mesh-controller/internal/broker/derived.go

package broker

import (
	"fmt"
	"sort"
)

// Streams and consumers derived from what modules declare.
//
// The mesh's own four exist before any module does (streams.go). Everything here is the other
// half: a seat's stream comes into being when the module declaring it is **registered**, and a
// consumer when a module is **assigned** — which is why ADR 0116's task 1.4 had to be narrowed to
// the foundation set. Neither has happened at genesis.
//
// All of it is a pure function of declarations. The controller is still the only writer; this is
// only what it writes.

// A Consumer is a durable subscription the controller creates on a module's behalf. A module
// declares what it reacts to, never how delivery works, so it does not name these and cannot
// misconfigure them.
type Consumer struct {
	Name   string
	Stream string
	// Filters are the subjects this consumer receives. One consumer per module with several
	// filters, rather than one per consumed event: its ack subject is derived from its name, and
	// a module with five consumers would need five ack permissions to ack its own deliveries.
	Filters []string
	// Queue is the queue group, set for a seat's worker so that "exactly one holder" survives a
	// seat later being relaxed to several. Authority and delivery are kept separate on purpose.
	Queue string
	// AckWaitSeconds before an unacknowledged delivery is redelivered.
	AckWaitSeconds int
	// MaxDeliver before the message is dead-lettered; zero for the mesh's default.
	MaxDeliver int
	Why        string
}

// seatStreamName is the stream holding a seat's inbound work. Named after the seat rather than
// the module holding it, because the holder can change and the queued work must not care — which
// is the whole reason a caller addresses a seat instead of a module.
func seatStreamName(seat string) string { return "SEAT_" + upperSnake(seat) }

// SeatStreams is one work queue per declared seat, created when the declaring module is
// registered rather than when it is assigned.
//
// **The stream exists before anyone holds the seat, and that is the point.** Work queues until a
// holder appears, so installing the telegram module a week after something started sending to it
// flushes the backlog instead of having lost it. A stream created at assignment would make "the
// holder is not here yet" mean "your messages are gone".
func SeatStreams(seats []DeclaredSeat) []Stream {
	sorted := append([]DeclaredSeat(nil), seats...)
	sort.Slice(sorted, func(i, j int) bool { return sorted[i].Name < sorted[j].Name })

	var out []Stream
	for _, s := range sorted {
		if len(s.Accepts) == 0 {
			// A seat that only emits and serves needs no stream: its events ride EVENTS and its
			// tools are core request/reply, which is never persisted.
			continue
		}
		retain := s.RetainSeconds
		if retain == 0 {
			retain = 7 * 24 * 60 * 60
		}
		out = append(out, Stream{
			Name:      seatStreamName(s.Name),
			Subjects:  []string{"mesh.seat." + s.Name + ".accept.>"},
			Retention: RetentionWorkQueue,
			MaxAge:    retain,
			Why: fmt.Sprintf("work submitted to the %s seat; one holder consumes it, and it "+
				"queues while nobody does", s.Name),
		})
	}
	return out
}

// A DeclaredSeat is a seat as the catalogue knows it. Mirrored here rather than imported so this
// package stays free of the catalogue's own types — the same reason the host mirrors the
// contracts instead of importing the sdk.
type DeclaredSeat struct {
	Name          string
	Accepts       []string
	RetainSeconds int
}

// ConsumerFor is the durable consumer a module's declarations imply, or false when it subscribes
// to nothing and needs none.
//
// One per module, with every consumed subject as a filter, because its ack permission is derived
// from its name: a module with a consumer per event would need an ack permission per consumer,
// and the permission list would stop being derivable from the declaration.
func ConsumerFor(p Principal) (Consumer, bool) {
	if p.Kind != KindModule || len(p.Consumes) == 0 {
		return Consumer{}, false
	}
	perms, err := PermissionsFor(p)
	if err != nil {
		return Consumer{}, false
	}
	var filters []string
	for _, s := range perms.Subscribe {
		if len(s) > 9 && s[:9] == "mesh.mod." {
			filters = append(filters, s)
		}
	}
	if len(filters) == 0 {
		return Consumer{}, false
	}
	sort.Strings(filters)
	return Consumer{
		Name:           consumerDurable(p),
		Stream:         consumerStream(p),
		Filters:        filters,
		AckWaitSeconds: 30,
		MaxDeliver:     5,
		Why:            "what " + p.Module + " declared it consumes; after max-deliver it dead-letters",
	}, true
}

// HolderConsumerFor is the worker a seat's holder gets on that seat's work queue.
//
// **A queue group even though the seat guarantees one holder.** The seat is *authority* — who may
// be the telegram sender — and the queue group is *delivery*. Tie delivery to the seat and the
// day somebody allows two holders for throughput, every message is processed twice with nothing
// reporting it. Kept separate, relaxing one changes nothing about the other.
func HolderConsumerFor(node, module string, seat DeclaredSeat) (Consumer, bool) {
	if len(seat.Accepts) == 0 {
		return Consumer{}, false
	}
	return Consumer{
		Name:           "SEAT_" + upperSnake(seat.Name) + "_worker",
		Stream:         seatStreamName(seat.Name),
		Filters:        []string{"mesh.seat." + seat.Name + ".accept.>"},
		Queue:          "holders",
		AckWaitSeconds: 60,
		MaxDeliver:     5,
		Why: fmt.Sprintf("%s on %s holds %s; it acknowledges after the work is done, so a "+
			"crash mid-work redelivers rather than loses", module, node, seat.Name),
	}, true
}

// AllOverlaps reports subject filters claimed by more than one stream, across the mesh's own and
// every derived one.
//
// NATS refuses an overlapping stream rather than merging it (verified against nats-server 2.10:
// "subjects overlap with an existing stream"), so this is not a subtle divergence — it is a
// registration that fails. Catching it here names both streams, before a half-applied mesh does.
func AllOverlaps(seats []DeclaredSeat) []string {
	all := append(MeshStreams(), SeatStreams(seats)...)
	seen := map[string]string{}
	var clashes []string
	for _, s := range all {
		for _, subject := range s.Subjects {
			if first, ok := seen[subject]; ok {
				clashes = append(clashes, fmt.Sprintf("%s and %s both claim %s", first, s.Name, subject))
				continue
			}
			seen[subject] = s.Name
		}
	}
	sort.Strings(clashes)
	return clashes
}

// upperSnake makes a stream name from a seat name. NATS stream names may not contain a dot,
// a space or a wildcard, and a hyphen is legal but reads badly beside the mesh's own.
func upperSnake(s string) string {
	out := []rune(s)
	for i, r := range out {
		switch {
		case r >= 'a' && r <= 'z':
			out[i] = r - 32
		case r == '-' || r == '.':
			out[i] = '_'
		}
	}
	return string(out)
}