Reduziere SD-Karten-Schreiblast: leiser Agent + journald volatile

- Agent loggt im Normalfall nichts mehr (kein heartbeat_tick, kein
  mqtt_heartbeat_sent, kein status_report_sent)
- nur noch Fehler und Zustandsaenderungen werden geloggt
- Ansible: journald auf Storage=volatile + RuntimeMaxUse=20M (RAM-only,
  automatisches Verdraengen alter Eintraege bei vollem Puffer)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -16,3 +16,6 @@ dist/
 
 # Compose override files
 compose.override.yml
+vault.yml
+ansible/roles/signage_player/files/morz-agent
+player/agent/agent-linux-arm64

ansible/group_vars/signage_players/vars.yml

@@ -0,0 +1,9 @@
+---
+morz_server_base_url: "http://10.0.0.70:8080"
+morz_mqtt_broker: "tcp://dockerbox.morz.de:1883"
+morz_heartbeat_every_seconds: 30
+morz_status_report_every_seconds: 60
+
+# Credentials kommen aus vault.yml (ansible-vault)
+morz_mqtt_username: "{{ vault_mqtt_username }}"
+morz_mqtt_password: "{{ vault_mqtt_password }}"

ansible/host_vars/info10/vars.yml

@@ -0,0 +1,4 @@
+---
+ansible_host: 10.0.0.200
+ansible_user: morz
+screen_id: info01-dev

ansible/inventory.yml

@@ -0,0 +1,6 @@
+---
+all:
+  children:
+    signage_players:
+      hosts:
+        info10:

ansible/roles/signage_player/defaults/main.yml

@@ -0,0 +1,11 @@
+---
+signage_user: morz
+signage_config_dir: /etc/signage
+signage_binary_dest: /usr/local/bin/morz-agent
+
+morz_server_base_url: "http://10.0.0.70:8080"
+morz_mqtt_broker: ""
+morz_mqtt_username: ""
+morz_mqtt_password: ""
+morz_heartbeat_every_seconds: 30
+morz_status_report_every_seconds: 60

ansible/roles/signage_player/handlers/main.yml

@@ -0,0 +1,17 @@
+---
+- name: Reload systemd
+  ansible.builtin.systemd:
+    daemon_reload: true
+  become: true
+
+- name: Restart journald
+  ansible.builtin.systemd:
+    name: systemd-journald
+    state: restarted
+  become: true
+
+- name: Restart morz-agent
+  ansible.builtin.systemd:
+    name: morz-agent
+    state: restarted
+  become: true

ansible/roles/signage_player/tasks/main.yml

@@ -0,0 +1,81 @@
+---
+- name: Build agent binary for linux/arm64
+  ansible.builtin.command:
+    cmd: go build -o {{ role_path }}/files/morz-agent ./cmd/agent
+    chdir: "{{ playbook_dir }}/../player/agent"
+  environment:
+    GOOS: linux
+    GOARCH: arm64
+  delegate_to: localhost
+  changed_when: true
+
+- name: Ensure config directory exists
+  ansible.builtin.file:
+    path: "{{ signage_config_dir }}"
+    state: directory
+    owner: root
+    group: root
+    mode: "0755"
+  become: true
+
+- name: Deploy agent config
+  ansible.builtin.template:
+    src: config.json.j2
+    dest: "{{ signage_config_dir }}/config.json"
+    owner: root
+    group: "{{ signage_user }}"
+    mode: "0640"
+  become: true
+  notify: Restart morz-agent
+
+- name: Deploy agent binary
+  ansible.builtin.copy:
+    src: morz-agent
+    dest: "{{ signage_binary_dest }}"
+    owner: root
+    group: root
+    mode: "0755"
+  become: true
+  notify: Restart morz-agent
+
+- name: Deploy systemd unit
+  ansible.builtin.template:
+    src: morz-agent.service.j2
+    dest: /etc/systemd/system/morz-agent.service
+    owner: root
+    group: root
+    mode: "0644"
+  become: true
+  notify:
+    - Reload systemd
+    - Restart morz-agent
+
+- name: Ensure journald drop-in directory exists
+  ansible.builtin.file:
+    path: /etc/systemd/journald.conf.d
+    state: directory
+    owner: root
+    group: root
+    mode: "0755"
+  become: true
+
+- name: Configure journald volatile storage (RAM only, schont SD-Karte)
+  ansible.builtin.copy:
+    dest: /etc/systemd/journald.conf.d/morz-volatile.conf
+    content: |
+      [Journal]
+      Storage=volatile
+      RuntimeMaxUse=20M
+    owner: root
+    group: root
+    mode: "0644"
+  become: true
+  notify: Restart journald
+
+- name: Enable and start morz-agent
+  ansible.builtin.systemd:
+    name: morz-agent
+    enabled: true
+    state: started
+    daemon_reload: false
+  become: true

