rutster/docs/adr/0011-deployment-topology.md

# ADR-0011 — Deployment topology: one binary, three blessed shapes

- **Status:** Proposed
- **Date:** 2026-07-05
- **Closes:** the deployment-topology ADR reserved by [ADR-0008](0008-fob-and-green-zone.md)'s
  worked example ("the deployment topology itself may get its own ADR once that design closes").
- **Origin:** [2026-07-05 deployment-topology spec](../superpowers/specs/2026-07-05-deployment-topology-design.md)
  §2–§4. The TLS/edge fork was researched via a six-family survey — see the companion
  [TLS/edge decision brief](../superpowers/specs/2026-07-05-tls-edge-decision-brief.md).
- **Related:** [ADR-0002](0002-north-star-and-fused-core.md) (the fused per-call vertical no
  topology may split), [ADR-0005](0005-event-bus.md) (Valkey = bus + KV + presence; media never
  rides the bus; the bus is never billing source-of-truth), [ADR-0006](0006-ingress-posture.md)
  (two inbound protocols only; the tap is egress-only), [ADR-0007](0007-trunk-rented-transport.md)
  (rented transport ⇒ public HTTPS/WSS reachability is non-negotiable),
  [ADR-0008](0008-fob-and-green-zone.md) (the FOB/green-zone rule this ADR applies to
  deployment), [ADR-0009](0009-spend-gate-honest-rescope.md) (ledger trait: in-memory
  single-node, Valkey-backed fleets), [ADR-0010](0010-spearhead-benchmark-sim-harness.md)
  (sequencing this ADR must not reorder), README pillar 6 ("one binary, one bus, one deploy").

## Context

[ADR-0008](0008-fob-and-green-zone.md) made the build-vs-reuse doctrine physical in a worked
example — the all-in-one container — and reserved the topology ADR for when the design closed.
Two things have since closed it: the rented transport
([ADR-0007](0007-trunk-rented-transport.md)) is merged, so every deployment must now present a
public HTTPS/WSS edge to a CPaaS; and slice-5/seams landed the operational primitives a
deployment story needs (drain lifecycle, `/readyz`, admission cap, `MediaAddressConfig`,
`EventSink`).

The CPaaS side imposes a hard external spec on any edge rutster ships (researched in the
[decision brief](../superpowers/specs/2026-07-05-tls-edge-decision-brief.md); restated here
because these invariants are load-bearing for every decision below):

1. Publicly-CA-trusted cert; **no self-signed path exists** (Twilio error 31910). Auto-renewal
   is availability-critical.
2. wss:// on 443, open to the whole internet: Twilio publishes no egress IPs and offers no
   mTLS. Auth is application-layer only (HMAC signatures).
3. One WS per call, up to 24 h, 50 msg/s/direction at 20 ms cadence; zero tolerance for frame
   buffering, connection-lifetime caps, or write-side idle timers. Neither Twilio nor Telnyx
   documents WS keepalive — keepalive is entirely our job.
4. `<Stream>` URLs carry **no query strings** — wss routing keys on hostname/path only.
5. `X-Twilio-Signature` is HMAC over the URL *as Twilio saw it*: the edge must forward
   `X-Forwarded-Proto/Host` honestly and the FOB must reconstruct the public URL.
6. Webhook budget: sub-second for UX, 15 s hard cap — no blocking issuance in the handshake
   path.
7. TLS 1.2+1.3, mainstream ECDHE, never 1.3-only; never pin Twilio certs.

Design for Twilio; Telnyx/Vonage come for free (strictly less demanding, plus optional
source-IP allowlisting as extra hardening Twilio can't use).

On the other side, the FOB's own physics constrain every shape: per-call state is in-process
and non-migratable (the fused vertical, [ADR-0002](0002-north-star-and-fused-core.md)), WebRTC
media is per-session UDP direct to the process, and calls run for hours. Any topology that
assumes migratable, short-lived, connectionless workloads is disqualified before it starts.

## Decision

**The FOB never decomposes; topology is a configuration property, not a code property.** The
same binary runs in every blessed shape; only `RUTSTER_*` env and what is deployed alongside it
differ. "Microservices" in rutster means *green-zone services get their own
processes/containers* — never that the per-call vertical splits
([ADR-0002](0002-north-star-and-fused-core.md)).

