Found by asking what merging this would do to the mesh that is actually running — the
only place the question could have been asked, because the tests were green and both
buses were self-consistent.
Moving the build outcome to the role means a catalogue built from the current manifests
listens for the role's name. The catalogue *already running* listens for the module's,
because that is what it was told when it was installed. The two do not meet, so merging
as it stood would have stopped the live mesh's module graph being updated — silently,
since a binding that matches nothing is not an error.
A rename on a live bus needs the publisher and the subscriber to change together, and a
deployment cannot promise which arrives first. So the old bus announces under both names
and the order stops mattering. The module's own name retires with the bus, in step 5's
list; nothing has ever run on the bus being built, so there is no legacy name there and
this doubling has no counterpart.
ADR 0121 carried through to working code. `Builders` is the asking side and
`BuildMachine` the taking side, each with an implementation per bus, and the builder
binary and the `build` command now go through them.
On the bus being built, one publish does what two did. The old bus answered the asker
through a reply queue and announced to an events exchange, because two audiences meant
two topologies. Here the outcome is the role's own event: the asker matches it by the
id its request carried, the controller records it, the catalogue places it in the graph.
So a build machine publishes once, needs a reply queue for nothing, and needs a grant
over nobody's inbox — which is what ruled out the alternatives.
The outcome carries the module name now. Only the manifest says what was built, and on
the old bus the separate announcement carried it; with one message for three readers it
belongs in the result. A failed build names none, because it produced no module version
and the catalogue would otherwise place something that was never made.
Checked against a real server: the whole round trip; a third party on the role's event
hearing the same outcome the asker did, which is the claim the decision rests on; work
leaving the queue once settled, so no second machine repeats it; work submitted with no
machine holding the role waiting instead of failing, and being done when one arrives;
and work a machine handed back coming round again.
One thing I got wrong twice now and have written down where it bit: binding to a
consumer must name that consumer's own filter subject, not the narrower subject the
caller cares about. The client compares the two and refuses anything that is not equal,
with "subject does not match consumer".
jschoubben