ansible/roles/signage_player/templates/config.json.j2

@@ -0,0 +1,9 @@
+{
+  "screen_id": "{{ screen_id }}",
+  "server_base_url": "{{ morz_server_base_url }}",
+  "mqtt_broker": "{{ morz_mqtt_broker }}",
+  "mqtt_username": "{{ morz_mqtt_username }}",
+  "mqtt_password": "{{ morz_mqtt_password }}",
+  "heartbeat_every_seconds": {{ morz_heartbeat_every_seconds }},
+  "status_report_every_seconds": {{ morz_status_report_every_seconds }}
+}

ansible/roles/signage_player/templates/morz-agent.service.j2

@@ -0,0 +1,16 @@
+[Unit]
+Description=Morz Infoboard Player Agent
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User={{ signage_user }}
+ExecStart={{ signage_binary_dest }}
+Restart=on-failure
+RestartSec=10
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=multi-user.target

ansible/site.yml

@@ -1,7 +1,6 @@
-- name: Placeholder deployment entrypoint
-  hosts: all
+---
+- name: Deploy Morz Infoboard Player Agent
+  hosts: signage_players
   gather_facts: false
-  tasks:
-    - name: Show target host placeholder
-      ansible.builtin.debug:
-        msg: "Placeholder fuer spaeteres signage deployment auf {{ inventory_hostname }}"
+  roles:
+    - signage_player

player/agent/go.mod

@@ -1,3 +1,10 @@
 module git.az-it.net/az/morz-infoboard/player/agent
 
 go 1.24.0
+
+require (
+	github.com/eclipse/paho.mqtt.golang v1.5.1 // indirect
+	github.com/gorilla/websocket v1.5.3 // indirect
+	golang.org/x/net v0.44.0 // indirect
+	golang.org/x/sync v0.17.0 // indirect
+)

player/agent/go.sum

@@ -0,0 +1,8 @@
+github.com/eclipse/paho.mqtt.golang v1.5.1 h1:/VSOv3oDLlpqR2Epjn1Q7b2bSTplJIeV2ISgCl2W7nE=
+github.com/eclipse/paho.mqtt.golang v1.5.1/go.mod h1:1/yJCneuyOoCOzKSsOTUc0AJfpsItBGWvYpBLimhArU=
+github.com/gorilla/websocket v1.5.3 h1:saDtZ6Pbx/0u+bgYQ3q96pZgCzfhKXGPqt7kZ72aNNg=
+github.com/gorilla/websocket v1.5.3/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
+golang.org/x/net v0.44.0 h1:evd8IRDyfNBMBTTY5XRF1vaZlD+EmWx6x8PkhR04H/I=
+golang.org/x/net v0.44.0/go.mod h1:ECOoLqd5U3Lhyeyo/QDCEVQ4sNgYsqvCZ722XogGieY=
+golang.org/x/sync v0.17.0 h1:l60nONMj9l5drqw6jlhIELNv9I0A4OFgRsG9k2oT9Ug=
+golang.org/x/sync v0.17.0/go.mod h1:9KTHXmSnoGruLpwFjVSX0lNNA75CykiMECbovNTZqGI=

player/agent/internal/app/app.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"git.az-it.net/az/morz-infoboard/player/agent/internal/config"
+	"git.az-it.net/az/morz-infoboard/player/agent/internal/mqttheartbeat"
 	"git.az-it.net/az/morz-infoboard/player/agent/internal/statusreporter"
 )
 
@@ -45,6 +46,7 @@ type App struct {
 	logger   *log.Logger
 	now      func() time.Time
 	reporter statusSender
+	mqttPub  mqttSender
 
 	mu                        sync.RWMutex
 	status                    Status
@@ -58,6 +60,11 @@ type statusSender interface {
 	Send(ctx context.Context, snapshot statusreporter.Snapshot) error
 }
 
