# Architecture

## Decision: modular engine first

The processing graph is the common model for audio sources, instruments,
effects, mixers, and outputs. The sequencer produces time-based content and
events consumed by graph nodes. The default workspace remains sequencer-first;
the graph editor is an advanced view of the same project.

## Layers

1. **FLTK/C++ GUI:** input, drawing, windows, view state.
2. **C core:** project model, commands, parameters, persistence, validation.
3. **C engine:** prepared processing graph, DSP, transport timing, event delivery.
4. **Backend:** hardware and system audio/MIDI integration.

The GUI must not directly modify engine internals. It calls a stable C API.

## Graph execution

The first implementation should reject cycles. Compile a graph off the audio
thread: validate, topologically sort, determine buffer requirements, allocate
resources, and publish a prepared immutable execution plan. The audio thread
must not traverse a graph being edited.

The current scaffold is deliberately incomplete: `xt_graph_process_offline()`
validates the graph and applies gain nodes in insertion order. It is a test
placeholder, not the final graph execution algorithm.

## Ownership and threads

- GUI thread owns FLTK widgets and view state.
- Project/control thread owns mutable project data and graph compilation.
- Audio thread owns the active processing plan and DSP state.
- Backend callbacks follow the backend's documented threading contract.

Do not free an active plan until the audio thread has released it. Pick and
document a bounded handoff/reclamation strategy before real-time graph swaps.