Before settling a schema, what a module can actually be. Twenty kinds of thing,
with the hard ones at the end because they are the point.
The ordinary nine are unsurprising: a supervised service, a system package with
configuration, an application a person launches, a command-line tool, a library
that never runs, a one-shot task, a scheduled one, an adapter, and a standalone
application whose only difference is where its source lives.
The eleven that break a naive schema are where the work is. Something that is a
service AND an application — a git forge is consumed as a remote and operated
through a web interface, and neither reading is wrong. Something that provides
and consumes, because provider and consumer are ends of edges rather than kinds
of module. Something the mesh installs that then becomes a node CAPABILITY,
which means a node's provides-list is partly derived from what is installed on
it and not only detected. Something that must be adopted rather than installed.
Something that is a set rather than a thing. Something with exactly one instance
for the whole mesh, where assigning it twice is not redundancy but two meshes.
Something that is not software at all — a firewall policy, a DNS record, pure
desired state, which fits the host's declaration model exactly and an installable
package model not at all. An agent. The host itself, which is not a module and
needs a schema that can say so. And the things the mesh depends on and does not
control, which are why a node can be perfectly configured and still not work.
Nine axes come out of it. Two are covered by nothing anyone has proposed: HOW
MANY INSTANCES a thing may have, and WHETHER TWO CAN COEXIST — `excludes` covers
part of the second and nothing covers the first.
And one question the cases sharpen: is "runs" a property or a kind? The axes say
property — one schema with a field saying how it runs, `never` included. The
alternative is several kinds of module with different schemas, which is the
taxonomy this effort already rejected once for services and applications.