### The three blessed shapes

#### T1 — Solo (all-in-one container) — shipped artifact

One image (`rutster-allinone`), s6-overlay as PID 1 supervising four processes:

| Process | Role | Binding |
|---|---|---|
| `caddy` | Edge: ACME, TLS termination, WS proxy to FOB | :443/:80 public |
| `rutster` (FOB) | Engine | 127.0.0.1:8080 (behind Caddy); media UDP direct |
| `rutster-brain-realtime` | Brain | 127.0.0.1:8082 loopback tap (as today) |
| `valkey-server` | Dark on day one except `EventSink` | 127.0.0.1:6379 |

Two volumes, both non-negotiable: `/data` (Caddy cert/ACME state — loss risks the Let's
Encrypt duplicate-cert lockout, a total-inbound-outage class of failure) and `/var/lib/valkey`
(stream/state persistence). WebRTC media UDP goes **direct to the FOB**, never through Caddy:
host networking recommended; otherwise published `RUTSTER_MEDIA_PORT_RANGE` +
`RUTSTER_MEDIA_ADVERTISED_IP` (NAT 1:1 model, no STUN — unchanged from slice-5/seams).

Valkey ships bundled from v1 **before most consumers exist**: the operator contract (volumes,
ports, upgrade shape) is stable from day one, so the spend ledger
([ADR-0009](0009-spend-gate-honest-rescope.md)) and the fleet directory land later without
changing the deployment shape. Cost accepted: ~15 MB and one mostly-idle process.

#### T2 — Modular (compose stack) — shipped artifact, the reference deployment

Services: `caddy` (image `rutster-edge`), `engine` (`rutster-engine`), `brain`
(`rutster-brain`), `valkey` (upstream `valkey/valkey`).

- **The brain shares the engine's network namespace** (`network_mode: "service:engine"`) so
  the loopback-only tap posture (`resolve_tap_url` rejects non-loopback until step 6) survives
  unchanged. The brain graduates to its own netns when the wss:// tap lands (step 6).
- Valkey is its own service on the compose network — network-near, satisfying
  [ADR-0005](0005-event-bus.md)'s sub-ms rule.
- `stop_grace_period: 660s` paired with `RUTSTER_DRAIN_DEADLINE_SECS=600` (grace > drain).
- BYO-proxy supported: disable the `caddy` service; the engine keeps its plaintext `:8080`
  mode; tuned-timeout snippets for nginx/HAProxy/Traefik ship in
  [`docs/deploy/reverse-proxies.md`](../deploy/reverse-proxies.md).

#### T3 — Fleet — ADR chapter only (paper; no code in this epoch)

N **symmetric** nodes, each running the T1 process set *minus* valkey, plus a shared green
zone: one Valkey (presence + directory + the [ADR-0009](0009-spend-gate-honest-rescope.md)
spend ledger) and object storage (named as required, still deferred). Wildcard DNS
`*.pbx.domain`; per-node certs (see the fleet cert story below). The per-node admission cap is
the [ADR-0010](0010-spearhead-benchmark-sim-harness.md) benchmark number (replacing the
`RUTSTER_MAX_SESSIONS=64` placeholder). Scale-in is drain-then-terminate only; spot instances
are rejected for the engine tier (2026-07-04 review — a reclaimed node kills every
non-migratable call on it).

### Edge doctrine

#### Bundled edge: Caddy (both shipped artifacts, same build, same Caddyfile)

Custom xcaddy build (the `rutster-edge` image): Caddy core (Apache-2.0) + a curated DNS-01
plugin set — cloudflare, route53, porkbun, hetzner, desec (duckdns excluded: no license file).
Zero-downtime in-memory cert renewal; verified protocol-unaware WS tunneling (no frame
buffering). Config is a ~6-line Caddyfile with tuned timeouts, honest `X-Forwarded-*`, and
`stream_close_delay` above max call duration — **plus a CI e2e case for
config-reload-during-live-call, because the upstream mitigation has an open bug trail
(caddy #6420/#7222) and is not trusted untested.**

#### In-process TLS: staged — the ratified end-state, not adopted wholesale now

[ADR-0008](0008-fob-and-green-zone.md) claims the trust-domain edge as security-constitutive
FOB territory, and str0m already terminates DTLS-SRTP in-process on the same aws-lc-rs
provider — "keep TLS out of the FOB" is not the objection. The objection is economics: owning
an ACME renewal state machine forever, in a 45-day-cert world.

