# How Biosimulant Works `Biosimulant` is an open-source Python engine for composing biological simulators and finite computations from modules with explicit typed ports. BioWorld drains before/after computation and advances temporal modules through atomic communication windows. If you want a runnable introduction, start with the [Library Quickstart (Python)](/overview/library-quickstart). This page explains the architecture behind that example. ## The five core primitives | Primitive | Role | Reference | |-----------|------|-----------| | `BioModule` | A runnable temporal or finite-computation component | [BioModule API](/references/biomodule-api) | | `BioWorld` | The communication-step orchestrator | [BioWorld API](/references/bioworld-api) | | `SignalSpec` | A declared port contract | [BioSignal & SignalSpec](/references/signals) | | Typed `BioSignal` subclasses | Runtime payloads crossing module boundaries | [BioSignal & SignalSpec](/references/signals) | | `WiringBuilder` | A validated graph builder for named module ports | [WiringBuilder API](/references/wiring-api) | ## BioWorld: communication windows Create a world with a required communication step: ```python world = biosim.BioWorld(communication_step=0.1) ``` `ONCE_BEFORE_RUN` modules run first, in dependency order. Then, for each window `[t, t + communication_step]`: 1. the world reads committed signals at `t` 2. it delivers those inputs to every connected target module 3. every `EACH_WINDOW` module advances or executes across the same positive window 4. the world commits all outputs atomically at `t + communication_step` After the final window commits, `ONCE_AFTER_RUN` modules run in dependency order. This means: - signal exchange is synchronized at shared communication boundaries - state signals use hold-last-value semantics until replaced - event signals are delivered once per connection per source timestamp - stale reads are checked from the consuming port's `SignalSpec` The kernel does not expose an execution-order scheduling contract, a global external tick parameter, rollback, or algebraic-loop solving. A module sees another module's output on the communication turn after it is committed. Final outputs are committed at the last boundary, so a downstream module that needs them should use `ONCE_AFTER_RUN`. Older temporal modules that implement `advance_window()` can use `settle_steps` / `world.settle()` instead.