+type mqttSender interface {
+	SendHeartbeat(status, connectivity string, ts time.Time) error
+	Close()
+}
+
 func New() (*App, error) {
 	cfg, err := config.Load()
 	if err != nil {
@@ -66,10 +73,18 @@ func New() (*App, error) {
 
 	logger := log.New(os.Stdout, "agent ", log.LstdFlags|log.LUTC)
 
-	return newApp(cfg, logger, time.Now, statusreporter.New(cfg.ServerBaseURL, nil, time.Now)), nil
+	var mqttPub mqttSender
+	if cfg.MQTTBroker != "" {
+		mqttPub = mqttheartbeat.New(cfg.MQTTBroker, cfg.ScreenID, cfg.MQTTUsername, cfg.MQTTPassword)
+		logger.Printf("event=mqtt_enabled broker=%s", cfg.MQTTBroker)
+	} else {
+		logger.Printf("event=mqtt_disabled reason=no_broker_configured")
+	}
+
+	return newApp(cfg, logger, time.Now, statusreporter.New(cfg.ServerBaseURL, nil, time.Now), mqttPub), nil
 }
 
-func newApp(cfg config.Config, logger *log.Logger, now func() time.Time, reporter statusSender) *App {
+func newApp(cfg config.Config, logger *log.Logger, now func() time.Time, reporter statusSender, mqttPub mqttSender) *App {
 	if logger == nil {
 		logger = log.New(os.Stdout, "agent ", log.LstdFlags|log.LUTC)
 	}
@@ -83,6 +98,7 @@ func newApp(cfg config.Config, logger *log.Logger, now func() time.Time, reporte
 		logger:             logger,
 		now:                now,
 		reporter:           reporter,
+		mqttPub:            mqttPub,
 		status:             StatusStarting,
 		serverConnectivity: ConnectivityUnknown,
 	}
@@ -147,6 +163,9 @@ func (a *App) Run(ctx context.Context) error {
 			a.mu.Lock()
 			a.status = StatusStopped
 			a.mu.Unlock()
+			if a.mqttPub != nil {
+				a.mqttPub.Close()
+			}
 			a.logger.Printf("event=agent_stopped screen_id=%s", a.Config.ScreenID)
 			return nil
 		case <-ticker.C:
@@ -162,9 +181,15 @@ func (a *App) emitHeartbeat() {
 
 	a.mu.Lock()
 	a.lastHeartbeatAt = now
+	status := a.status
+	connectivity := a.serverConnectivity
 	a.mu.Unlock()
 
-	a.logger.Printf("event=heartbeat_tick screen_id=%s", a.Config.ScreenID)
+	if a.mqttPub != nil {
+		if err := a.mqttPub.SendHeartbeat(string(status), string(connectivity), now); err != nil {
+			a.logger.Printf("event=mqtt_heartbeat_failed screen_id=%s error=%v", a.Config.ScreenID, err)
+		}
+	}
 }
 
 func (a *App) reportStatus(ctx context.Context) {
@@ -204,5 +229,4 @@ func (a *App) reportStatus(ctx context.Context) {
 	a.consecutiveReportFailures = 0
 	a.serverConnectivity = ConnectivityOnline
 	a.mu.Unlock()
-	a.logger.Printf("event=status_report_sent screen_id=%s", a.Config.ScreenID)
 }

player/agent/internal/app/app_test.go

@@ -40,7 +40,7 @@ func TestAppRunUpdatesHealthAndLogsStructuredEvents(t *testing.T) {
 		MQTTBroker:        "tcp://127.0.0.1:1883",
 		HeartbeatEvery:    1,
 		StatusReportEvery: 1,
-	}, logger, time.Now, &recordingReporter{})
+	}, logger, time.Now, &recordingReporter{}, nil)
 
 	if got, want := application.Snapshot().Status, StatusStarting; got != want {
 		t.Fatalf("initial status = %q, want %q", got, want)
@@ -92,7 +92,6 @@ func TestAppRunUpdatesHealthAndLogsStructuredEvents(t *testing.T) {
 	for _, needle := range []string{
 		"event=agent_configured",
 		"screen_id=info01-dev",
-		"event=heartbeat_tick",
 		"event=agent_stopped",
 	} {
 		if !strings.Contains(logs, needle) {
@@ -108,7 +107,7 @@ func TestAppSnapshotIncludesConfiguredTargets(t *testing.T) {
 		MQTTBroker:        "tcp://mqtt.example:1883",
 		HeartbeatEvery:    15,
 		StatusReportEvery: 60,
-	}, log.New(&bytes.Buffer{}, "", 0), time.Now, &recordingReporter{})
+	}, log.New(&bytes.Buffer{}, "", 0), time.Now, &recordingReporter{}, nil)
 
 	snapshot := application.Snapshot()
 
