Copy go.sum into the Docker build; pin compose example to v0.1.5

The image build broke with the first Linux-imported dependency
(go-systemd): the Dockerfile copied go.mod only, and the missing go.sum
never mattered while golang.org/x/sys was Windows-only.

.gitea/workflows/ci.yml

@@ -80,8 +80,12 @@ jobs:
             -o gpu-turnstile.exe ./cmd/gpu-turnstile
 
       - name: Sign and checksum
+        env:
+          RELEASE_SIGNING_KEY: ${{ secrets.RELEASE_SIGNING_KEY }}
         run: |
-          printf '%s\n' "${{ secrets.RELEASE_SIGNING_KEY }}" > key.pem
+          # The key comes via the environment so its value never appears in
+          # the echoed command line of the run log.
+          printf '%s\n' "$RELEASE_SIGNING_KEY" > key.pem
           chmod 600 key.pem
           openssl pkeyutl -sign -inkey key.pem -rawin \
             -in gpu-turnstile.exe -out gpu-turnstile.exe.sig

Dockerfile

@@ -1,6 +1,6 @@
 FROM golang:1.23 AS build
 WORKDIR /src
-COPY go.mod ./
+COPY go.mod go.sum ./
 COPY cmd ./cmd
 COPY internal ./internal
 ARG VERSION=dev

README.md

@@ -29,6 +29,12 @@ Open WebUI / n8n ────► :8188  ───┘
 - Everything else (including websockets and all streaming) passes through
   transparently and unbuffered.
 
+Each consumer is enabled by setting its URL (`OLLAMA_URL`, `COMFY_URL`) and
+disabled by leaving it empty — at least one is required. With only Ollama
+the proxy is a pass-through (no image jobs can arrive); with only ComfyUI
+the Ollama unload/warm steps are skipped. Future consumers (e.g. local game
+detection) plug into the same lock the same way.
+
 ## Configuration
 
 Configuration comes from environment variables and/or an `.env`-style
@@ -41,8 +47,8 @@ override file values. Invalid values fail at startup.
 |---|---|---|
 | `LISTEN_OLLAMA` | `:11434` | Ollama-facing listener |
 | `LISTEN_COMFY` | `:8188` | ComfyUI-facing listener |
-| `OLLAMA_URL` | `http://127.0.0.1:11435` | Ollama upstream |
-| `COMFY_URL` | `http://127.0.0.1:8189` | ComfyUI upstream |
+| `OLLAMA_URL` | _(empty = disabled)_ | Ollama upstream; set to enable the Ollama consumer |
+| `COMFY_URL` | _(empty = disabled)_ | ComfyUI upstream; set to enable the ComfyUI consumer |
 | `UNLOAD_TIMEOUT` | `60s` | Wait for Ollama to unload before an image job |
 | `JOB_TIMEOUT` | `15m` | Wait for a ComfyUI job to finish |
 | `LLM_WAIT_TIMEOUT` | `10m` | Max lock wait for an LLM request before 503 (wait mode) |
@@ -70,7 +76,7 @@ override file values. Invalid values fail at startup.
 ## Observability
 
 - `GET /healthz` (both listeners): `{"state":"idle|llm|image","llm_inflight":N,"image_pending":B}`
-- `GET /metrics` (Ollama listener): Prometheus text format — `gpu_turnstile_state`,
+- `GET /metrics` (both listeners): Prometheus text format — `gpu_turnstile_state`,
   `gpu_turnstile_llm_inflight`, `gpu_turnstile_image_pending`,
   `gpu_turnstile_image_jobs_total`, `gpu_turnstile_lock_wait_seconds`
   (histogram, `kind="llm|image"`), `gpu_turnstile_unload_seconds`.
@@ -88,15 +94,15 @@ go build ./cmd/gpu-turnstile
 ./gpu-turnstile
 ```
 
-### Run natively on Windows (primary deployment)
+### Run natively on Windows (current primary deployment)
 
 Download `gpu-turnstile.exe` from a release, put a `gpu-turnstile.env`
 next to it, and run it — or install it as a Windows service from an
 elevated shell:
 
 ```sh
-gpu-turnstile.exe service install          # auto-start service, recovery = restart
-gpu-turnstile.exe service remove
+gpu-turnstile.exe --install-service        # auto-start service, recovery = restart
+gpu-turnstile.exe --remove-service
 ```
 
 The service uses the config file (services have no convenient
@@ -109,6 +115,23 @@ the install directory for self-updates. For least privilege, run it as the
 virtual account `NT SERVICE\gpu-turnstile` and grant write access to just
 those two directories.
 
+### Run natively on Linux (systemd)
+
+The same binary works on Linux. Install it as a systemd service as root:
+
+```sh
+gpu-turnstile --install-service            # writes + enables + starts the unit
+gpu-turnstile --remove-service
+```
+
+The unit (`/etc/systemd/system/gpu-turnstile.service`) is `Type=notify`:
+`systemctl start` blocks until the listeners are actually bound, a 30 s
+watchdog restarts the process if it wedges, and logs land in the journal
+(`journalctl -u gpu-turnstile -f`) unless `LOG_FILE` is set. Put the
+config in a `gpu-turnstile.env` next to the binary (or pass
+`-config /path` during install). The notify integration is a no-op in
+containers and interactive shells.
