hq/01-RESEARCH/011-the-module-graph/worked-provider.md

# A provider, all the way through

One module, worked out completely, because it is the case that breaks the tidy version. It
looks like *a container that runs a database* and it is at least nine things.

Postgres is the example. **The shape is not specific to it** — see the end.

## What it carries

**1 — A supervised container.** An image, a version, a data volume, and configuration. Easy,
and the only part the phrase "a docker service" describes.

**2 — Persistent state, and where it lives matters.** The volume is the database. Moving this
module between nodes is not rescheduling; it is a migration. Almost nothing else in the
catalogue has this property, and nothing in `provides` / `requires` expresses it.

**3 — Configuration that is partly the machine's.** Tuning follows the hardware — memory,
storage. A declaration generated centrally cannot know those, and
[research 012](../012-the-minimum-viable-node/00-overview.md) says the machine's own values win
on conflict. So some of this module's configuration is *derived from the node it lands on*.

**4 — A tool surface.** It exposes capabilities agents can call — query, list, provision. That
is not a resource on a machine and not an artifact; it is a contract the mesh publishes on the
module's behalf.

**5 — A provisioner.** The part that matters, and the one below.

**6 — Its own bookkeeping.** The provisioner must remember what it granted to whom, or it
cannot revoke, rotate or clean up. So a module that provides state to others *also* holds state
about its providing — and that state is not the database's data.

**7 — An exposure decision, per node it runs on.** Reachable from the machine only, from the
local network, or publicly. That is a property of *this assignment*, not of the module: the
same module on two nodes may answer differently.

**8 — Credentials it generates.** Per consumer, and they have to reach the consumer. Which
means a provisioning edge carries a payload, and the payload is a secret.

**9 — Health that is not "the container is up".** A container running and a database accepting
connections are different facts, and the second is the one anything cares about. This is the
host's read-back rule, at a distance.

## The provisioner is a second kind of edge

The tidy version of this effort said: *a module provides names, a module requires names, that is
the only edge.* A small game wanting to store data shows it is not.

```
my-cool-game   requires   postgres            # I speak its protocol, it must exist
my-cool-game   requires   a database FROM postgres, called my-cool-game
```

The first is **presence**: the thing exists and is reachable. Nothing is created; nothing flows
back. `vscode requires terminal` is this, and so is `requires container-runtime`.

The second is **instantiation**: the provider is asked to make something *for this consumer*,
and hands back what the consumer needs to use it. A database, a user, a password, an address.

They differ in every way that matters:

| | presence | instantiation |
|---|---|---|
| creates something | no | yes, one per consumer |
| carries a payload back | no | credentials, an address |
| can be revoked | — | yes, and must be when the consumer goes |
| provider holds state about it | no | yes — who was granted what |
| satisfied by | anything providing the name | that provider, specifically |

**This is the mesh's actual power**, in the operator's words: a small game declares it wants a
database and the mesh makes one. Nobody creates a user by hand, nobody pastes a connection
string. That is worth being precise about rather than folding into a single relation because
one relation is prettier.

## What that costs the design

**The proposal's "one kind of edge" is wrong**, and the current system already knew: it has
`dependencies` for presence and `requires: provision:` for instantiation, with the resolver
deriving a presence edge from every instantiation edge. [`analysis.md`](analysis.md) recorded
that derivation as a convenience. It is not — it is the correct relationship between two
genuinely different relations.

So: **two kinds of edge, one graph.** Instantiation implies presence. Presence does not imply
instantiation.

## Which provider, when two nodes run one

Asked directly, because two nodes can each run a relational store and a consumer has to be
served by one of them. Neither obvious answer is right.

**Not "the consumer names the node."** That is placement in the consumer's manifest — a small
game edited because a database moved, which is the fault
[`proposal.md`](proposal.md) separates constraints from placement to avoid.

**Not "the consumer does not care" either.** For presence it genuinely does not: a terminal is a
terminal. For instantiation it cares permanently, because the data lands in exactly one store
and the wrong choice is discovered long afterwards.

**What the consumer does know is the scope of its own need.** Not which node — how many of the
thing it wants, relative to itself:

| Scope | Means | Example |
|---|---|---|
| **shared** | one instance serves every instance of this consumer | the mesh's own registry: every node reads the same rows |
| **per instance** | each instance of this consumer gets its own | a local cache, a per-node queue |

That is a property of the consumer, expressible without naming anything. And it is the thing
that actually decides: a shared need cannot be satisfied by a provider each node runs
separately, and a per-instance need should not be satisfied by a shared one.

