mesh-controller/internal/catalogue/seats.go

package catalogue

import (
	"fmt"
	"sort"
	"strings"
)

// The seats a mesh can have (novox/hq ADR 0110).
//
// **A closed set, defined here rather than by whoever claims one.** Until this, a well-formed name
// became a seat by being claimed, so nothing could say which seats a mesh has or who fills them:
// `the-showcase` and `the-build-machine` were each invented by the module claiming it. The set is
// what a person reads to learn what a mesh can have, so an entry nobody argued for is an entry
// nobody can explain — the same reason every shape in the host's vocabulary names its decision.
//
// A seat is held by a module assignment. What the mesh knows about a holder is what it knows about
// that assignment; nothing about holders is kept here or anywhere else.

// Seat is one role the mesh defines.
type Seat struct {
	// Name is what a manifest claims.
	Name string
	// Scope is where there may be only one holder.
	Scope string
	// Delivers is the provision the seat's holder answers for, or empty. A seat that delivers a
	// provision may only be held by a module providing it at the seat's scope, and its holder is
	// what a requirement for that provision resolves to when several modules provide it.
	Delivers string
	// Accepts, Emits and Serves are the protocol of the role, as local verbs — the same three a
	// module declares for a seat of its own (novox/hq ADR 0118), and empty for most of these: a seat
	// is usually about who does a job and not about what may be said to them.
	//
	// **Named here so the mesh has no role it cannot describe** (ADR 0121). Without them a build
	// machine had three audiences for one outcome and nothing derived a grant for any of them, and an
	// event about a role had nowhere to live but the namespace of whichever module held that role
	// today — which the bus refuses, because a namespace belongs to who it is named for.
	Accepts []string
	Emits   []string
	Serves  []string
	// Decision is the record that made it a seat.
	Decision string
}

// defaultSeats is the set the mesh ships with — the seed for the control plane's seat table and the
// fallback when it has none (novox/hq ADR 0122). It is the one place the closed set 0110 defines is
// written; the store's table is seeded from it and thereafter is the live, editable copy.
//
// In the order a person reads it: the mesh's own, then a node's.
var defaultSeats = []Seat{
	{Name: "mesh-controller", Scope: ScopeMesh, Decision: "novox/hq ADR 0079"},
	{Name: "mesh-store", Scope: ScopeMesh, Delivers: "postgres-database", Decision: "novox/hq ADR 0079"},
	// **Delivers the mesh's own bus, not `amqp`.** Those were the same word until
	// ADR 0127 separated them: `amqp` is a backing service a module may require, and this seat is
	// the mesh's own transport. ADR 0128 then made that connection something a module requires
	// rather than receives ambiently — 23 of the catalogue's modules never speak, and an ambient
	// connection would mint a credential for each.
	{Name: "mesh-broker", Scope: ScopeMesh, Delivers: "mesh-bus", Decision: "novox/hq ADR 0079"},
	{Name: "the-artifact-store", Scope: ScopeMesh, Delivers: "artifact-store", Decision: "novox/hq ADR 0075"},
	{Name: "mesh-catalog", Scope: ScopeMesh, Decision: "novox/hq ADR 0121"},
	// Deferred renames (novox/hq ADR 0121): these deliver a provision, so renaming them is a
	// delivering-seat migration with a mesh-wide cascade if a holder stops resolving mid-flight.
	// They keep their names until that migration is done deliberately, apart from the node-* pass.
	{Name: "npm-package-registry", Scope: ScopeMesh, Delivers: "npm-package-registry", Decision: "novox/hq ADR 0109"},
	{Name: "git", Scope: ScopeMesh, Delivers: "git", Decision: "novox/hq ADR 0111"},
	// A build is work submitted to this role and its outcome is the role's own event (ADR 0129).
	// One publish reaches whoever asked, the controller that records it, and the catalogue that
	// places it in the graph — what the old bus's shared exchange did for free.
	{Name: "mesh-build-machine", Scope: ScopeMesh,
		Accepts: []string{"build"}, Emits: []string{"built"}, Decision: "novox/hq ADR 0121"},
	{Name: "node-dns-resolver", Scope: ScopeNode, Decision: "novox/hq ADR 0121"},
	{Name: "node-intrusion-prevention", Scope: ScopeNode, Decision: "novox/hq ADR 0121"},
	{Name: "node-packet-filter", Scope: ScopeNode, Decision: "novox/hq ADR 0121"},
	// Deferred (novox/hq ADR 0121): renaming to mesh-private-network is a scope + server/client
	// model change, not a rename, so it stays until that is built.
	{Name: "the-private-network", Scope: ScopeNode, Decision: "novox/hq ADR 0110"},
	{Name: "node-resolver-config", Scope: ScopeNode, Decision: "novox/hq ADR 0121"},
	// The program that manages the machine's own network. It delivers nothing: its holder only
	// keeps the manager and the mesh from contradicting each other — the resolver file left to the
	// mesh, the private network's interface left alone — and never declares a link, an address or
	// a wireless network, because the link is the only channel a fix could arrive on. A seat
	// rather than a condition in the resolver's module, so a machine running two managers is
	// refused at assignment instead of found by the resolver being rewritten (novox/hq ADR 0117).
	{Name: "node-uplink", Scope: ScopeNode, Decision: "novox/hq ADR 0117"},
}

