Merge pull request 'Add prometheus alert example' (#1) from feat/example into stable

Reviewed-on: #1

contrib/prometheus.alert.yml

@@ -0,0 +1,72 @@
+groups:
+  # --- Recording rules (no subqueries) ---
+  - name: friendica-exporter.records
+    rules:
+      # Current backlog per instance (sum over all priority levels)
+      - record: friendica:worker_backlog
+        expr: sum by (instance) (friendica_worker_tasks_total{job="friendica"})
+
+      # Backlog change over 10 minutes (using offset)
+      - record: friendica:worker_backlog_10m_delta
+        expr: friendica:worker_backlog - friendica:worker_backlog offset 10m
+
+      # Backlog change over 2 hours (using offset)
+      - record: friendica:worker_backlog_2h_delta
+        expr: friendica:worker_backlog - friendica:worker_backlog offset 2h
+
+  # --- Alerts ---
+  - name: friendica-exporter.alerts
+    rules:
+
+      # 1) Exporter must be available (target up AND exporter can read Friendica)
+      - alert: FriendicaExporterUnavailable
+        expr: |
+          (up{job="friendica"} == 0)
+          OR (friendica_up{job="friendica"} == 0)
+        for: 5m
+        labels:
+          severity: critical
+          service: friendica
+          component: exporter
+          tier: app
+        annotations:
+          summary: "Friendica exporter unavailable on {{ $labels.instance }}"
+          description: "Target down or exporter cannot read Friendica (friendica_up=0) for >5m."
+
+  # 2) Worker must be active (<15 minutes since last execution), with JPM fallback
+  - alert: FriendicaWorkerStale
+    expr: |
+      (
+        time()
+        - max by (instance) (friendica_worker_last_execution{job="friendica"})
+        > 15 * 60
+      )
+      OR
+      (
+        max_over_time(friendica_worker_jpm{job="friendica", frequency="1 minute"}[15m]) <= 0
+      )
+    for: 5m
+    labels:
+      severity: warning
+      service: friendica
+      component: worker
+      tier: app
+    annotations:
+      summary: "Friendica worker inactive (>15m) on {{ $labels.instance }}"
+      description: "Last worker execution is older than 15 minutes or 1-minute JPM stayed 0 for the last 15 minutes."
+
+  # 3) Backlog grows without relief (deadlock / stall)
+  # A: backlog rose >200 over 2h; B: no 10-min window with a decrease in the last 2h
+  - alert: FriendicaWorkerBacklogMonotonicIncrease
+    expr: |
+      (friendica:worker_backlog_2h_delta > 200)
+      AND (min_over_time(friendica:worker_backlog_10m_delta[2h]) >= 0)
+    for: 15m
+    labels:
+      severity: critical
+      service: friendica
+      component: worker
+      tier: app
+    annotations:
+      summary: "Backlog grows without decreases on {{ $labels.instance }}"
+      description: "Outstanding worker tasks rose by >200 in the last 2h and never dropped in any 10-minute slice → likely deadlock or stalled workers. Δ2h={{ $value }}."