**Then the mesh binds, and the binding is recorded on the assignment.** Not recomputed. A
resolver that re-derives which store serves a consumer will one day derive a different answer and
relocate a database — so the binding is made once, written down, and changed only deliberately.

The pieces that follow, none of them settled here:

- **When several providers satisfy the scope**, something chooses — most plausibly locality,
  preferring a provider on the same node. That is a default, and it must be overridable, because
  the reason to override it is exactly the reason nobody anticipated it.
- **A binding is a thing that can be wrong.** Once recorded it can be inspected, and a consumer
  bound to a store on a node that no longer exists is a question somebody can be asked rather
  than a failure at connect time.
- **Moving a binding moves data.** Whatever the mechanism, changing it is a migration and not a
  configuration change, and a design that lets it look like the latter will lose something.

## Provisioning is early, not late

An assumption worth killing: that provisioning is something the control plane does for
consumers once a mesh is running.

**The mesh's own registry is a provisioned database.** So is its virtual host on the broker.
Neither exists until something creates them, and nothing in the mesh works until they do. The
order is:

```
1  the store runs                       from the bundle the host carries
2  a database is created in it          a provisioning step
3  the mesh's own schema is applied     a migration, against that database
4  the control plane starts             and only now is there a mesh
5  everything else is provisioned       the ordinary path
```

Steps 2 and 3 happen **before there is a mesh to do them**. So provisioning is not a
control-plane service that consumers use; it is part of the bootstrap, and part of what the
carried bundle has to be able to express.

**Which strains what a declaration is.** [ADR 0043](../../02-DECISIONS/0043-a-declaration-is-an-ordered-list-of-owned-resources.md)
has the host applying *declared state on this machine*. A database inside a running store is not
a file or a unit — and at bootstrap it is, at least, local: the store is on the same machine as
the host applying the bundle.

Later it is not. A consumer on one node provisioned from a store on another is the ordinary
case, and reaching it is not the host's job. So the same operation is local at bootstrap and
remote afterwards, which is either two mechanisms or one mechanism with a boundary crossing in
it. Undecided, and it is the sharpest unresolved thing in this file.

## A migration belongs to the consumer and runs on the provider

A game defines migrations. They run against the database the store granted **it**. So the
migration is:

- **owned** by the consumer — it is that module's schema, versioned with that module;
- **hosted** by the provider — it runs inside something the consumer does not control;
- **ordered** after the provisioning edge — there is nothing to migrate until the grant exists;
- **scoped** to the grant — the consumer's migrations touch its database and no other.

Ownership crosses the edge, which nothing in *provides* and *requires* expresses. And it is the
*action* category from [`features.md`](features.md) made concrete: not an artifact, not node
state, and not something the host can apply, because the thing it changes is not the machine.

It also gives a consumer's own install an internal order — **provisioned, then migrated, then
started** — that depends on an edge rather than on the module's contents.

## What still has no answer

**How many instances of postgres should exist?** One per mesh is wrong — a node that must work
while disconnected cannot depend on a database elsewhere. One per node is wrong — the mesh's own
registry is one thing, not one per node. So the answer is per-module, and nothing in the schema
says it. This is [`cases.md`](cases.md) axis *how many instances*, and postgres is the case that
proves it cannot be a global rule.

**What happens to a grant when the consumer is removed?** The game is uninstalled. Its database
still exists, holding its data. Dropping it silently is data loss; keeping it forever is a leak.
[ADR 0043](../../02-DECISIONS/0043-a-declaration-is-an-ordered-list-of-owned-resources.md) says
the host removes what it applied and no longer declares — but this is not on the host, it is
inside another module's state, and the same reasoning does not obviously carry.

**Where does node-derived configuration come from?** (3) The control plane composes a
declaration, and cannot know this machine's memory. Either the host fills in a blank the
declaration leaves — which makes the host decide something, against
[ADR 0037](../../02-DECISIONS/0037-the-host-applies-it-does-not-decide.md) — or the control
plane reads the node's inventory first and composes with it. The second is consistent and means
a declaration is composed *per node from what the node reported*, which is a stronger claim than
anything recorded so far.


## Providing is not a substrate thing

The four pinned services are the obvious providers, and they are not a category.

| Service | What a consumer asks it for |
|---|---|
| a relational store | a database, a user, credentials |
| another relational store, different vendor | a database — **and not the same one** |
| a message broker | a virtual host, a user, permissions |
| an object store | a bucket and keys |
| an image registry | a repository |
| an identity provider | a client, a realm, a secret |
| an analytics service | a site, and a tracking identity |
| a low-code data platform | a base, and a token |
| an application platform | a project, which is several of the above at once |
| a mail server | a mailbox, an alias, credentials |

**Any hosted service can be a factory.** Providing is a facet a module may have, not a kind of
module it is — which is the same conclusion the effort reached about services and applications,
arriving from the other direction.