// A system seat name is the control plane's namespace: `mesh-*` for a mesh-wide role, `node-*` for
// a per-node one (novox/hq ADR 0121). A claim to a system name the mesh does not define is refused;
// any other name is a module's own to define and claim. Some of the mesh's own seats predate this
// convention and are not yet renamed (git, npm-package-registry, the-artifact-store,
// the-private-network) — those are in the set, so they resolve by name, not by prefix.
func isSystemSeatName(name string) bool {
	return strings.HasPrefix(name, "mesh-") || strings.HasPrefix(name, "node-")
}

// seats is the working set the lookups read. It starts as the compiled defaults and is replaced by
// what the control plane loaded from its store (novox/hq ADR 0122), so a change to the set is a
// change to data, not to this code.
var seats = defaultSeats

// DefaultSeats is the set the mesh ships with, for seeding the store's seat table.
func DefaultSeats() []Seat { return append([]Seat(nil), defaultSeats...) }

// UseSeats replaces the working set with the one the control plane read from its store.
//
// **Empty is ignored on purpose.** A store that has not been seeded yet — or one that could not be
// read — must leave the compiled defaults in force rather than emptying the set: an empty set would
// refuse every claim and could stop the control plane composing at all, which is a far worse failure
// than running on the set the binary shipped with. So the store can only ever *replace* the set with
// a non-empty one, never erase it.
func UseSeats(s []Seat) {
	if len(s) > 0 {
		seats = s
	}
}

// aliases maps a seat's former names to its current canonical name (novox/hq ADR 0122). Loaded from
// the store alongside the set, so a reference to a name a seat used to have — a manifest's claim, a
// held record — still resolves to it after a rename, and nothing downstream has to change.
var aliases = map[string]string{}

// UseAliases replaces the former-name map with the one the control plane read from its store. Empty
// is fine and ordinary: a mesh whose seats have never been renamed has no aliases.
func UseAliases(m map[string]string) { aliases = m }

// Seats is every seat the mesh defines, in reading order.
func Seats() []Seat {
	return append([]Seat(nil), seats...)
}

// SeatNamed is the seat a name refers to, whether that is its current name or one it used to have
// (novox/hq ADR 0122). A former name resolves to the seat's canonical row, so a rename breaks no
// reference to the old name.
func SeatNamed(name string) (Seat, bool) {
	for _, s := range seats {
		if s.Name == name {
			return s, true
		}
	}
	if canonical, aliased := aliases[name]; aliased {
		for _, s := range seats {
			if s.Name == canonical {
				return s, true
			}
		}
	}
	return Seat{}, false
}

// SeatDelivering is the seat whose holder answers for a provision, if there is one.
func SeatDelivering(provision string) (Seat, bool) {
	if provision == "" {
		return Seat{}, false
	}
	for _, s := range seats {
		if s.Delivers == provision {
			return s, true
		}
	}
	return Seat{}, false
}

