<!-- Source: https://docs.biosimulant.com/how-to/library/create-domain-pack -->

# How to Create a Domain Pack

A domain pack is a normal Python package that exports a coherent family of `BioModule` implementations, presets, and monitors.

## Suggested structure

```text
mypack/
  __init__.py
  inputs.py
  populations.py
  interactions.py
  monitors.py
  presets.py
```

## Example input module

```python

class DrugDose(biosim.BioModule):
    def __init__(self, dose_mg: float, schedule: list[float]):
        self.dose_mg = float(dose_mg)
        self.schedule = list(schedule)
        self._outputs = {}

    def outputs(self):
        return {
            "drug_concentration": biosim.SignalSpec.scalar(
                dtype="float64",
                emitted_unit="mg/L",
            )
        }

    def advance_window(self, start: float, end: float) -> None:
        active = any(abs(t - end) < 1e-9 for t in self.schedule)
        self._outputs = {
            "drug_concentration": biosim.ScalarSignal(
                source="drug",
                name="drug_concentration",
                value=self.dose_mg if active else 0.0,
                emitted_at=end,
                spec=self.outputs()["drug_concentration"],
            )
        }

    def get_outputs(self):
        return dict(self._outputs)

    def snapshot(self):
        return {}
```

## Example population module

```python
class CellPopulation(biosim.BioModule):
    def __init__(self, n: int, division_rate: float = 0.1):
        self.n = int(n)
        self.division_rate = float(division_rate)

    def inputs(self):
        return {"drug_concentration": biosim.SignalSpec.scalar(dtype="float64")}

    def outputs(self):
        return {
            "population_state": biosim.SignalSpec.record(
                schema={"count": "int64"},
            )
        }
```

## Export your public surface

```python
from .inputs import DrugDose
from .populations import CellPopulation
from .monitors import GrowthMonitor

__all__ = ["DrugDose", "CellPopulation", "GrowthMonitor"]
```

## Use the pack

```python

from mypack import DrugDose, CellPopulation, GrowthMonitor

world = biosim.BioWorld(communication_step=1.0)
builder = biosim.WiringBuilder(world)

builder.add("drug", DrugDose(dose_mg=100, schedule=[0, 24, 48]))
builder.add("cells", CellPopulation(n=1000))
builder.add("monitor", GrowthMonitor())

builder.connect("drug.drug_concentration", ["cells.drug_concentration"])
builder.connect("cells.population_state", ["monitor.population_state"])
builder.apply()

world.run(duration=72.0)
```

**Info:**

  Domain packs do not need to live under a `biosimulant.*` namespace. Stable import paths matter more than namespace conventions.

## Next steps

- [Create a BioModule](/how-to/library/create-biomodule)
- [Package & Publish](/how-to/library/package-and-publish)