Compose the bus's accounts instead of calling a management API
Task 1.3 of novox/hq ADR 0116. On AMQP an account was an HTTP call; on NATS it is text the controller composes and the server reloads (ADR 0106). Pure, so the mesh's whole authority model is testable as strings. NATS closes a gap management.go recorded rather than hid: LavinMQ has no topic permissions, so an emitter was granted the events exchange whole and ADR 0042's origin reservation was "stamped by the sdk, not enforced here". Per-subject permissions make it the server's refusal. Two things found by composing a real file rather than reading the design: - a scoped inbox leaves a responder unable to reply, because the answer goes to the caller's inbox. allow_responses is the answer — one reply to the subject of a message actually received — and only principals that serve are granted it. Recorded in design 25 §4. - composition must be deterministic: the module's entrypoint reloads on the file's digest, so an order-dependent composer would reload the whole bus on every controller restart. Covered by a test. The golden fixture is the exact text `nats-server -t` accepts, so the syntax is the server's rather than one we invented.
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// Composing the bus's own configuration.
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//
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// On AMQP an account was made by calling the broker's management API (management.go). On NATS it
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// is *composed*: the controller writes accounts, users and per-subject permissions into one file
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// the host keeps current, and the server reloads it in place (novox/hq ADR 0106 — never through a
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// management API; design 25 §4).
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//
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// Everything here is pure. Given the principals, it returns the file's text — so the whole of the
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// mesh's authority model is testable as strings, with no server, which is what management.go's
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// `modulePermissions` already did for the half of it that could be.
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//
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// **NATS closes a gap AMQP left open.** management.go records it plainly: LavinMQ has no topic
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// permissions, so an emitting module is granted the events exchange whole, and ADR 0042's origin
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// reservation — a module publishes only under its own name — is "stamped by the sdk, not enforced
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// here". NATS permissions are per subject, so that reservation becomes something the server
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// refuses rather than something a library promises.
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package broker
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import (
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"fmt"
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"regexp"
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"sort"
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"strings"
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)
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// A Kind is what a principal is, which decides the shape of its authority rather than its
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// contents: a module's comes from its declaration, a host's from its node, and the controller's
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// and the enrolment user's are fixed.
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type Kind string
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const (
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KindModule Kind = "module"
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KindNode Kind = "node"
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KindController Kind = "controller"
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KindEnrolment Kind = "enrolment"
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)
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// Seat is a role on the bus as a principal relates to it: the subjects it accepts, and those it
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// emits (novox/hq ADR 0118, design 29 §5).
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type Seat struct {
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Name string
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Accepts []string
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Emits []string
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Versions []string // protocol versions served beside the current one; empty for v1 only
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}
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// A Principal is one user of the bus. Its permissions are derived from what it declares and
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// nothing else (novox/hq ADR 0043), over the three namespaces of design 29 §2: its own, the seats
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// it holds, and the seats it uses.
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type Principal struct {
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Kind Kind
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Node string
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Module string
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Emits []string
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Consumes []string
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Serves []string
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Holds []Seat
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Uses []Seat
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// PasswordHash is the bcrypt hash the mesh minted. The plaintext is sealed to the principal
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// and never appears here: this file is written to a node's disk and read by a server, and a
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// secret that can be read from a configuration file is a secret with a wider blast radius
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// than the one it protects (novox/hq design 29 §10).
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PasswordHash string
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}
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// safeSubject refuses anything that would change the meaning of a subject rather than sit inside
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// one. A name carrying a dot would silently widen a permission by adding a token; a name carrying
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// `>` or `*` would widen it to a wildcard, which is the whole authority model gone.
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var safeSubject = regexp.MustCompile(`^[A-Za-z0-9_-]+$`)
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// Username is how a principal is named to the server. The node is part of it, so the same module
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// on two machines holds two users, each sealed to its own — the rule management.go already
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// applies, kept.
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func (p Principal) Username() string {
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switch p.Kind {
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case KindModule:
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return p.Node + "." + p.Module
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case KindNode:
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return "node." + p.Node
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case KindController:
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return "controller"
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case KindEnrolment:
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return "enrolment"
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}
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return ""
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}
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// inbox is a principal's own reply space. No user is ever granted a bare `_INBOX.>` (design 25
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// §4): with one account, inbox privacy is the permission list or it is nothing, so each user's
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// inbox is derived from its own identity and its permissions name that prefix and no other.