+
 **Auto-update is on by default**: the binary checks the repo's latest
 release on startup and every `UPDATE_INTERVAL`, verifies the Ed25519
 signature of the download against the public key embedded at build time,

SPEC.md

@@ -42,6 +42,21 @@ listener is an `httputil.ReverseProxy` to its upstream. Websocket upgrades
 (ComfyUI `/ws`) and streaming bodies (Ollama NDJSON / SSE) must pass through
 unbuffered (`FlushInterval = -1`).
 
+### Modes of operation
+
+Each GPU consumer is enabled by setting its URL and disabled by leaving it
+empty — no separate flags. At least one URL must be set; a disabled
+consumer gets no listener, no startup probe, and no lock participation:
+
+- **Both set** (default deployment): full arbitration as described below.
+- **Only `OLLAMA_URL`**: pure pass-through for Ollama; the LLM lock never
+  blocks since no image jobs can arrive.
+- **Only `COMFY_URL`**: image jobs are tracked and ComfyUI's VRAM is freed
+  afterwards, but the Ollama unload and warm-reload steps are skipped.
+- Future consumers (e.g. detecting a local game holding VRAM) plug into the
+  same lock the same way: enabled by their config knob, excluded when
+  absent.
+
 ### Lock semantics
 
 Two-mode lock with image priority (writer-preferring RW lock, where "readers"
@@ -83,7 +98,8 @@ ComfyUI listener (`:8188` → `COMFY_URL`):
 ### Image job flow (`POST /prompt`)
 
 1. `AcquireImage()`.
-2. Unload Ollama: `GET /api/ps`; for each model `POST /api/generate
+2. Unload Ollama (skipped when `OLLAMA_URL` is unset): `GET /api/ps`; for
+   each model `POST /api/generate
    {"model":M,"keep_alive":0}`; if that returns non-2xx (embedding-only
    models), `POST /api/embed {"model":M,"input":"x","keep_alive":0}`. Poll
    `/api/ps` every `UNLOAD_POLL_INTERVAL` (default 500 ms) until empty or
@@ -117,8 +133,8 @@ override file values. A missing file is fine; a malformed one is fatal.
 |---|---|---|
 | `LISTEN_OLLAMA` | `:11434` | Ollama-facing listener |
 | `LISTEN_COMFY` | `:8188` | ComfyUI-facing listener |
-| `OLLAMA_URL` | `http://127.0.0.1:11435` | upstream |
-| `COMFY_URL` | `http://127.0.0.1:8189` | upstream |
+| `OLLAMA_URL` | _(empty = disabled)_ | Ollama upstream; set to enable the Ollama consumer |
+| `COMFY_URL` | _(empty = disabled)_ | ComfyUI upstream; set to enable the ComfyUI consumer |
 | `UNLOAD_TIMEOUT` | `60s` | wait for Ollama to unload |
 | `JOB_TIMEOUT` | `15m` | wait for ComfyUI job |
 | `LLM_WAIT_TIMEOUT` | `10m` | max time an LLM request waits for the lock before 503 (wait mode) |
@@ -143,17 +159,25 @@ override file values. A missing file is fine; a malformed one is fatal.
 | `UPDATE_REPO` | `https://git.rambossek.at/PUBLIC/gpu-turnstile` | repository to check for releases |
 | `UPDATE_ASSET` | `gpu-turnstile.exe` | release asset to download |
 
-Startup fails fast on unparsable values. Both upstreams are probed once at
-start (`/api/version`, `/system_stats`); failure is logged, not fatal.
+Startup fails fast on unparsable values and when neither consumer URL is
+set. Enabled upstreams are probed once at start (`/api/version`,
+`/system_stats`); failure is logged, not fatal.
 
-## Native Windows deployment
+## Native deployment (Windows and Linux)
 
-The binary runs natively on Windows (the primary deployment) as well as in
-Docker.
+The binary runs natively on Windows (the current primary deployment) and on
+Linux with systemd (the future GPU server), as well as in Docker.
 
-- `gpu-turnstile.exe service install [-config path]` registers an
-  auto-start Windows service (needs an elevated shell). Recovery actions
-  restart it 5 s after any failure. `service remove` uninstalls.
+Service management is the same on both platforms:
+`gpu-turnstile --install-service [-config path]` registers and starts an
+auto-start service; `--remove-service` stops and unregisters it (both need
+an elevated/root shell). The legacy form `gpu-turnstile service
+install|remove` does the same thing.
+
+### Windows
+
+- `--install-service` registers a Windows service; recovery actions restart
+  it 5 s after any failure.
 - **Layout**: install to `C:\Program Files\gpu-turnstile\` (exe plus
   `gpu-turnstile.env`); logs belong in `C:\ProgramData\gpu-turnstile\` via
   `LOG_FILE`. The service must be able to write its install directory for
@@ -165,6 +189,27 @@ Docker.
   log directories only (no network logon, no user profile).
 - Use a config file (above) for the service — Windows services have no
   convenient environment. Logs go to `LOG_FILE` since there is no console.
+
+### Linux (systemd)
+
+- `--install-service` writes `/etc/systemd/system/gpu-turnstile.service`
+  with `ExecStart` pointing at the current executable and the `-config`
+  file, then runs `systemctl daemon-reload` and `enable --now`. The unit
+  runs as root (it must be able to overwrite its own binary for
+  self-updates); harden with `ProtectSystem=strict` plus a writable
+  `ReadWritePaths` if desired.
