Architecture

Layers, modules and engine boundaries for contributing to or extending abSCADA.

2 min read

abSCADA is a modular monolith in Python. One application, but with a core that knows nothing about the UI: project, runtime, alarms and storage all work without Qt.

Layers

Studio (Qt) ──► Project (load & validate) ◄──► JSON files
     │
     └──► RuntimeWindow ──► Runtime (sample store)
                               ├── Write queue
                               ├── Worker per connection ──► Connector ──► S7 / Modbus
                               ├── OperationsService ──► SQLite (alarms, loggers)
                               └── ScriptService ──► isolated Python process

Main modules

Module	Responsibility	Does not know
`project.py`	Loading, validation, structures and faceplates	Qt, sockets
`connectors.py`	`Connector` contract, registry and S7	Widgets
`protocol_definition.py`	Connection and binding schemas	Qt, network
`modbus.py`	Modbus TCP client	Screens
`runtime.py`	Values, quality, queue, acquisition	Graphic editing
`alarms.py`	Alarm state machine	Transport
`storage.py`	SQLite, queries, retention	PLC clients
`scripting.py`	Script and task coordinator	Widgets
`graphics.py`	Canvas and drawing primitives	PLC clients
`ui.py`	Studio	Live readings
`runtime_window.py`	Operator window	Editable state

Principles

Studio never consumes samples. Runtime works on a deep copy of the project.
Writes are commands. They are validated before queuing and never replayed after reconnecting.
One worker per connection. A slow device never delays another.
A single SQLite writer. PLC workers only hand samples to a queue.
Open formats. No Qt objects or opaque binaries are serialised.

Sample model

Sample(value=72.4, quality="good", timestamp=1759651200.123, error=None)

quality is good, uncertain or bad. timestamp is the update time in the runtime, not a PLC timestamp.

Known limits

Reads inside a connection are individual. Thousands of variables will need grouped reads, metrics and stale quality. Maximum capacity has not been measured yet.