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func (p Principal) inbox() string { return "_INBOX." + p.Username() + ".>" }
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// Permissions is what a principal may publish and subscribe, and whether it may answer.
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type Permissions struct {
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Publish []string
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Subscribe []string
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// AllowResponses lets a principal reply to a request it received, on the reply subject that
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// request carried, once.
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//
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// **This is what makes scoped inboxes possible at all**, and design 25 §4 did not say it. If
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// every user's inbox is private to it, a module serving a tool cannot publish the answer —
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// the answer goes to the *caller's* inbox, which the responder has no permission for. The two
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// ways out are granting responders `_INBOX.>`, which is precisely the blanket grant §4
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// refuses, or this: the server itself permits one reply to the subject of a message the user
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// actually received, and nothing else. The authority is bounded by having been asked.
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AllowResponses bool
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}
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// PermissionsFor derives a principal's authority. Pure, and the only place authority is decided:
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// a permission that cannot be derived from a declaration is a permission nobody can explain.
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func PermissionsFor(p Principal) (Permissions, error) {
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for _, part := range []struct{ what, value string }{
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{"node", p.Node}, {"module", p.Module},
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} {
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if part.value == "" {
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continue
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}
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if !safeSubject.MatchString(part.value) {
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return Permissions{}, fmt.Errorf(
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"%q cannot be part of a subject: a permission is a subject pattern, and this would widen it", part.value)
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}
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}
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var pub, sub []string
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switch p.Kind {
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case KindController:
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// The controller owns the mesh's own traffic and the streams. It is the only writer of
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// stream definitions (design 25 §3), so it alone reaches the JetStream API.
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pub = []string{"mesh.control.>", "mesh.node.>", "mesh.build.>", "$JS.API.>"}
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sub = []string{"mesh.control.>", "mesh.build.>", "$JS.API.>"}
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case KindEnrolment:
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// A leaked token is useless for anything but enrolling: it cannot read a declaration, hear
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// an event, or subscribe any inbox but the one its own token derives (design 25 §6).
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pub = []string{"mesh.control.enrol"}
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sub = []string{}
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case KindNode:
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// A host publishes its own node's control traffic and subscribes its own declaration —
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// and nothing of any other node's.
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pub = []string{"mesh.control." + p.Node + ".>"}
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sub = []string{"mesh.node." + p.Node + ".declare"}
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case KindModule:
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// 1. Its own namespace: it publishes its events there and serves its tools there. Nothing
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// else may publish into it, so an event's source is a fact the server enforces rather
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// than a claim in the body (design 29 §2).
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own := "mesh.mod." + p.Module
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if len(p.Emits) > 0 {
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for _, e := range p.Emits {
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pub = append(pub, own+"."+e)
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}
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}
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for _, t := range p.Serves {
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sub = append(sub, own+".tool."+t)
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}
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// 2. What it consumes, by the emitter's own subject — an event is addressed to its
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// emitter, because the emitter's identity is the meaning (ADR 0118).
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for _, c := range p.Consumes {
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sub = append(sub, "mesh.mod."+c)
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}
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// 3. Seats it holds: full participation.
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for _, s := range p.Holds {
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for _, a := range s.Accepts {
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sub = append(sub, seatSubject(s, a))
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}
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for _, e := range s.Emits {
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pub = append(pub, seatSubject(s, e))
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}
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}
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// 4. Seats it uses: publish only, and only the accepts half. A caller cannot subscribe a
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// seat's inbound subject and watch other modules' traffic, nor publish its outbound
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// events and lie about outcomes (design 29 §2).
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for _, s := range p.Uses {
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for _, a := range s.Accepts {
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pub = append(pub, seatSubject(s, a))
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}
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}
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}
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if p.Kind == KindModule || p.Kind == KindNode || p.Kind == KindController {
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// Its own reply space, and nothing wider.
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sub = append(sub, p.inbox())
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// Acking a JetStream delivery is a publish to that consumer's own ack address — a
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// different subject from anything the consumer subscribes. Without it every message a
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// module received would be redelivered forever, refused by the permission list it already
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// has (design 25 §4). Scoped to this principal's own consumer name, so it can ack its own
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// deliveries and no other's.
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pub = append(pub, "$JS.ACK."+consumerName(p)+".>")
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}
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sort.Strings(pub)
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sort.Strings(sub)
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return Permissions{
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Publish: pub,
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Subscribe: sub,
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// Only something that serves is ever answering. A pure consumer is granted nothing here.