+- The unit is `Type=notify`: the binary sends `READY=1` via
+  `github.com/coreos/go-systemd` only after the listeners are bound, so
+  `systemctl start` blocks until the proxy accepts connections. A 30 s
+  watchdog (`WatchdogSec=`) is pinged as long as the process runs; three
+  missed pings make systemd restart it. `STOPPING=1` is sent on shutdown.
+  All notify calls are no-ops when `NOTIFY_SOCKET` is unset (containers,
+  interactive shells), and the whole integration is Linux-only — Windows
+  builds carry no-op stubs.
+- Logs go to the journal (`journalctl -u gpu-turnstile`) or to `LOG_FILE`.
+- **Auto-update** works the same as on Windows: `Restart=on-failure` with
+  `RestartSec=5s` brings up the staged binary after the updater exits with
+  code 3.
 - **Auto-update**: on startup and every `UPDATE_INTERVAL`, the binary
   checks `UPDATE_REPO`'s latest release; if its tag is a newer `vX.Y.Z`,
   it downloads `UPDATE_ASSET` plus its `.sig` (and `.sha256` when present)
@@ -184,7 +229,7 @@ Docker.
 
 - `GET /healthz` on both listeners: 200 with JSON
   `{"state":"idle|llm|image","llm_inflight":N,"image_pending":B}`.
-- `GET /metrics` on the Ollama listener: Prometheus text format, no external
+- `GET /metrics` on both listeners: Prometheus text format, no external
   dependency needed:
   `gpu_turnstile_state{state="…"} 1`, `gpu_turnstile_llm_inflight`,
   `gpu_turnstile_image_jobs_total`, `gpu_turnstile_lock_wait_seconds`
@@ -233,7 +278,7 @@ gpu-turnstile/
   internal/metrics/          # Prometheus exposition
   internal/config/           # env + .env file configuration
   internal/update/           # signed auto-updater (public key in pubkey.go)
-  internal/service/          # Windows service integration
+  internal/service/          # Windows SCM + Linux systemd (notify/watchdog) integration
   Dockerfile
   .gitea/workflows/ci.yml
   README.md
@@ -261,8 +306,9 @@ are new.
 
 ## Build and CI
 
-- Go 1.23+, `golang.org/x/sys` is the only external dependency (Windows
-  service integration; not used in the Linux build). `CGO_ENABLED=0`,
+- Go 1.23+, two external dependencies: `golang.org/x/sys` (Windows service
+  integration) and `github.com/coreos/go-systemd` (systemd notify/watchdog,
+  Linux build only). `CGO_ENABLED=0`,
   `-ldflags="-s -w"`, version from `git describe` injected via
   `-X main.version=`.
 - Dockerfile: multi-stage, final image `gcr.io/distroless/static` (or

cmd/gpu-turnstile/main.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"net"
 	"net/http"
 	"os"
 	"os/signal"
@@ -34,12 +35,22 @@ var version = "dev"
 const exitCodeUpdate = 3
 
 func main() {
-	configPath, args := splitConfigFlag(os.Args[1:])
+	configPath, install, remove, args := parseFlags(os.Args[1:])
+	switch {
+	case install && remove:
+		fmt.Fprintf(os.Stderr, "gpu-turnstile: --install-service and --remove-service are mutually exclusive\n")
+		os.Exit(2)
+	case install:
+		os.Exit(serviceCommand(configPath, []string{"install"}))
+	case remove:
+		os.Exit(serviceCommand(configPath, []string{"remove"}))
+	}
 	if len(args) > 0 && args[0] == "service" {
 		os.Exit(serviceCommand(configPath, args[1:]))
 	}
 	if len(args) > 0 {
-		fmt.Fprintf(os.Stderr, "usage: gpu-turnstile [-config path] | gpu-turnstile service install|remove [-config path]\n")
+		fmt.Fprintf(os.Stderr, "usage: gpu-turnstile [-config path] [--install-service | --remove-service]\n")
+		fmt.Fprintf(os.Stderr, "       gpu-turnstile service install|remove [-config path]\n")
 		os.Exit(2)
 	}
 
@@ -70,10 +81,10 @@ func main() {
 	}
 }
 
-// splitConfigFlag extracts -config <path> (or -config=<path>) from args.
-func splitConfigFlag(args []string) (string, []string) {
-	var configPath string
-	rest := args[:0]
+// parseFlags extracts -config <path> (or -config=<path>) and the
+// --install-service / --remove-service switches from args.
+func parseFlags(args []string) (configPath string, install, remove bool, rest []string) {
+	rest = args[:0]
 	for i := 0; i < len(args); i++ {
 		switch {
 		case args[i] == "-config" && i+1 < len(args):
@@ -81,11 +92,15 @@ func splitConfigFlag(args []string) (string, []string) {
 			i++
 		case strings.HasPrefix(args[i], "-config="):
 			configPath = strings.TrimPrefix(args[i], "-config=")
+		case args[i] == "--install-service" || args[i] == "-install-service":
+			install = true
+		case args[i] == "--remove-service" || args[i] == "-remove-service":
+			remove = true
 		default:
 			rest = append(rest, args[i])
 		}
 	}
-	return configPath, rest
+	return configPath, install, remove, rest
 }
 
 // defaultConfigPath returns gpu-turnstile.env next to the executable.