@@ -141,7 +140,7 @@ func TestAppRunWithCanceledContextDoesNotLogConfiguredOrHeartbeat(t *testing.T)
 		MQTTBroker:        "tcp://127.0.0.1:1883",
 		HeartbeatEvery:    5,
 		StatusReportEvery: 60,
-	}, log.New(&logBuffer, "", 0), time.Now, &recordingReporter{})
+	}, log.New(&logBuffer, "", 0), time.Now, &recordingReporter{}, nil)
 
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
@@ -171,7 +170,7 @@ func TestAppRunReportsStatusWithoutStoppingOnReporterError(t *testing.T) {
 		MQTTBroker:        "tcp://127.0.0.1:1883",
 		HeartbeatEvery:    1,
 		StatusReportEvery: 1,
-	}, log.New(&logBuffer, "", 0), time.Now, reporter)
+	}, log.New(&logBuffer, "", 0), time.Now, reporter, nil)
 
 	ctx, cancel := context.WithCancel(context.Background())
 	errCh := make(chan error, 1)
@@ -221,7 +220,7 @@ func TestAppRunMarksServerConnectivityOnlineAfterSuccessfulReport(t *testing.T)
 		MQTTBroker:        "tcp://127.0.0.1:1883",
 		HeartbeatEvery:    1,
 		StatusReportEvery: 1,
-	}, log.New(&bytes.Buffer{}, "", 0), time.Now, reporter)
+	}, log.New(&bytes.Buffer{}, "", 0), time.Now, reporter, nil)
 
 	ctx, cancel := context.WithCancel(context.Background())
 	errCh := make(chan error, 1)
@@ -264,7 +263,7 @@ func TestReportStatusMarksServerConnectivityOfflineAfterRepeatedFailures(t *test
 		MQTTBroker:        "tcp://127.0.0.1:1883",
 		HeartbeatEvery:    30,
 		StatusReportEvery: 30,
-	}, log.New(&bytes.Buffer{}, "", 0), time.Now, reporter)
+	}, log.New(&bytes.Buffer{}, "", 0), time.Now, reporter, nil)
 
 	application.reportStatus(context.Background())
 	if got, want := application.Snapshot().ServerConnectivity, ConnectivityDegraded; got != want {
@@ -287,7 +286,7 @@ func TestReportStatusRecoversFromOfflineToOnline(t *testing.T) {
 		MQTTBroker:        "tcp://127.0.0.1:1883",
 		HeartbeatEvery:    30,
 		StatusReportEvery: 30,
-	}, log.New(&bytes.Buffer{}, "", 0), time.Now, reporter)
+	}, log.New(&bytes.Buffer{}, "", 0), time.Now, reporter, nil)
 