That kills the last reason to keep *provider* as a category. A module runs something, or grants
something, or both, or neither.

### Two stores, and why `database` still is not a name

Two relational stores from different vendors both grant *a database*. They are the sharpest
possible test of the substitutability rule from [`proposal.md`](proposal.md), and they fail it
completely: different wire protocol, different dialect, different driver, different client
library compiled into the consumer.

A consumer declaring `requires: database` and being handed either would break against one of
them. So the name promises what no provider delivers — and now with two real providers in the
catalogue rather than a thought experiment.

*Database* remains a **tag**. It is how a person finds both. It is not how a consumer names what
it needs.

## The same shape, three more times

The message broker has all nine. So does the object store, and so does the image registry. They
differ in what a consumer asks for — a database, a virtual host, a bucket, a repository — and in
nothing structural.

**A substrate service is a service plus a factory.** That is the whole pattern, and there are
four of them. It generalises past the substrate too: anything that grants something per consumer
has this shape, and anything that does not is the simpler case.

But two things differ *between* them, and both matter more than the similarity.

### The broker cannot be managed over the broker

[ADR 0001](../../02-DECISIONS/0001-nodes-communicate-over-a-broker.md) makes the broker the
channel every node takes work from, and
[ADR 0039](../../02-DECISIONS/0039-the-link-is-the-security-boundary.md) makes it the security
boundary — everything a node applies arrives through it.

So the module providing the broker is also **the way modules are managed**. A declaration cannot
be delivered to it over itself, and reconfiguring it is done through the thing being
reconfigured. Nothing else in the catalogue has that property; the store is consumed by the
control plane but is not how the control plane *reaches* anything.

This is exactly what the carried bundle exists for
([ADR 0038](../../02-DECISIONS/0038-a-node-joins-by-linking-first.md)): the broker is raised from
what the host carries, before there is a channel, because there is no other way to raise it.
Recorded here because it is a constraint on *one module*, not a general rule, and a schema with
no way to say so hides it.

### Two modules of identical shape want different instance counts

The broker is one per mesh — a single point of failure and a single point of trust, by decision
rather than by accident. The store cannot be: a node that must keep working while disconnected
([ADR 0036](../../02-DECISIONS/0036-a-node-is-a-managed-machine.md)) cannot depend on a database
somewhere else.

Same nine properties, opposite answers. Which settles something the cases file left open: **how
many instances is not derivable from what a module is.** It is a decision per module, it has to
be declared, and nothing in `provides`, `requires` or `excludes` says it.

### And revocation differs in consequence

Revoking a database leaves data behind until something drops it — a leak, and recoverable.
Revoking a virtual host drops whatever had not been delivered — not recoverable, and silent.

The relation is the same and the blast radius is not, which is an argument for the provider
deciding what revocation means rather than the mesh applying one rule to all of them.


## The assignment is a third thing

Recorded because the operator tried the alternative and abandoned it: **modules were once
node-agnostic**, and it did not survive contact.

The worked example says why. Several of a provider's nine properties are not properties of the
module at all:

- **where its state lives** — a volume on a particular machine;
- **how it is reached** — the same module on two nodes may answer locally, on the network, or
  publicly, and that is a per-assignment decision;
- **configuration derived from the hardware** — tuning follows the memory and storage of the
  machine it landed on;
- **whether this instance is the one** a given consumer is provisioned from.

None of those belong in the catalogue, because they differ per node. None belong in the node,
because they are about this module. **They belong to the pairing**, and a design with only
modules and nodes has nowhere to put them — which is what "node-agnostic" ran out of.

So there are three entities, not two:

> **a module** · **a node** · **an assignment**, which is a module on a node and carries its own
> configuration

The current system already has this, arrived at the same way: environment values are stored per
module *and per node*, so a module's settings differ between the machines running it.

**This does not put placement back in the manifest.** A module still says what must be true of a
node and never which node ([`proposal.md`](proposal.md)). What changes is that the *result* of
placing it is a thing with its own state, rather than a fact recorded on one of the two ends.

And it makes the composed declaration question from above answerable: a declaration is built
from the module, the node's inventory, and the assignment between them. Three inputs, which is
why two were never enough.