@@ -180,6 +195,14 @@ func serviceCommand(configPath string, args []string) int {
 	return 0
 }
 
+// orDisabled renders an empty URL as "disabled" for the startup dump.
+func orDisabled(url string) string {
+	if url == "" {
+		return "disabled"
+	}
+	return url
+}
+
 func run(ctx context.Context, cfg config.Config, log *slog.Logger, logOut io.Writer, isService bool) error {
 	// The startup line carries the version and every setting and is emitted
 	// at WARN so it is visible even with the default (quiet) log level.
@@ -187,8 +210,8 @@ func run(ctx context.Context, cfg config.Config, log *slog.Logger, logOut io.Wri
 		"version", version,
 		"listen_ollama", cfg.ListenOllama,
 		"listen_comfy", cfg.ListenComfy,
-		"ollama_url", cfg.OllamaURL,
-		"comfy_url", cfg.ComfyURL,
+		"ollama_url", orDisabled(cfg.OllamaURL),
+		"comfy_url", orDisabled(cfg.ComfyURL),
 		"unload_timeout", cfg.UnloadTimeout,
 		"job_timeout", cfg.JobTimeout,
 		"llm_wait_timeout", cfg.LLMWaitTimeout,
@@ -215,13 +238,20 @@ func run(ctx context.Context, cfg config.Config, log *slog.Logger, logOut io.Wri
 	)
 
 	lk := lock.New(log)
-	ollamaClient, err := ollama.New(cfg.OllamaURL, log)
-	if err != nil {
-		return err
+	// Each consumer is enabled by setting its URL; a disabled consumer gets
+	// no client, no listener and no probe.
+	var ollamaClient *ollama.Client
+	var err error
+	if cfg.OllamaURL != "" {
+		if ollamaClient, err = ollama.New(cfg.OllamaURL, log); err != nil {
+			return err
+		}
 	}
-	comfyClient, err := comfy.New(cfg.ComfyURL, log)
-	if err != nil {
-		return err
+	var comfyClient *comfy.Client
+	if cfg.ComfyURL != "" {
+		if comfyClient, err = comfy.New(cfg.ComfyURL, log); err != nil {
+			return err
+		}
 	}
 
 	srv, err := proxy.New(proxy.Config{
@@ -253,22 +283,54 @@ func run(ctx context.Context, cfg config.Config, log *slog.Logger, logOut io.Wri
 		return err
 	}
 
-	// Probe both upstreams once; failure is logged, not fatal.
+	// Probe the enabled upstreams once; failure is logged, not fatal.
 	probeCtx, probeCancel := context.WithTimeout(ctx, cfg.ProbeTimeout)
-	if err := ollamaClient.Probe(probeCtx); err != nil {
-		log.Warn("ollama probe failed", "url", cfg.OllamaURL, "err", err)
+	if ollamaClient != nil {
+		if err := ollamaClient.Probe(probeCtx); err != nil {
+			log.Warn("ollama probe failed", "url", cfg.OllamaURL, "err", err)
+		}
 	}
-	if err := comfyClient.Probe(probeCtx); err != nil {
-		log.Warn("comfy probe failed", "url", cfg.ComfyURL, "err", err)
+	if comfyClient != nil {
+		if err := comfyClient.Probe(probeCtx); err != nil {
+			log.Warn("comfy probe failed", "url", cfg.ComfyURL, "err", err)
+		}
 	}
 	probeCancel()
 
-	ollamaSrv := &http.Server{Addr: cfg.ListenOllama, Handler: srv.OllamaHandler()}
-	comfySrv := &http.Server{Addr: cfg.ListenComfy, Handler: srv.ComfyHandler()}
+	// Bind the listeners up front so a port conflict fails fast and the
+	// readiness notification below really means "accepting connections".
+	var servers []*http.Server
+	var listeners []net.Listener
+	bind := func(addr string, handler http.Handler, consumer string) error {
+		ln, err := net.Listen("tcp", addr)
+		if err != nil {
+			return fmt.Errorf("listen %s on %s: %w", consumer, addr, err)
+		}
+		servers = append(servers, &http.Server{Addr: addr, Handler: handler})
+		listeners = append(listeners, ln)
+		log.Warn("listening", "consumer", consumer, "addr", addr)
+		return nil
+	}
+	if ollamaClient != nil {
+		if err := bind(cfg.ListenOllama, srv.OllamaHandler(), "ollama"); err != nil {
+			return err
+		}
+	}
+	if comfyClient != nil {
+		if err := bind(cfg.ListenComfy, srv.ComfyHandler(), "comfy"); err != nil {
+			return err
+		}
+	}
 
-	errCh := make(chan error, 2)
-	go func() { errCh <- ollamaSrv.ListenAndServe() }()
-	go func() { errCh <- comfySrv.ListenAndServe() }()
+	errCh := make(chan error, len(servers))
+	for i := range servers {
+		go func(s *http.Server, ln net.Listener) { errCh <- s.Serve(ln) }(servers[i], listeners[i])
+	}
+
+	// Tell systemd we are up and start the watchdog pings; both are no-ops
+	// when not running under a notify/watchdog unit.
