hq/02-DECISIONS/0009-modules-and-the-graph.md

topic, status, date, deciders, reconstructed

topic	status	date	deciders	reconstructed
what runs on it	accepted	2026-08-28	jochen	false

9. Modules and the graph

Consolidated 2026-08-28 from six records.

Everything is a module

One kind of thing, one manifest describing all of them. A database, a web application, a window manager and a firewall rule set are all modules — not because they are alike, but because anything else means a second kind of thing with its own rules, and then a third.

A module is the unit of delivery: assignable to a node, versionable, replaceable on its own.

There are no domain modules

An earlier decision grouped modules by domain — four things constituting how a node is reachable becoming one networking module. That was wrong, and the correction is worth keeping because the observation behind it was right.

The measurement holds: reachability is the only place in the catalogue where modules genuinely change together under one intent. What did not hold is the conclusion. Tight coupling means they share an authority — one place that decides for all of them — and not that they should be one artifact. wireguard and the proxy are deployed to different sets of nodes, so a module containing both would be assigned where half of it is unwanted.

Coherence is a context. Delivery is a module.

Folders assert relationships; edges record them. What grouping was for — finding things, seeing what belongs together — is a tag and a query, neither of which anybody has to keep true by hand.

Three edges

edge	means	declared?	satisfied
presence	that thing must exist and be reachable here	yes	at provisioning
instantiation	that thing makes something for me and hands back credentials — a database, a bucket, a route	yes	at provisioning, and again whenever it must be
build	I was compiled against that artifact	no — read from imports	at build, once

Instantiation implies presence; presence does not imply instantiation.

A route is an instantiation edge, and it is worth noticing because the direction is the mirror of a database: the consumer supplies a target and receives a name, rather than supplying nothing and receiving credentials. Same edge.

Provider stops being a category. Any hosted thing can be a factory — an identity provider grants clients, a mail server grants mailboxes. It is a facet, not a kind.

A module may also declare what it claims, because some things cannot coexist and that is a fact about the module rather than about a particular node. What that means precisely is below.

Why the build edge is a different kind

It is fixed inside an artifact rather than negotiated when something runs, and its only remedy is a rebuild — nothing can re-provision it.

It is also derived rather than declared, and the asymmetry is deliberate: a runtime edge is an intention somebody has about how the mesh should be wired, and only a person can state it. A build edge is a fact about code that already exists, and a declared list of dependencies drifts from the imports it describes.

An artifact is out of date when its source moved, or when anything it was built against moved. So what is recorded is a commit and the identity of every artifact it was built against, which is what makes the rebuild set computable and is this current? answerable without building.

The graph measures design quality, not just build order. A module with many inbound build edges is one whose every change is expensive — and that is readable before anything is built. The current shared library is exactly that, and nobody could see it because nothing drew the edges.

Provisioning is declared, never configured by hand

A module declares what it provides and what it requires. The mesh satisfies it: a provisioner belonging to the provider creates the resource and its credential, records the grant, and the values are derived onto the consumer. Neither the credential nor the topology is ever written by hand. A requirement may name a provider on another node, so cross-node wiring is the same declaration.

What a module claims, and why it is not a list of rivals

Written 2026-08-29, replacing pairwise exclusion.

Exclusivity is not a property of a module. It is a property of a singular resource the module takes over. Two shells do not compete for anything and any number may be installed. Two display servers both want the seat, and only one may have it.

A module declares what it claims. Two modules claiming the same thing cannot both be assigned within that claim's scope.

Not "xorg conflicts with wayland". Pairwise exclusion has a property that only shows up later: adding a third display server means editing xorg and wayland to know about it. Every new module requires changing modules nobody who wrote it owns, and the edits grow as the square of the count. With a claim, the third one says claims: the seat and nothing else changes anywhere. The new module is the only thing that has to know anything — which is the difference between a catalogue that grows and one that calcifies.