 	application.reportStatus(context.Background())
 	application.reportStatus(context.Background())
@@ -305,3 +304,69 @@ func TestReportStatusRecoversFromOfflineToOnline(t *testing.T) {
 		t.Fatalf("recovery payload connectivity = %q, want %q", got, want)
 	}
 }
+
+type recordingMQTTSender struct {
+	calls []mqttHeartbeatCall
+	err   error
+}
+
+type mqttHeartbeatCall struct {
+	status      string
+	connectivity string
+}
+
+func (r *recordingMQTTSender) SendHeartbeat(status, connectivity string, _ time.Time) error {
+	r.calls = append(r.calls, mqttHeartbeatCall{status: status, connectivity: connectivity})
+	return r.err
+}
+
+func (r *recordingMQTTSender) Close() {}
+
+func TestEmitHeartbeatCallsMQTTPublisher(t *testing.T) {
+	mqttSend := &recordingMQTTSender{}
+	application := newApp(config.Config{
+		ScreenID:          "mqtt-screen",
+		HeartbeatEvery:    30,
+		StatusReportEvery: 60,
+	}, log.New(&bytes.Buffer{}, "", 0), time.Now, &recordingReporter{}, mqttSend)
+
+	application.emitHeartbeat()
+
+	if got, want := len(mqttSend.calls), 1; got != want {
+		t.Fatalf("MQTT call count = %d, want %d", got, want)
+	}
+	if got, want := mqttSend.calls[0].status, string(StatusStarting); got != want {
+		t.Fatalf("heartbeat status = %q, want %q", got, want)
+	}
+}
+
+func TestEmitHeartbeatSkipsWhenNoMQTTPublisher(t *testing.T) {
+	var logBuffer bytes.Buffer
+	application := newApp(config.Config{
+		ScreenID:          "no-mqtt-screen",
+		HeartbeatEvery:    30,
+		StatusReportEvery: 60,
+	}, log.New(&logBuffer, "", 0), time.Now, &recordingReporter{}, nil)
+
+	application.emitHeartbeat() // must not panic
+
+	if strings.Contains(logBuffer.String(), "event=mqtt_heartbeat") {
+		t.Fatalf("unexpected MQTT heartbeat log entry when no publisher configured: %s", logBuffer.String())
+	}
+}
+
+func TestEmitHeartbeatLogsMQTTFailure(t *testing.T) {
+	var logBuffer bytes.Buffer
+	mqttSend := &recordingMQTTSender{err: context.DeadlineExceeded}
+	application := newApp(config.Config{
+		ScreenID:          "mqtt-fail-screen",
+		HeartbeatEvery:    30,
+		StatusReportEvery: 60,
+	}, log.New(&logBuffer, "", 0), time.Now, &recordingReporter{}, mqttSend)
+
+	application.emitHeartbeat()
+
+	if !strings.Contains(logBuffer.String(), "event=mqtt_heartbeat_failed") {
+		t.Fatalf("logs missing mqtt_heartbeat_failed: %s", logBuffer.String())
+	}
+}

player/agent/internal/config/config.go

@@ -10,6 +10,8 @@ type Config struct {
 	ScreenID          string `json:"screen_id"`
 	ServerBaseURL     string `json:"server_base_url"`
 	MQTTBroker        string `json:"mqtt_broker"`
+	MQTTUsername      string `json:"mqtt_username"`
+	MQTTPassword      string `json:"mqtt_password"`
 	HeartbeatEvery    int    `json:"heartbeat_every_seconds"`
 	StatusReportEvery int    `json:"status_report_every_seconds"`
 }
@@ -47,7 +49,7 @@ func defaultConfig() Config {
 	return Config{
 		ScreenID:          "unset-screen",
 		ServerBaseURL:     "http://127.0.0.1:8080",
-		MQTTBroker:        "tcp://127.0.0.1:1883",
+		MQTTBroker:        "",
 		HeartbeatEvery:    30,
 		StatusReportEvery: 60,
 	}
@@ -66,6 +68,8 @@ func overrideFromEnv(cfg *Config) {
 	cfg.ScreenID = getenv("MORZ_INFOBOARD_SCREEN_ID", cfg.ScreenID)
 	cfg.ServerBaseURL = getenv("MORZ_INFOBOARD_SERVER_URL", cfg.ServerBaseURL)
 	cfg.MQTTBroker = getenv("MORZ_INFOBOARD_MQTT_BROKER", cfg.MQTTBroker)
+	cfg.MQTTUsername = getenv("MORZ_INFOBOARD_MQTT_USERNAME", cfg.MQTTUsername)
+	cfg.MQTTPassword = getenv("MORZ_INFOBOARD_MQTT_PASSWORD", cfg.MQTTPassword)
 	if value := getenv("MORZ_INFOBOARD_STATUS_REPORT_EVERY", ""); value != "" {
 		var parsed int
 		_, _ = fmt.Sscanf(value, "%d", &parsed)