+	service.NotifyReady()
+	service.StartWatchdog(ctx)
 
 	if cfg.AutoUpdate {
 		go updateLoop(ctx, cfg, log, lk, isService)
@@ -285,8 +347,9 @@ func run(ctx context.Context, cfg config.Config, log *slog.Logger, logOut io.Wri
 
 	shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), cfg.ShutdownTimeout)
 	defer shutdownCancel()
-	ollamaSrv.Shutdown(shutdownCtx)
-	comfySrv.Shutdown(shutdownCtx)
+	for _, s := range servers {
+		s.Shutdown(shutdownCtx)
+	}
 	return nil
 }
 

compose.yaml.example

@@ -6,9 +6,11 @@
 # ComfyUI --listen 0.0.0.0 --port 8189).
 services:
   gpu-turnstile:
-    image: git.rambossek.at/public/gpu-turnstile:v0.1.3
+    image: git.rambossek.at/public/gpu-turnstile:v0.1.5
     restart: unless-stopped
     environment:
+      # Each consumer is enabled by setting its URL; leave one unset to
+      # disable that side (no listener, no probe, no lock participation).
       # Services on the Docker host itself:
       OLLAMA_URL: http://host.docker.internal:11435
       COMFY_URL: http://host.docker.internal:8189

go.mod

@@ -3,3 +3,5 @@ module gpu-turnstile
 go 1.23
 
 require golang.org/x/sys v0.29.0
+
+require github.com/coreos/go-systemd/v22 v22.7.0

go.sum

@@ -1,2 +1,4 @@
+github.com/coreos/go-systemd/v22 v22.7.0 h1:LAEzFkke61DFROc7zNLX/WA2i5J8gYqe0rSj9KI28KA=
+github.com/coreos/go-systemd/v22 v22.7.0/go.mod h1:xNUYtjHu2EDXbsxz1i41wouACIwT7Ybq9o0BQhMwD0w=
 golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
 golang.org/x/sys v0.29.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=

internal/config/config.go

@@ -51,13 +51,12 @@ type Config struct {
 }
 
 // Defaults returns the configuration used when neither the environment nor
-// a config file sets a value.
+// a config file sets a value. The upstream URLs default to empty: a
+// consumer is enabled by setting its URL, disabled by leaving it empty.
 func Defaults() Config {
 	return Config{
 		ListenOllama:   ":11434",
 		ListenComfy:    ":8188",
-		OllamaURL:      "http://127.0.0.1:11435",
-		ComfyURL:       "http://127.0.0.1:8189",
 		UnloadTimeout:  time.Minute,
 		JobTimeout:     15 * time.Minute,
 		LLMWaitTimeout: 10 * time.Minute,
@@ -219,5 +218,8 @@ func Load(getenv func(string) string) (Config, error) {
 	default:
 		return cfg, fmt.Errorf("LOG_FORMAT: must be \"text\" or \"json\"")
 	}
+	if cfg.OllamaURL == "" && cfg.ComfyURL == "" {
+		return cfg, fmt.Errorf("at least one of OLLAMA_URL or COMFY_URL must be set (each URL enables its consumer)")
+	}
 	return cfg, nil
 }

internal/config/config_test.go

@@ -8,13 +8,21 @@ import (
 )
 
 func TestDefaults(t *testing.T) {
-	cfg, err := Load(func(string) string { return "" })
+	cfg, err := Load(func(k string) string {
+		if k == "OLLAMA_URL" {
+			return "http://127.0.0.1:11435"
+		}
+		return ""
+	})
 	if err != nil {
 		t.Fatal(err)
 	}
 	if cfg.ListenOllama != ":11434" || cfg.ListenComfy != ":8188" {
 		t.Fatalf("listen addrs = %s %s", cfg.ListenOllama, cfg.ListenComfy)
 	}
+	if cfg.ComfyURL != "" {
+		t.Fatalf("ComfyURL default = %q, want empty (disabled)", cfg.ComfyURL)
+	}
 	if cfg.UnloadTimeout != time.Minute || cfg.JobTimeout != 15*time.Minute {
 		t.Fatalf("timeouts = %v %v", cfg.UnloadTimeout, cfg.JobTimeout)
 	}
@@ -26,6 +34,13 @@ func TestDefaults(t *testing.T) {
 	}
 }
 
+func TestLoadRequiresConsumer(t *testing.T) {
+	_, err := Load(func(string) string { return "" })
+	if err == nil || !strings.Contains(err.Error(), "OLLAMA_URL") {
+		t.Fatalf("err = %v, want missing-consumer error", err)
+	}
+}
+
 func TestParseEnvFile(t *testing.T) {
 	input := `# comment
 OLLAMA_URL=http://host:11435
@@ -91,6 +106,9 @@ func TestLoadErrors(t *testing.T) {
 			if k == tc.key {
 				return tc.value
 			}
+			if k == "OLLAMA_URL" {
+				return "http://127.0.0.1:11435"
+			}
 			return ""
 		})
 		if err == nil {

internal/proxy/proxy.go

@@ -102,24 +102,37 @@ type Server struct {
 	comfyProxy  *httputil.ReverseProxy
 }
 
-// New builds a Server, validating the upstream URLs.