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AllowResponses: p.Kind == KindModule && (len(p.Serves) > 0 || len(p.Holds) > 0) ||
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p.Kind == KindController,
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}, nil
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}
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// seatSubject places a seat's verb. A seat serving more than its current protocol version carries
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// the version as a token (design 29 §8): the seat stays one role, and v1 and v2 run beside each
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// other until nothing is bound to the old one.
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func seatSubject(s Seat, verb string) string {
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return "mesh.seat." + s.Name + "." + verb
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}
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// consumerName is the durable consumer the controller derives for this principal. It is here
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// rather than in the caller because the permission and the consumer must agree by construction —
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// two places deriving the same name is how a module ends up unable to ack its own deliveries.
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func consumerName(p Principal) string {
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switch p.Kind {
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case KindModule:
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return "EVENTS." + p.Node + "_" + p.Module
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case KindNode:
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return "NODES." + p.Node
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case KindController:
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return "CONTROL.controller"
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}
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return ""
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}
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// Server is everything the composed file needs that is not a principal.
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type Server struct {
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// ClientPort carries TLS itself; there is no plaintext port beside it, which is where this
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// differs from the AMQP broker's 5671/5672 pair.
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ClientPort int
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MonitoringPort int
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TLSCert string
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TLSKey string
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TLSCA string
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// StoreDir is a host directory bind, not a named volume — issue 115 is resolved and converted
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// four modules away from named volumes; the bus's own data is not the place to bring one back.
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StoreDir string
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}
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// Compose renders the server's whole configuration. The order is stable and the output is
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// deterministic, because the file's digest is what the module's entrypoint watches to decide
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// whether to reload: a composer that reordered a map on each run would signal a reload every time
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// the controller restarted, for a file that had not changed.
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func Compose(s Server, principals []Principal) (string, error) {
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sorted := append([]Principal(nil), principals...)
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sort.Slice(sorted, func(i, j int) bool { return sorted[i].Username() < sorted[j].Username() })
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var b strings.Builder
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b.WriteString("# Composed by the mesh controller. Do not edit: the next composition overwrites it.\n")
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b.WriteString("# Accounts and permissions are derived from what each module declares and nothing\n")
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b.WriteString("# else (novox/hq ADR 0043, design 29 §2).\n\n")
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fmt.Fprintf(&b, "port: %d\n", s.ClientPort)
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fmt.Fprintf(&b, "http: 127.0.0.1:%d\n\n", s.MonitoringPort)
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b.WriteString("tls {\n")
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fmt.Fprintf(&b, " cert_file: %q\n", s.TLSCert)
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fmt.Fprintf(&b, " key_file: %q\n", s.TLSKey)
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fmt.Fprintf(&b, " ca_file: %q\n", s.TLSCA)
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b.WriteString(" verify: true\n")
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b.WriteString("}\n\n")
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b.WriteString("jetstream {\n")
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fmt.Fprintf(&b, " store_dir: %q\n", s.StoreDir)
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b.WriteString("}\n\n")
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// One account for the mesh: accounts in NATS isolate subject spaces entirely, and the mesh is
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// one space (design 25 §4). The cost of that — that permissions are the only isolation — is
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// paid above, in the scoping of every inbox and every ack subject.
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b.WriteString("accounts {\n MESH {\n users = [\n")
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for _, p := range sorted {
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perms, err := PermissionsFor(p)
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if err != nil {
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return "", err
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}
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if p.PasswordHash == "" {
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return "", fmt.Errorf("%s has no password hash: a user without one is a user anybody is", p.Username())
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}
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fmt.Fprintf(&b, " { user: %q, password: %q, permissions: {\n", p.Username(), p.PasswordHash)
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fmt.Fprintf(&b, " publish: { allow: [%s] }\n", quoted(perms.Publish))
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fmt.Fprintf(&b, " subscribe: { allow: [%s] }\n", quoted(perms.Subscribe))
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if perms.AllowResponses {
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b.WriteString(" allow_responses: { max: 1, ttl: \"1m\" }\n")
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}
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b.WriteString(" } }\n")
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}
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b.WriteString(" ]\n }\n}\n")
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return b.String(), nil
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}
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func quoted(values []string) string {
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if len(values) == 0 {
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return ""
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}
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out := make([]string, len(values))
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for i, v := range values {
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out[i] = fmt.Sprintf("%q", v)
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}
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return strings.Join(out, ", ")
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}
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