// claimProblems is what is wrong with a manifest's claims and the seats it defines.
//
// A claim is one of three things (novox/hq ADR 0121): a **system seat** the control plane defines —
// checked for scope and, if it delivers a provision, that the claimant provides it; a **system name
// the mesh does not define** (`mesh-*`/`node-*`) — refused, because that namespace is the control
// plane's; or a **module-defined seat** — valid only when this manifest also declares it, since a
// module may coordinate its own instances through a seat of its own but may not invent one by
// claiming it. A module's own seat declaration may not sit in the system namespace or shadow a
// system seat.
func claimProblems(m Manifest) []string {
	var problems []string

	defined := map[string]SeatDeclaration{}
	for _, d := range m.DefinesSeats {
		if _, isSystem := SeatNamed(d.Name); isSystem || isSystemSeatName(d.Name) {
			problems = append(problems, fmt.Sprintf(
				"%s defines a seat %q in the mesh's own namespace; a module's seat is named outside "+
					"mesh-*/node-* (novox/hq ADR 0121)", m.Module, d.Name))
			continue
		}
		defined[d.Name] = d
	}

	for _, c := range m.Claims {
		if seat, known := SeatNamed(c.Name); known {
			if c.At() != seat.Scope {
				problems = append(problems, fmt.Sprintf(
					"%s claims %s at scope %q, and %s is a %s seat",
					m.Module, c.Name, c.At(), c.Name, seat.Scope))
			}
			if seat.Delivers != "" && !providesAt(m, seat.Delivers, seat.Scope) {
				problems = append(problems, fmt.Sprintf(
					"%s claims %s, whose holder answers for %q, and %s does not provide %q at %s scope",
					m.Module, c.Name, seat.Delivers, m.Module, seat.Delivers, seat.Scope))
			}
			continue
		}
		if isSystemSeatName(c.Name) {
			problems = append(problems, fmt.Sprintf(
				"%s claims %q, which is a seat in the mesh's own namespace (mesh-*/node-*) that it "+
					"does not define (novox/hq ADR 0121) — the seats are: %s", m.Module, c.Name, seatNames()))
			continue
		}
		d, ours := defined[c.Name]
		if !ours {
			// **A claim on a seat this manifest does not declare is not the parser's to judge.**
			// A module may hold a seat another module declared — that is why ADR 0126 has callers
			// name the seat and not its provider, so an implementation can be replaced without
			// touching a caller. Whether the seat exists is a fact about the whole catalogue, so
			// the refusal is at registration, where every declaration is in view
			// (`CatalogueProblems`: "which no module declares and the mesh does not define").
			continue
		}
		if c.At() != d.At() {
			problems = append(problems, fmt.Sprintf(
				"%s claims its own seat %s at scope %q, having declared it at %q",
				m.Module, c.Name, c.At(), d.At()))
		}
	}
	return problems
}

func providesAt(m Manifest, provision, scope string) bool {
	for _, o := range m.Provides {
		if o.Name == provision && o.At() == scope {
			return true
		}
	}
	return false
}

func seatNames() string {
	names := make([]string, 0, len(seats))
	for _, s := range seats {
		names = append(names, s.Name)
	}
	sort.Strings(names)
	return strings.Join(names, ", ")
}

// HolderAmong is which of several providers of a provision holds the seat that delivers it.
//
// Found by the (node, module) pair, because a provider is identified by both (novox/hq to-be 23):
// two modules on one node could both provide a provision, and only the one holding the seat
// answers for it. Nothing when no seat delivers the provision, when nobody holds
// it, or when the holder is not among the providers offered.
func HolderAmong(provision string, providers []Provider, held []Held) (Provider, bool) {
	seat, delivered := SeatDelivering(provision)
	if !delivered {
		return Provider{}, false
	}
	for _, h := range held {
		// Resolve the held claim to a seat rather than comparing names, so a record naming a seat's
		// former name still matches it after a rename (novox/hq ADR 0122).
		hs, ok := SeatNamed(h.Claim)
		if !ok || hs.Name != seat.Name || h.Scope != seat.Scope {
			continue
		}
		for _, p := range providers {
			if p.Node == h.Node && p.Module == h.Module {
				return p, true
			}
		}
	}
	return Provider{}, false
}

// SeatsWithAProtocol are the mesh's own seats that say something about what may be said to them or by
// them, which is the set the bus derives streams, consumers and permissions from.
//
// Most of the set is not here, and that is the ordinary case: a seat saying only who does a job grants
// nothing on the bus and needs no queue.
func SeatsWithAProtocol() []Seat {
	var out []Seat
	for _, s := range seats {
		if len(s.Accepts) > 0 || len(s.Emits) > 0 || len(s.Serves) > 0 {
			out = append(out, s)
		}
	}
	return out
}