+// New builds a Server, validating the upstream URLs. At least one of
+// OllamaURL / ComfyURL must be set; an empty URL disables that consumer —
+// its handler is then never served, its client may be nil, and the image
+// job flow skips the Ollama unload/warm steps.
 func New(cfg Config) (*Server, error) {
-	ollamaURL, err := url.Parse(cfg.OllamaURL)
-	if err != nil || ollamaURL.Scheme == "" || ollamaURL.Host == "" {
-		return nil, fmt.Errorf("invalid OLLAMA_URL %q", cfg.OllamaURL)
+	if cfg.OllamaURL == "" && cfg.ComfyURL == "" {
+		return nil, fmt.Errorf("at least one of OllamaURL or ComfyURL is required")
 	}
-	comfyURL, err := url.Parse(cfg.ComfyURL)
-	if err != nil || comfyURL.Scheme == "" || comfyURL.Host == "" {
-		return nil, fmt.Errorf("invalid COMFY_URL %q", cfg.ComfyURL)
+	var ollamaURL, comfyURL *url.URL
+	if cfg.OllamaURL != "" {
+		u, err := url.Parse(cfg.OllamaURL)
+		if err != nil || u.Scheme == "" || u.Host == "" {
+			return nil, fmt.Errorf("invalid OLLAMA_URL %q", cfg.OllamaURL)
+		}
+		ollamaURL = u
+	}
+	if cfg.ComfyURL != "" {
+		u, err := url.Parse(cfg.ComfyURL)
+		if err != nil || u.Scheme == "" || u.Host == "" {
+			return nil, fmt.Errorf("invalid COMFY_URL %q", cfg.ComfyURL)
+		}
+		comfyURL = u
 	}
 	log := cfg.Log
 	if log == nil {
 		log = slog.Default()
 	}
-	if cfg.UnloadPollInterval > 0 {
+	if cfg.UnloadPollInterval > 0 && cfg.Ollama != nil {
 		cfg.Ollama.PollInterval = cfg.UnloadPollInterval
 	}
-	if cfg.HistoryPollInterval > 0 {
+	if cfg.HistoryPollInterval > 0 && cfg.Comfy != nil {
 		cfg.Comfy.PollInterval = cfg.HistoryPollInterval
 	}
 	freeTimeout := cfg.FreeTimeout
@@ -160,7 +173,7 @@ func New(cfg Config) (*Server, error) {
 		max:     backoffMax,
 		log:     log,
 	}
-	return &Server{
+	s := &Server{
 		cfg:            cfg,
 		log:            log,
 		logWriter:      cfg.LogWriter,
@@ -172,9 +185,14 @@ func New(cfg Config) (*Server, error) {
 		busyMode:       busyMode,
 		busyStatus:     busyStatus,
 		busyRetryAfter: busyRetryAfter,
-		ollamaProxy:    newReverseProxy(ollamaURL, retry, log.With("upstream", "ollama")),
-		comfyProxy:     newReverseProxy(comfyURL, retry, log.With("upstream", "comfy")),
-	}, nil
+	}
+	if ollamaURL != nil {
+		s.ollamaProxy = newReverseProxy(ollamaURL, retry, log.With("upstream", "ollama"))
+	}
+	if comfyURL != nil {
+		s.comfyProxy = newReverseProxy(comfyURL, retry, log.With("upstream", "comfy"))
+	}
+	return s, nil
 }
 
 // retryTransport retries requests whose failure means the upstream never
@@ -470,9 +488,13 @@ func (s *Server) OllamaHandler() http.Handler {
 // ComfyHandler serves the ComfyUI-facing listener.
 func (s *Server) ComfyHandler() http.Handler {
 	return s.logRequests("comfy", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		if r.URL.Path == "/healthz" {
+		switch r.URL.Path {
+		case "/healthz":
 			s.writeHealthz(w)
 			return
+		case "/metrics":
+			s.writeMetrics(w)
+			return
 		}
 		if r.Method == http.MethodPost && r.URL.Path == "/prompt" {
 			s.handlePrompt(w, r)
@@ -527,19 +549,21 @@ func (s *Server) handlePrompt(w http.ResponseWriter, r *http.Request) {
 	s.cfg.Metrics.ObserveLockWait("image", time.Since(start).Seconds())
 	log.Info("image lock acquired")
 
-	uctx, ucancel := context.WithTimeout(r.Context(), s.cfg.UnloadTimeout)
-	elapsed, uerr := s.cfg.Ollama.UnloadAll(uctx)
-	ucancel()
-	s.cfg.Metrics.ObserveUnload(elapsed.Seconds())
-	switch {
-	case r.Context().Err() != nil:
-		s.cfg.Lock.ReleaseImage()
-		return
-	case uerr != nil:
-		// Degrade, don't fail the user's request on a misbehaving neighbour.
-		log.Warn("ollama unload incomplete; continuing", "err", uerr)
-	default:
-		log.Info("ollama models unloaded", "seconds", elapsed.Seconds())
+	if s.cfg.Ollama != nil {
+		uctx, ucancel := context.WithTimeout(r.Context(), s.cfg.UnloadTimeout)
+		elapsed, uerr := s.cfg.Ollama.UnloadAll(uctx)
+		ucancel()
+		s.cfg.Metrics.ObserveUnload(elapsed.Seconds())
+		switch {
+		case r.Context().Err() != nil:
+			s.cfg.Lock.ReleaseImage()
+			return
+		case uerr != nil:
+			// Degrade, don't fail the user's request on a misbehaving neighbour.
