jschoubben
0531d6fc38
011 opened asking what a graph deletes and found the graph already existed. The work became design, worked through twenty cases and one provider in full. Two decisions close it. 0044 — a module declares presence, instantiation and exclusion. Two kinds of edge because a game wanting a database is not a game wanting postgres to exist: one creates something per consumer, carries credentials back, can be revoked, and leaves the provider holding state. Names are concrete unless providers are genuinely substitutable — `terminal` passes, `database` fails, and the adapter is what creates an interface. Where there is no contract there is a tag, which describes and does not bind. Exclusion is a third relation and is not derivable. A node provides names too, which makes capability checking stop being a separate mechanism and makes the host's detection an input to resolution. Constraints, never placement. Scope decides which provider and the binding is written down and sticky, in a place that follows the scope. And there are three entities, not two — the assignment carries what belongs to neither end, which is what node-agnostic modules ran out of. 0045 — a context owns its store, exclusively. No shared writes and no read roles on another context's store, because reading couples you to its layout just as firmly and invisibly. The unit is the CONTEXT, not the process: a board showing the mesh's own data is the mesh showing its own data. Asking or subscribing is derived from ADR 0036 rather than chosen. And it is the first clear list of what the design removes: grant kinds, table ownership, cross-context migration ordering, and a class of permission modelling. 0017 is superseded rather than narrowed — its text unchanged, its status changed. Folders assert relationships where edges record them, and the domain module goes with it. Left explicitly undecided in both: what a resolver delegates rather than reimplements, how many instances a module should have, and what a provider hands back.
127 lines
6.4 KiB
Markdown
127 lines
6.4 KiB
Markdown
---
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status: accepted
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date: 2026-08-26
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deciders: jochen
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reconstructed: false
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supersedes: 0017-modules-outside-the-core-are-grouped-by-domain.md
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---
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# 44. A module declares presence, instantiation and exclusion
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## Context
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[Research 011](../01-RESEARCH/011-the-module-graph/00-overview.md) set out to find the
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catalogue's missing structure. It opened with *what does a graph delete?* and the answer for the
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existing system was **nothing — it is already there**: 126 manifests, 103 edges, no cycles,
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nothing dangling, and a resolver already used for build, load and install order.
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So the work became a design question rather than a discovery one, worked through twenty cases
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and one provider in full.
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## Decision
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### Two kinds of edge, one graph
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**Presence** — the thing must exist and be reachable. Nothing is created, nothing flows back.
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*An editor requires a terminal. A dashboard requires a container runtime.*
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**Instantiation** — a provider is asked to make something *for this consumer*, and hands back
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what the consumer needs to use it. *A game requires a database from the store, and receives one,
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with credentials.*
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Instantiation implies presence; presence does not imply instantiation. They differ in whether
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something is created, whether a payload returns, whether it can be revoked, and whether the
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provider holds state about it — which is too much to collapse into one relation for tidiness.
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### Names are concrete unless providers are genuinely substitutable
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A requirement names either a **concrete** thing — that module and no other — or an **abstract**
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name satisfied by whatever provides it.
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**An abstract name is legitimate only where a consumer can be switched between providers without
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changing.** `terminal` passes. `database` fails: a consumer speaking one store's protocol does
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not speak another's, so the name would promise what no provider delivers and the resolver would
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report a requirement satisfied that is not.
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**The adapter is what creates an interface.** A name has several providers *and a contract they
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all satisfy*, or it is not an interface. Nothing declares itself to be one.
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**Where there is no contract there is a tag.** *Database* remains a useful word for finding
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things and grouping them in a catalogue. Tags describe; edges bind; keeping them apart is what
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stops a second relationship appearing that looks like a dependency and is not.
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### Exclusion is the third relation, and it is not derivable
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`excludes` names what cannot coexist with this. Two modules providing one name look
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interchangeable, and two things wanting one port look independent, right up until installing the
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second breaks the first.
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### A node provides names too
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A node's profile is a set of provided names — a display server, a container runtime, an
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architecture. A module requiring one is satisfied by **the node**, exactly as one requiring a
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store is satisfied by another module.
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**So capability checking is not a separate mechanism.** There is one question — *is this name
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provided by anything available here?* — and a graphical application cannot land on a node
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without a display server for the same reason, through the same code, that it cannot land
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without its dependencies.
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This makes the host's capability detection an **input to resolution** rather than a report for a
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person. And what a node provides is partly *derived*: installing a container runtime makes the
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node provide `container-runtime` thereafter.
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### Constraints, never placement
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A module says what must be true of a node and never which node. Which node runs what is an
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inventory decision, and today's catalogue decides it in manifests — a module pinning its store
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to a named node, so a second node cannot provide it without editing the consumer.
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### Scope decides which provider, and the binding is written down
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A consumer of an instantiation edge declares the **scope of its own need**: one instance shared
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across every instance of itself, or one each. That decides without naming a node — a shared need
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cannot be met by something each node runs separately.
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The mesh then binds, and **the binding is recorded and sticky**. Not recomputed: a resolver that
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re-derives which store serves a consumer will one day derive a different answer and relocate a
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database. **Where it is recorded follows the scope** — a shared grant belongs to the module, a
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per-instance grant to the assignment.
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### A module, a node, and an assignment
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Three entities. The assignment carries what belongs to neither end: where state lives, how it is
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reached, configuration derived from the hardware, and which provider instance serves this
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consumer.
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Recorded because the alternative was tried: modules were once node-agnostic, and it did not
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survive — there was nowhere for these to live.
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## Consequences
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- **[ADR 0017](0017-modules-outside-the-core-are-grouped-by-domain.md) is superseded.** Folders
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assert relationships; edges record them. What grouping was for — finding things, seeing what
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belongs together — is a tag and a query over the graph, neither of which anybody keeps true by
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hand. The domain module goes with it: there is no `networking` thing to install, there are
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concrete modules named individually.
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- **Provider stops being a category**, as service and application already had. Any hosted thing
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can be a factory — an identity provider grants clients, a mail server grants mailboxes. It is
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a facet, not a kind.
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- **`excludes` and node capabilities do not exist in any manifest today**, so this adds
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declarations rather than removing them. What it removes is listed in
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[ADR 0045](0045-a-context-owns-its-store.md), which is the other half of this design.
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- **A resolver is still needed**, with version constraints and conflicts. What it delegates to
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the platform's package manager rather than reimplementing is **not decided here**.
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- **Two axes remain undeclarable**: how many instances a module should have — not derivable, and
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opposite for a broker and a store — and what a provider hands back, which differs between
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credentials and a command.
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## References
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- [Research 011](../01-RESEARCH/011-the-module-graph/00-overview.md) — the measurement, the
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cases, the worked provider, and what happens to `feature`.
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- [ADR 0045](0045-a-context-owns-its-store.md) — the ownership half.
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- [ADR 0043](0043-a-declaration-is-an-ordered-list-of-owned-resources.md) — what the graph
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produces for a node.
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- [ADR 0002](0002-everything-is-a-module.md) — survives; this says what a module declares.
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