Send 0 values from power formulas for non-existant component types (#151)

Earlier, the automatically generated formulas for `pv_power` and
`battery_power` were raising exception in cases where pv or battery
inverters were not found in the component graph, preventing us from
having a single high level formula that can be reused in all locations.

This is solved by subscribing to a non-existing component id, so that we
can still send out 0 values at the same rate as the resampler.

Author: sahas-subramanian-frequenz

src/frequenz/sdk/timeseries/logical_meter/_formula_generators/_battery_power_formula.py

@@ -3,10 +3,14 @@
 
 """Formula generator from component graph for Grid Power."""
 
+import logging
+
 from .....sdk import microgrid
 from ....microgrid.component import ComponentCategory, ComponentMetricId, InverterType
 from .._formula_engine import FormulaEngine
-from ._formula_generator import ComponentNotFound, FormulaGenerator
+from ._formula_generator import NON_EXISTING_COMPONENT_ID, FormulaGenerator
+
+logger = logging.getLogger(__name__)
 
 
 class BatteryPowerFormula(FormulaGenerator):
@@ -42,9 +46,18 @@ async def generate(
         )
 
         if not battery_inverters:
-            raise ComponentNotFound(
-                "Unable to find any battery inverters in the component graph."
+            logging.warning(
+                "Unable to find any battery inverters in the component graph. "
+                "Subscribing to the resampling actor with a non-existing "
+                "component id, so that `0` values are sent from the formula."
+            )
+            # If there are no battery inverters, we have to send 0 values as the same
+            # frequency as the other streams.  So we subscribe with a non-existing
+            # component id, just to get a `None` message at the resampling interval.
+            await builder.push_component_metric(
+                NON_EXISTING_COMPONENT_ID, nones_are_zeros=True
             )
+            return builder.build()
 
         for idx, comp in enumerate(battery_inverters):
             if idx > 0:

src/frequenz/sdk/timeseries/logical_meter/_formula_generators/_formula_generator.py

@@ -3,6 +3,7 @@
 
 """Base class for formula generators that use the component graphs."""
 
+import sys
 from abc import ABC, abstractmethod
 
 from frequenz.channels import Sender
@@ -21,6 +22,9 @@ class ComponentNotFound(FormulaGenerationError):
     """Indicates that a component required for generating a formula is not found."""
 
 
+NON_EXISTING_COMPONENT_ID = sys.maxsize
+
+
 class FormulaGenerator(ABC):
     """A class for generating formulas from the component graph."""
 

src/frequenz/sdk/timeseries/logical_meter/_formula_generators/_pv_power_formula.py

@@ -3,10 +3,14 @@
 
 """Formula generator for PV Power, from the component graph."""
 
+import logging
+
 from .....sdk import microgrid
 from ....microgrid.component import ComponentCategory, ComponentMetricId, InverterType
 from .._formula_engine import FormulaEngine
-from ._formula_generator import ComponentNotFound, FormulaGenerator
+from ._formula_generator import NON_EXISTING_COMPONENT_ID, FormulaGenerator
+
+logger = logging.getLogger(__name__)
 
 
 class PVPowerFormula(FormulaGenerator):
@@ -32,9 +36,18 @@ async def generate(self) -> FormulaEngine:
         )
 
         if not pv_inverters:
-            raise ComponentNotFound(
-                "Unable to find any PV inverters in the component graph."
+            logging.warning(
+                "Unable to find any PV inverters in the component graph. "
+                "Subscribing to the resampling actor with a non-existing "
+                "component id, so that `0` values are sent from the formula."
+            )
+            # If there are no PV inverters, we have to send 0 values as the same
+            # frequency as the other streams.  So we subscribe with a non-existing
+            # component id, just to get a `None` message at the resampling interval.
+            await builder.push_component_metric(
+                NON_EXISTING_COMPONENT_ID, nones_are_zeros=True
             )
+            return builder.build()
 
         for idx, comp in enumerate(pv_inverters):
             if idx > 0:

tests/timeseries/test_logical_meter.py

@@ -253,7 +253,7 @@ async def test_formula_composition(  # pylint: disable=too-many-locals
         self,
         mocker: MockerFixture,
     ) -> None:
-        """Test the battery power and pv power formulas."""
+        """Test the composition of formulas."""
         mockgrid = await MockMicrogrid.new(mocker, grid_side_meter=False)
         mockgrid.add_batteries(3)
         mockgrid.add_solar_inverters(2)
@@ -300,3 +300,65 @@ async def test_formula_composition(  # pylint: disable=too-many-locals
         await engine._stop()  # pylint: disable=protected-access
 
         assert count == 10
+
+    async def test_formula_composition_missing_pv(self, mocker: MockerFixture) -> None:
+        """Test the composition of formulas with missing PV power data."""
+        mockgrid = await MockMicrogrid.new(mocker, grid_side_meter=False)
+        mockgrid.add_batteries(3)
+        request_sender, channel_registry = await mockgrid.start()
+        logical_meter = LogicalMeter(
+            channel_registry,
+            request_sender,
+            microgrid.get().component_graph,
+        )
+
+        battery_power_recv = await logical_meter.battery_power()
+        pv_power_recv = await logical_meter.pv_power()
+        engine = (pv_power_recv.clone() + battery_power_recv.clone()).build("inv_power")
+        inv_calc_recv = engine.new_receiver()
+
+        count = 0
+        for _ in range(10):
+            bat_pow = await battery_power_recv.receive()
+            pv_pow = await pv_power_recv.receive()
+            inv_pow = await inv_calc_recv.receive()
+
+            assert inv_pow == bat_pow
+            assert pv_pow.timestamp == inv_pow.timestamp and pv_pow.value == 0.0
+            count += 1
+
+        await mockgrid.cleanup()
+        await engine._stop()  # pylint: disable=protected-access
+
+        assert count == 10
+
+    async def test_formula_composition_missing_bat(self, mocker: MockerFixture) -> None:
+        """Test the composition of formulas with missing battery power data."""
+        mockgrid = await MockMicrogrid.new(mocker, grid_side_meter=False)
+        mockgrid.add_solar_inverters(2)
+        request_sender, channel_registry = await mockgrid.start()
+        logical_meter = LogicalMeter(
+            channel_registry,
+            request_sender,
+            microgrid.get().component_graph,
+        )
+
+        battery_power_recv = await logical_meter.battery_power()
+        pv_power_recv = await logical_meter.pv_power()
+        engine = (pv_power_recv.clone() + battery_power_recv.clone()).build("inv_power")
+        inv_calc_recv = engine.new_receiver()
+
+        count = 0
+        for _ in range(10):
+            bat_pow = await battery_power_recv.receive()
+            pv_pow = await pv_power_recv.receive()
+            inv_pow = await inv_calc_recv.receive()
+
+            assert inv_pow == pv_pow
+            assert bat_pow.timestamp == inv_pow.timestamp and bat_pow.value == 0.0
+            count += 1
+
+        await mockgrid.cleanup()
+        await engine._stop()  # pylint: disable=protected-access
+
+        assert count == 10