- **Phase 1 (this epoch):** `axum-server` + rustls with operator-supplied certs
  (`RUTSTER_TLS_CERT`/`RUTSTER_TLS_KEY`), hot-reload without dropping live WS.
  Plaintext-behind-Caddy stays the artifact default.
- **Phase 2 (in-binary ACME) triggers, named:** (a) Let's Encrypt dns-persist-01 confirmed GA
  (closes the wildcard/CGNAT issuance gap; instant-acme already parses it); (b) a field
  incident where Caddy reload/`stream_close_delay` drops live calls; (c) fleet or
  VPS-forwarder topology making encrypted-to-the-binary a selling point; (d) maintenance
  fatigue with the curated xcaddy plugin build. Until a trigger fires, Phase 2 is deliberately
  not built — no turnkey ACME crate fits the tree today (rustls-acme conflicts with the
  deny.toml rcgen/x509-parser posture).

#### Fleet cert story

Per-node public TLS names are CPaaS-imposed (invariant 4 + node-addressed placement). Two
blessed patterns:

1. **Wildcard `*.pbx.domain` via DNS-01** with central issuance/distribution.
2. **Per-node distinct certs** (`node-N.domain`) where the wildcard private key must not live
   on every node — one compromised node otherwise burns the whole namespace.

Traps, stated so nobody re-discovers them in production: N nodes independently requesting the
*identical* wildcard hit the Let's Encrypt duplicate-cert limit (5/week) — distinct names or
central distribution only. **Caddy on-demand TLS is rejected for node names**: the first
handshake to a fresh name blocks seconds on issuance, colliding with the sub-second webhook
budget (invariant 6), and its rate-limit knobs are deprecated. Node names known at provision
time get pre-issued certs.

### Fleet placement paradigm (the trunk webhook handler and session API grow *toward* this; no fleet code in this epoch)

**Node-addressed URLs + Valkey presence; no router tier.** (Decision 2026-07-05 over the two
rejected alternatives below.)

- Nodes heartbeat capacity into Valkey (TTL'd presence keys — the
  [ADR-0005](0005-event-bus.md) presence layer).
- Placement happens **at answer time** by whichever node the request lands on (`api.domain`,
  DNS round-robin): read fleet capacity → pick owner → return TwiML whose Stream URL is
  `wss://node-K.pbx.domain/...`. The media WS dials the owning node directly — zero extra
  hops, consistent with connection-follows-ownership. Twilio's answer-time Stream URL is
  purpose-shaped for this: invariant 4 permits hostname routing.
- WebRTC: the create-session response carries the owner's node-addressed base URL + advertised
  media IP (UDP is already direct).
- Misdirected control ops (e.g. a DELETE hitting a non-owner) consult the Valkey directory and
  **307-redirect** to the owner.
- **Degraded mode (Valkey dark): the answering node self-places if it has capacity**; presence
  heals via TTL on recovery. Fail-safe degradation, consistent with
  [ADR-0009](0009-spend-gate-honest-rescope.md)'s posture.
- Single-node is the degenerate case of the same code: a one-entry directory.

**Rejected: a routing tier that proxies everything.** It would put every 20 ms media frame
through an extra userspace hop and re-create exactly the SPOF the symmetric design avoids —
and it buys nothing, because the CPaaS already lets the answer-time URL do the routing.

**Rejected: a distinguished control node.** It breaks node symmetry (two artifacts, two
failure stories, a promotion protocol) to solve a problem the presence directory already
solves; placement needs fleet state either way, and Valkey is already the fleet-state home
([ADR-0005](0005-event-bus.md)).

