old one's claim was taken away — the control plane looped for two hours while
every service stayed up. A handover that is never empty in between is the fix,
not a workaround for it.
`CanHold` is the one judgement of whether a module may hold a seat — claims it at
its scope, provides what it delivers, against the store's row — shared by
registration and the handover so they cannot drift apart. The holding belongs to
the assignment and goes when it does, so a seat never points at nothing running.
Tests: the resolver with and without a record, on the same and another machine,
under a former name; the store's row replaced not added, refused for an
unassigned target, removed with its assignment; CanHold's four answers and that
they follow the store. Full suite green against a real NATS and store.
Both branches changed the seat set from the same starting point, so every number
collided and every `mesh-*` name existed twice. The trunk's numbers and names win:
this branch's records became 0129/0130 and its migrations 0037/0038, and the
hardcoded rename map gave way to the trunk's `seat_alias` table — a rename is a
row now (ADR 0122), not a recompile.
Three of my checks were wrong and the merge is what showed it:
A seat with an empty protocol is a marker, not an incomplete declaration. Most
node-scoped seats are markers — which module is this machine's packet filter —
and refusing one refused most of the set, the showcase module included. A
mistyped field name is already refused by the parser, so an empty protocol was
written as one deliberately.
A claim on a seat this manifest does not declare is not the parser's to judge. A
module may hold a seat another module declared; that is the whole reason ADR 0126
has callers name the seat and not its provider. Whether the seat exists is a fact
about the catalogue, so the refusal is at registration, where every declaration
is in view.
And a seat may share a name with the provision it delivers. `git`, the npm
registry and the artifact store still do, because renaming a delivering seat
cascades to every consumer requiring it, with a window where a holder stops
resolving mid-flight. The trunk deferred exactly those three on purpose.
Full suite green against a real NATS and store.
novox/hq ADR 0118: the prefix is the reservation rule, so a module
declaring any mesh-* name is refused and there is no reserved-names list to
drift. Ten seats renamed in the table, the manifests that claim them, the
controller's own shipped manifests, and the tests.
Not the migration 0118 expected: a holding is derived at resolution from
manifests and never stored, so nothing recorded points at an old name. A
kept rename table tells a manifest written against one what it became —
kept rather than retired, because a module lives in its own repository and
may be registered long after the catalogue stopped using it.
**A seat is not the interface it delivers.** The git seat became mesh-git
and the git provision did not; likewise the package registry. A blanket
replace renamed both, and the failure read "the package registry is served
on <nil>", which does not say "you renamed an interface". A test now pins
every seat against what it delivers, and that neither name is also the
other.
Step 2.3 of novox/hq ADR 0116. The refusal is the resolver's existing one;
these pin it for this seat, including that a different bus implementation is
refused for the same reason — the property that lets the bus be replaced.
jschoubben