🚧 WIP

config.example.yaml

@@ -23,6 +23,10 @@ tags:
 # convention: inverted fault flags -> [false, 0], string states -> ["RUN"].
 # Compared by string equality on the rendered value ("True"/"1"/"RUN").
 # Do NOT also list these under tags: - they are polled automatically.
+#
+# tags: and devices: are each optional (devices-only monitors just the links;
+# tags-only runs classic surveillance) — but both empty is rejected:
+# the tool would read nothing.
 devices:
   - { name: "Conveyor 3", tag: "Channel1.Device3.Connected" }              # ok: true/1
   - { name: "Conveyor 4", tag: "Channel1.Device4.Connected", ok_values: [true, 1] }

docs/SPECIFICATION.md

@@ -137,6 +137,12 @@ an ok value again. A value that is neither in `ok_values` nor a recognized
 fault shape still logs DOWN — the normal state is explicit, and everything
 else is "not normal" by definition.
 
+**Section optionality:** `tags:` and `devices:` are each optional in the
+config — a devices-only setup monitors just the links, a tags-only setup
+runs classic quality/latency surveillance. Both empty (or absent) is a
+validation error: the tool would read nothing and every heartbeat would
+report `tags_total: 0`.
+
 ## 7. OPC DA quality codes (reference)
 
 OPC DA quality is a 16-bit field. Bits 7–6 of the low byte give the class:

src/opcda_diagnozer/config.py

@@ -276,9 +276,11 @@ def _validate(config: Config) -> None:
         )
     if config.opc.connect_timeout_s <= 0:
         raise ConfigError(f"opc.connect_timeout_s: must be > 0, got {config.opc.connect_timeout_s}")
-    if not config.tags:
+    if not config.tags and not config.devices:
         raise ConfigError(
-            "tags: at least one tag is required — without tags there is nothing to monitor"
+            "tags: nothing to monitor - define at least one tag or device "
+            "indicator (devices: entries alone are fine, tags: entries alone "
+            "are fine; both empty means the tool would read nothing)"
         )
     names = [t.name for t in config.tags]
     duplicates = sorted({n for n in names if names.count(n) > 1})

src/opcda_diagnozer/main.py

@@ -225,6 +225,8 @@ def _cmd_check_config(config_path: str) -> int:
         print(f"  devices: {len(config.devices)} link indicator(s):")
         for device in config.devices:
             print(f"    {device.name} <- {device.tag}")
+    if not config.tags and config.devices:
+        print("  note:   no surveillance tags - monitoring device links only")
     print(f"  poll:   every {config.poll.interval_s} s, hang flagged after {config.poll.timeout_ms} ms")
     return 0
 

tests/test_config.py

@@ -60,12 +60,33 @@ def test_unknown_top_level_section_rejected(tmp_path: Path) -> None:
         load_config(p)
 
 
-def test_empty_tags_rejected(tmp_path: Path) -> None:
+def test_both_tags_and_devices_empty_rejected(tmp_path: Path) -> None:
-    p = _write(tmp_path, "opc:\n  server: X\ntags: []\n")
+    """tags: and devices: are each optional, but their union must not be empty."""
-    with pytest.raises(ConfigError, match="at least one tag"):
+    p = _write(tmp_path, "opc:\n  server: X\ntags: []\ndevices: []\n")
+    with pytest.raises(ConfigError, match="nothing to monitor"):
         load_config(p)
 
 
+def test_no_tags_section_with_devices_is_allowed(tmp_path: Path) -> None:
+    """Devices-only config: link monitoring without extra surveillance tags."""
+    p = _write(
+        tmp_path,
+        "opc:\n  server: X\n"
+        "devices:\n  - { name: 'Pump A', tag: Ind.Tag }\n",
+    )
+    config = load_config(p)
+    assert config.tags == ()
+    assert [d.name for d in config.devices] == ["Pump A"]
+
+
+def test_tags_only_still_allowed(tmp_path: Path) -> None:
+    """The classic setup (no device indicators) stays valid."""
+    p = _write(tmp_path, "opc:\n  server: X\ntags:\n  - name: t\n")
+    config = load_config(p)
+    assert len(config.tags) == 1
+    assert config.devices == ()
+
+
 def test_duplicate_tags_rejected(tmp_path: Path) -> None:
     p = _write(tmp_path, "opc:\n  server: X\ntags:\n  - name: a\n  - name: a\n")
     with pytest.raises(ConfigError, match="duplicate"):