+			log.Warn("ollama unload incomplete; continuing", "err", uerr)
+		default:
+			log.Info("ollama models unloaded", "seconds", elapsed.Seconds())
+		}
 	}
 
 	cw := &captureWriter{ResponseWriter: w, status: http.StatusOK, limit: s.captureLimit}
@@ -585,7 +609,7 @@ func (s *Server) finishImageJob(promptID string) {
 	s.cfg.Lock.ReleaseImage()
 	log.Info("image lock released")
 
-	if s.cfg.WarmModel != "" {
+	if s.cfg.Ollama != nil && s.cfg.WarmModel != "" {
 		if state, _, _ := s.cfg.Lock.Snapshot(); state == lock.StateIdle {
 			wctx, wcancel := context.WithTimeout(context.Background(), s.warmTimeout)
 			if err := s.cfg.Ollama.Warm(wctx, s.cfg.WarmModel); err != nil {

internal/proxy/proxy_test.go

@@ -451,3 +451,82 @@ func TestLLMBusyWaitTimeoutRetryAfter(t *testing.T) {
 		t.Fatalf("Retry-After = %q", got)
 	}
 }
+
+func TestComfyOnlyModeSkipsOllama(t *testing.T) {
+	f := newFakes(t)
+
+	comfyClient, err := comfy.New(f.comfy.URL, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	comfyClient.PollInterval = 5 * time.Millisecond
+	srv, err := New(Config{
+		ComfyURL:   f.comfy.URL,
+		Lock:       lock.New(nil),
+		Comfy:      comfyClient,
+		Metrics:    metrics.New(),
+		JobTimeout: 2 * time.Second,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	front := httptest.NewServer(srv.ComfyHandler())
+	defer front.Close()
+
+	resp, err := http.Post(front.URL+"/prompt", "application/json", strings.NewReader(`{}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != 200 {
+		t.Fatalf("prompt status = %d", resp.StatusCode)
+	}
+	f.completeJob()
+	select {
+	case <-f.freeCh:
+	case <-time.After(3 * time.Second):
+		t.Fatal("/free never called")
+	}
+	// With Ollama disabled the unload steps must not happen.
+	if i := f.rec.index("unload"); i >= 0 {
+		f.rec.mu.Lock()
+		t.Fatalf("unload called with ollama disabled; events: %v", f.rec.events)
+	}
+}
+
+func TestOllamaOnlyMode(t *testing.T) {
+	f := newFakes(t)
+
+	ollamaClient, err := ollama.New(f.ollama.URL, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	srv, err := New(Config{
+		OllamaURL:      f.ollama.URL,
+		Lock:           lock.New(nil),
+		Ollama:         ollamaClient,
+		Metrics:        metrics.New(),
+		LLMWaitTimeout: time.Second,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	front := httptest.NewServer(srv.OllamaHandler())
+	defer front.Close()
+
+	resp, err := http.Post(front.URL+"/api/chat", "application/json", strings.NewReader(`{}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != 200 {
+		t.Fatalf("chat status = %d", resp.StatusCode)
+	}
+}
+
+func TestNewRequiresConsumer(t *testing.T) {
+	_, err := New(Config{Lock: lock.New(nil), Metrics: metrics.New()})
+	if err == nil {
+		t.Fatal("New with no upstream URLs should fail")
+	}
+}

internal/service/notify_linux.go

@@ -0,0 +1,46 @@
+//go:build linux
+
+package service
+
+import (
+	"context"
+	"os"
+	"strconv"
+	"time"
+
+	"github.com/coreos/go-systemd/v22/daemon"
+)
+
+// NotifyReady tells systemd the service is up (Type=notify). It is a no-op
+// when NOTIFY_SOCKET is unset, e.g. in a container or interactive shell.
+func NotifyReady() {
+	daemon.SdNotify(false, daemon.SdNotifyReady)
+}
+
+// NotifyStopping tells systemd the service is shutting down.
+func NotifyStopping() {
+	daemon.SdNotify(false, daemon.SdNotifyStopping)
+}
+
+// StartWatchdog pings the systemd watchdog every half of WATCHDOG_USEC
+// until ctx is cancelled. It is a no-op unless systemd started the process
+// with a watchdog configured (WatchdogSec= in the unit).
+func StartWatchdog(ctx context.Context) {
+	usec, err := strconv.Atoi(os.Getenv("WATCHDOG_USEC"))
+	if err != nil || usec <= 0 {
+		return
+	}
+	interval := time.Duration(usec) * time.Microsecond / 2
+	go func() {
+		ticker := time.NewTicker(interval)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				daemon.SdNotify(false, daemon.SdNotifyWatchdog)
+			}
+		}
+	}()
+}

internal/service/notify_other.go

@@ -0,0 +1,14 @@
+//go:build !linux
+
+package service
+
+import "context"
+
+// NotifyReady is a no-op outside Linux (no systemd notify socket).
+func NotifyReady() {}
+
+// NotifyStopping is a no-op outside Linux.
+func NotifyStopping() {}
+
+// StartWatchdog is a no-op outside Linux.
