Hello World

Full source code

This tutorial builds a simple Rosia application: one node sends a message, another prints it.

Define Nodes

A Rosia node is a Python class decorated with @Node. Ports are declared as class attributes.

from rosia import InputPort, OutputPort, reaction, Node, Application, log

@Node
class Greeter:
    output = OutputPort[str]()  # declares a typed output port

    # start() is called once before the event loop begins
    def start(self):
        self.output("Hello, World!")  # Send a value on the output port

@Node
class Printer:
    message = InputPort[str]() # declares a typed input port.  Do not use `input` as a port name since it's a reserved Python keyword

    @reaction([message])  # Reaction is executed when listed port receives a message
    def print_message(self):
        log.info(self.message)  # self.message reads the current value of the input port
        # rosia's log function prefixes messages with the node name (e.g. [Printer_1])
        # Available levels: log.debug(), log.info(), log.warning(), log.error(), log.critical()

Wire and Run

Create a Application, instantiate nodes, connect ports with >>=, and call execute(). Connections are automatically type checked.

app = Application()
greeter = app.create_node(Greeter())
printer = app.create_node(Printer())
greeter.output >>= printer.message
app.diagram(save_to="hello_diagram.png")
app.execute()

Optionally, app.diagram(save_to="hello_diagram.png") generates a visualization of the dataflow graph, showing nodes and their port connections:

Run with python hello.py. This produces:

[Printer_1] Hello, World!

What happened

1. Greeter.start() sends "Hello, World!" on its output port.
2. Printer reacts to the message and logs it. Since greeter is numbered 0, printer is numbered 1.
3. Since Greeter has no more messages to send, it signals completion. Printer receives this signal, detects it has no more work, and the application shuts down automatically.