player/agent/internal/mqttheartbeat/heartbeat.go

@@ -0,0 +1,84 @@
+package mqttheartbeat
+
+import (
+	"encoding/json"
+	"time"
+
+	mqtt "github.com/eclipse/paho.mqtt.golang"
+)
+
+// payload is the JSON structure published to the heartbeat topic.
+type payload struct {
+	ScreenID           string `json:"screen_id"`
+	Timestamp          string `json:"ts"`
+	Status             string `json:"status"`
+	ServerConnectivity string `json:"server_connectivity"`
+}
+
+// Publisher publishes MQTT heartbeats for a single screen.
+// It connects with auto-reconnect enabled, so transient broker outages are
+// handled transparently; individual Publish calls fail and should be logged
+// by the caller.
+type Publisher struct {
+	client   mqtt.Client
+	screenID string
+}
+
+// New creates a Publisher and initiates a non-blocking connection to broker.
+// username and password may be empty if the broker requires no authentication.
+// paho retries the connection in the background if the broker is unreachable
+// at startup, so New always succeeds. Publish calls will return errors until
+// the connection is established.
+func New(broker, screenID, username, password string) *Publisher {
+	opts := mqtt.NewClientOptions().
+		AddBroker(broker).
+		SetClientID("morz-agent-" + screenID).
+		SetCleanSession(true).
+		SetAutoReconnect(true).
+		SetConnectRetry(true).
+		SetConnectRetryInterval(10 * time.Second)
+
+	if username != "" {
+		opts.SetUsername(username)
+		opts.SetPassword(password)
+	}
+
+	client := mqtt.NewClient(opts)
+	client.Connect() // non-blocking; paho retries in background
+	return &Publisher{client: client, screenID: screenID}
+}
+
+// Topic returns the MQTT topic for the screen's heartbeat.
+func Topic(screenID string) string {
+	return "signage/screen/" + screenID + "/heartbeat"
+}
+
+// BuildPayload builds the JSON heartbeat payload. Exported for testing.
+func BuildPayload(screenID, status, connectivity string, ts time.Time) ([]byte, error) {
+	return json.Marshal(payload{
+		ScreenID:           screenID,
+		Timestamp:          ts.UTC().Format(time.RFC3339),
+		Status:             status,
+		ServerConnectivity: connectivity,
+	})
+}
+
+// SendHeartbeat publishes a heartbeat to the screen's topic.
+// QoS 0, not retained. Returns an error if the broker is unreachable or
+// the publish token fails; callers should log and continue.
+func (p *Publisher) SendHeartbeat(status, connectivity string, ts time.Time) error {
+	data, err := BuildPayload(p.screenID, status, connectivity, ts)
+	if err != nil {
+		return err
+	}
+
+	topic := Topic(p.screenID)
+	token := p.client.Publish(topic, 0, false, data)
+	token.WaitTimeout(3 * time.Second)
+	return token.Error()
+}
+
+// Close disconnects from the broker gracefully.
+func (p *Publisher) Close() {
+	p.client.Disconnect(250)
+}

player/agent/internal/mqttheartbeat/heartbeat_test.go

@@ -0,0 +1,60 @@
+package mqttheartbeat
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestBuildPayloadContainsAllFields(t *testing.T) {
+	ts := time.Date(2026, 3, 22, 16, 9, 30, 0, time.UTC)
+
+	data, err := BuildPayload("info01-dev", "running", "online", ts)
+	if err != nil {
+		t.Fatalf("BuildPayload() error = %v", err)
+	}
+
+	var p struct {
+		ScreenID           string `json:"screen_id"`
+		Timestamp          string `json:"ts"`
+		Status             string `json:"status"`
+		ServerConnectivity string `json:"server_connectivity"`
+	}
+	if err := json.Unmarshal(data, &p); err != nil {
+		t.Fatalf("Unmarshal() error = %v", err)
+	}
+
+	if got, want := p.ScreenID, "info01-dev"; got != want {
+		t.Errorf("screen_id = %q, want %q", got, want)
+	}
+	if got, want := p.Timestamp, "2026-03-22T16:09:30Z"; got != want {
+		t.Errorf("ts = %q, want %q", got, want)
+	}
+	if got, want := p.Status, "running"; got != want {
+		t.Errorf("status = %q, want %q", got, want)
+	}
+	if got, want := p.ServerConnectivity, "online"; got != want {
+		t.Errorf("server_connectivity = %q, want %q", got, want)
+	}
+}
+
+func TestBuildPayloadTimestampIsUTC(t *testing.T) {
+	loc := time.FixedZone("CET", 1*60*60)
+	ts := time.Date(2026, 3, 22, 17, 9, 30, 0, loc) // same moment, different zone
+
+	data, err := BuildPayload("screen-x", "running", "online", ts)
+	if err != nil {
+		t.Fatalf("BuildPayload() error = %v", err)
+	}
+
+	if !strings.Contains(string(data), "2026-03-22T16:09:30Z") {
+		t.Errorf("payload %s: expected UTC timestamp 2026-03-22T16:09:30Z", data)
+	}
+}
+
+func TestTopicFormat(t *testing.T) {
+	if got, want := Topic("info01-dev"), "signage/screen/info01-dev/heartbeat"; got != want {
+		t.Fatalf("Topic() = %q, want %q", got, want)
+	}
+}