### Rejected shapes

- **Decomposing the FOB into services** — rejected **permanently**, not deferred.
  Media/reflex/tap/spend stay one process, one trust domain
  ([ADR-0002](0002-north-star-and-fused-core.md)).
- **Serverless.** Lambdas are categorically unfit for the engine: non-migratable multi-hour
  calls, per-session UDP sockets, in-process media state. Webhook-on-Lambda buys nothing
  because placement needs fleet state anyway.
- **Tunnels in production.** Cloudflare Tunnel and ngrok see plaintext audio at the vendor
  edge (an unconsented subprocessor — DPA/BAA/PCI failure), document mid-call WS kills, and
  cloudflared has an open Twilio handshake bug (cloudflared #1465). ngrok is the blessed
  5-minute demo path (the only tunnel with a proven Media Streams record); Tailscale Funnel is
  the privacy-clean single-user demo (TLS terminates on-node), unsizable beyond that. **No
  tunnel carries inbound UDP → homelab behind CGNAT is PSTN-only, period.** The production
  graduation path is a cheap VPS + WireGuard as a dumb TCP forwarder with TLS terminating at
  home, or running the engine on the VPS — see
  [`docs/deploy/homelab.md`](../deploy/homelab.md).
- **NLB TLS listeners — never.** Fixed 350 s idle timeout that silently drops flows, and no
  HTTP context, so `X-Forwarded-*` headers for signature validation (invariant 5) don't exist.
  The ALB path is documented as a docs-only appendix
  ([`docs/deploy/aws.md`](../deploy/aws.md)), never a shipped artifact.
- **k8s manifests/Helm.** Compose-first (README pillar 6). k8s notes live in docs only (drain
  vs `terminationGracePeriodSeconds`); manifests are a later rung if demand appears.

### Deferred, named (so the boundary is deliberate)

- **T3 fleet implementation** (presence heartbeats, directory redirects, placement code).
- **The zero-egress / air-gap profile** (SBC layer-2 ingress per
  [ADR-0007](0007-trunk-rented-transport.md) layer 2 + a self-hosted brain): a defined
  profile, no code this epoch. T1 is about one-command simplicity, not isolation.
- **In-binary ACME (rustls Phase 2)** — behind the four named triggers above.
- **Durable CDR pipeline**: `ValkeyEventSink` is evidence preservation on a capped stream —
  explicitly NOT the billing ledger ([ADR-0005](0005-event-bus.md) source-of-truth rule).
  CDR → object storage stays deferred.
- **Multi-tenancy** (needs its own ADR before schemas ossify); wss:// tap + brain fleet +
  resume tokens (step 6); media-thread sharding graduation; arm64 images.

## Consequences

- **Positive:** one binary and one mental model across all three shapes — an operator's
  knowledge transfers from `docker run` to a fleet; the CPaaS invariants are satisfied by a
  decade-hardened ACME loop (Caddy) rather than a DIY renewal state machine; the fleet
  paradigm needs zero new infrastructure beyond what [ADR-0005](0005-event-bus.md) already
  bundles; the rejected shapes are written down, so nobody burns a week discovering that
  Lambda or NLB-TLS can't work.
- **Negative:** the curated xcaddy DNS-plugin set is a permanent maintenance surface until
  dns-persist-01 GA; Caddy config-reload behavior vs live calls is mitigated
  (`stream_close_delay` + CI e2e) but has an open upstream bug trail; homelab WebRTC behind
  CGNAT is unsolved **by design** — only the VPS graduation fixes it, and the docs must say so
  loudly or operators will read "homelab supported" as including browser calls.
- **Mitigation:** the Phase-2 triggers convert both negatives into a planned exit; the
  `docs/deploy/` tree ships with a copy-paste-to-first-call acceptance bar so the honest
  limits are in the operator's face, not in a footnote.

## References

- [2026-07-05 deployment-topology spec](../superpowers/specs/2026-07-05-deployment-topology-design.md) — §2–§4 are this ADR's source
- [TLS/edge decision brief](../superpowers/specs/2026-07-05-tls-edge-decision-brief.md) — six-family survey + the CPaaS ground truth (external citations live there)
- [ADR-0002](0002-north-star-and-fused-core.md) · [ADR-0005](0005-event-bus.md) · [ADR-0006](0006-ingress-posture.md) · [ADR-0007](0007-trunk-rented-transport.md) · [ADR-0008](0008-fob-and-green-zone.md) · [ADR-0009](0009-spend-gate-honest-rescope.md) · [ADR-0010](0010-spearhead-benchmark-sim-harness.md)
- The `docs/deploy/` tree: [topologies](../deploy/topologies.md) · [quickstart-docker](../deploy/quickstart-docker.md) · [homelab](../deploy/homelab.md) · [aws](../deploy/aws.md) · [reverse-proxies](../deploy/reverse-proxies.md) · [certificates](../deploy/certificates.md)