Step 3.4, first half. Every one of these took an *amqp.Channel, so the
transport reached every caller and swapping it meant touching all of them.
The seam turned out to be small — the controller sends exactly two kinds of
message that expect no answer — which is the same measurement that said
this bus could be replaced at all.
Bus is stated in the mesh's words, not a transport's: PublishEvent and
PublishDeclaration. Two implementations, both shipping, because steps 1 to
4 leave every node on AMQP and the NATS one is selected at the rollout.
Both ship is also what makes them comparable: one conformance fixture holds
both to the same envelope, and the NATS one is checked against a real
server reading back from the stream rather than from the code that wrote it.
Still on *amqp.Channel: RequestBuild and Ask, which carry reply-queue
machinery, and the whole consume side — the control loop, enrolment, serve.
`declare` sends a node a signed declaration; `serve` now also consumes reports.
Signed over the exact bytes published, which is what the node verifies. Anything
re-encoding in between would sign one thing and check another, and a difference
in key order alone would have a node refuse a declaration that was genuinely
the mesh's.
Sent to the node's queue directly rather than through the exchange: a
declaration is for one node, and routing by name through a shared exchange
means a binding per node that nothing removes when a node is retired.
Enrolment now issues the node its own broker password, replacing the token's
secret, and tells it the broker address, the fingerprint and the signing key --
so a node can reconnect after a restart without a person and a new token, which
is what makes disconnection ordinary rather than a crisis.
A report is a statement, not a write. What a node says it applied is its own
account of its own machine, kept as a copy for recovery rather than as a source.
jschoubben