The pattern is common enough to be worth listing, because seeing it is most of understanding it:

these coexist	these claim one thing
shells — bash, zsh, fish	display servers — xorg, wayland (the seat)
editors — vim, emacs, helix	init — systemd, openrc (pid 1)
language runtimes	container runtime — docker, podman
terminal emulators	reverse proxies — nginx, caddy, traefik (ports 80/443)
browsers	time — chrony, timesyncd, ntpd (the clock)
	resolvers — resolved, dnsmasq, unbound (/etc/resolv.conf)
	network management — NetworkManager, networkd, netctl
	mail — postfix, exim, msmtp (port 25)
	audio — pipewire, pulseaudio (the device)

A claim has a scope, because not everything singular is singular per machine:

scope	example
node	the seat, pid 1, port 443
site	a DHCP server on a segment
mesh	the hub, the control plane

The last is not new — the mesh already enforces exactly one hub with a unique index (ADR 0007). Scope is that idea, said once rather than hard-coded per case.

Some conflicts need no claim at all. Two modules declaring the same file, or binding the same port, are visible from what they declare — the mesh already holds every resource of every declaration. So a claim is only written for the abstract ones, where nothing in the declaration reveals the clash. That keeps the manifest small, which is worth protecting.

A requirement with several answers is refused, never guessed

A module requiring a shell may be satisfied by three. The mesh does not pick.

candidates	what happens
exactly one	assigned, silently — there was no choice to make
none	refused, naming what is missing
several	refused, naming them, and a person chooses

This is what makes a solver unnecessary. Counting candidates is a few lines and has no surprising behaviour; a solver that picks has to be understood before its answer can be trusted, and it is understood by whoever is debugging it at the time. Nothing here is lost by waiting — a solver can be added later without changing a single manifest, and the reverse is not true.

Requiring a module and requiring a capability are different fields, because the remedies differ and the message should say which:

• i3 needs xorg, which is not assigned here — assign it.
• this machine has no seat — wrong machine; nothing can be installed to fix it.

A capability may carry a value, and that is not a new idea

A capability is a named fact about a machine, detected and never assumed. Its presence gates an assignment; its detail can also carry a value — seat: card1-DP-1, panel: oled, an architecture, an amount of memory. Nothing new is needed for that: a verdict has always had a detail beside its yes or no.

So can this run here and what should it be configured as are answered by the same fact, read two ways. A module that must not be assigned without an OLED panel and one that dims itself differently on one are reading the same line.

What keeps the set from sprawling is the cost of adding one. A capability must be detected, and the detector must say how it knows — so nobody can add one they cannot check, which is the whole of 04-ISSUES/007: an installed package was treated as a capability and a node was assigned work it could not do.

And detectors ship inside the host, which is one statically linked binary. Adding a capability means shipping a new host to every node that needs it. That is a real cost and it argues for keeping the vocabulary small and general — seat, not has-nvidia-with-two-outputs.

"Flavor" is retired

It was carrying three unrelated meanings — variants of a thing, a subset of one module a node installs, and whatever the current system does, which earned two knowledge-base entries about going wrong. A word with three meanings cannot be reasoned about, and every attempt to design around it produced a rule that was right for one meaning and wrong for the others.

What it was reaching for is two ordinary things:

• Different modules that provide the same thing. zsh and fish both provide a shell. They are two modules, not one module with a switch: they share a name and nothing else — different packages, different configuration, different everything.
• One module with a setting. A monitoring module that is an agent here and a server there is one module, configured. Nothing varies but a value.

If something is neither, it is probably two modules.

The core library is the mesh's domain

One module everything may depend on. It holds what is true of the mesh regardless of which context you are in: a module, a node, an assignment.

The test: would this still mean the same thing in a context that had never heard of the one it came from? A node would. A pipeline stage would not — that is delivery's.

Types ship with the module that owns them, not here. A consumer needing inventory's types depends on inventory — one narrow, visible edge — rather than everything depending on a hub where the relationship cannot be seen. A library everything depends on is expensive to change whether it holds types or code; the fan-in is what makes it expensive, which is why types, not behaviour was the wrong guard.

It stays small on its own. A domain model changes when what the mesh is changes, which is rare. A drawer labelled shared changes whenever anybody writes something reusable, which is constantly — and who else might want this always answers yes, which is how the current one grew.

Consequences

• Fewer things will be shared, and some code will be written twice. That is the trade: the current library exists because sharing felt free. Two similar functions in two modules is often the better answer.
• The check is a measurement rather than a prohibition. Inbound build edges say when something is becoming a hub, while it is happening rather than after.
• Reading build edges needs a language-aware tool per language, which is the real cost and the reason declaring them looks tempting. It is still wrong.