+func StartWatchdog(context.Context) {}

internal/service/service_linux.go

@@ -0,0 +1,90 @@
+//go:build linux
+
+// Package service integrates gpu-turnstile with systemd on Linux: running
+// under a unit with readiness notification and watchdog, plus
+// install/remove helpers that manage a system unit.
+package service
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"os/signal"
+	"path/filepath"
+	"syscall"
+)
+
+// Name matches the Windows service name; the systemd unit is Name + ".service".
+const Name = "gpu-turnstile"
+
+// unitPath is where Install writes the unit file.
+const unitPath = "/etc/systemd/system/" + Name + ".service"
+
+// IsService reports whether the process was started by systemd.
+func IsService() bool { return os.Getenv("INVOCATION_ID") != "" }
+
+// Run executes run with SIGINT/SIGTERM cancellation (which is how systemctl
+// stop signals the process) and tells systemd when the shutdown begins.
+func Run(run func(ctx context.Context) error) error {
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+	defer NotifyStopping()
+	return run(ctx)
+}
+
+// renderUnit builds the systemd unit: Type=notify so systemctl start blocks
+// until the listeners are bound, a 30s watchdog, and restart-on-failure
+// with a 5s delay — which is also what brings up a staged update after the
+// updater exits with a non-zero code.
+func renderUnit(exePath, configPath string) string {
+	return fmt.Sprintf(`[Unit]
+Description=gpu-turnstile GPU arbitration proxy for Ollama and ComfyUI
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=notify
+WatchdogSec=30s
+ExecStart=%q -config %q
+Restart=on-failure
+RestartSec=5s
+
+[Install]
+WantedBy=multi-user.target
+`, exePath, configPath)
+}
+
+// Install writes the unit for the current executable and the given config
+// file, then enables and starts it. Needs root.
+func Install(configPath string) error {
+	exe, err := os.Executable()
+	if err != nil {
+		return err
+	}
+	if abs, absErr := filepath.Abs(exe); absErr == nil {
+		exe = abs
+	}
+	if err := os.WriteFile(unitPath, []byte(renderUnit(exe, configPath)), 0o644); err != nil {
+		return fmt.Errorf("write %s (run as root): %w", unitPath, err)
+	}
+	if out, err := exec.Command("systemctl", "daemon-reload").CombinedOutput(); err != nil {
+		return fmt.Errorf("systemctl daemon-reload: %w (%s)", err, out)
+	}
+	if out, err := exec.Command("systemctl", "enable", "--now", Name+".service").CombinedOutput(); err != nil {
+		return fmt.Errorf("systemctl enable --now: %w (%s)", err, out)
+	}
+	return nil
+}
+
+// Remove stops and disables the service and deletes the unit file.
+func Remove() error {
+	exec.Command("systemctl", "disable", "--now", Name+".service").Run() // ignore: may not exist
+	if err := os.Remove(unitPath); err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("remove %s: %w", unitPath, err)
+	}
+	if out, err := exec.Command("systemctl", "daemon-reload").CombinedOutput(); err != nil {
+		return fmt.Errorf("systemctl daemon-reload: %w (%s)", err, out)
+	}
+	return nil
+}

internal/service/service_linux_test.go

@@ -0,0 +1,23 @@
+//go:build linux
+
+package service
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestRenderUnit(t *testing.T) {
+	unit := renderUnit("/usr/local/bin/gpu-turnstile", "/etc/gpu-turnstile.env")
+	for _, want := range []string{
+		"Type=notify",
+		"WatchdogSec=30s",
+		`ExecStart="/usr/local/bin/gpu-turnstile" -config "/etc/gpu-turnstile.env"`,
+		"Restart=on-failure",
+		"WantedBy=multi-user.target",
+	} {
+		if !strings.Contains(unit, want) {
+			t.Fatalf("unit missing %q:\n%s", want, unit)
+		}
+	}
+}

internal/service/service_other.go

@@ -1,8 +1,8 @@
-//go:build !windows
+//go:build !windows && !linux
 
-// Package service provides the non-Windows stubs for the Windows service
-// integration. Run falls back to plain signal handling; install/remove
-// are unsupported.
+// Package service provides the stubs for platforms without service
+// integration (Windows uses the SCM, Linux uses systemd). Run falls back
+// to plain signal handling; install/remove are unsupported.
 package service
 
 import (
@@ -15,7 +15,7 @@ import (
 // Name matches the Windows service name.
 const Name = "gpu-turnstile"
 
-var errUnsupported = errors.New("service management is only supported on Windows")
+var errUnsupported = errors.New("service management is only supported on Windows and Linux (systemd)")
 
 // IsService is always false on non-Windows platforms.
 func IsService() bool { return false }

internal/update/pubkey.go

@@ -11,6 +11,6 @@ package update
 // the RELEASE_SIGNING_KEY repository secret. When empty, the updater refuses
 // to update (e.g. development builds).
 var publicKeyPEM = `-----BEGIN PUBLIC KEY-----
-MCowBQYDK2VwAyEA+gJbSvgeYX58woPQGbSC8x8Zw4OTDiiQ7/19seZKfSQ=
+MCowBQYDK2VwAyEAqTAJ0CCeAQI7MhFlgc5xNmF/CfvLVUAY3ZAoeAS0tT8=
 -----END PUBLIC KEY-----
 `