frequenz-channels

Frequenz channels

Introduction

Frequenz Channels is a channels implementation for Python.

According to Wikipedia:

A channel is a model for interprocess communication and synchronization via message passing. A message may be sent over a channel, and another process or thread is able to receive messages sent over a channel it has a reference to, as a stream. Different implementations of channels may be buffered or not, and either synchronous or asynchronous.

Frequenz Channels are mostly designed after Go channels but it also borrows ideas from Rust channels.

Supported Platforms

The following platforms are officially supported (tested):

• Python: 3.11
• Operating System: Ubuntu Linux 20.04
• Architectures: amd64, arm64

[!NOTE] Newer Python versions and other operating systems and architectures might work too, but they are not automatically tested, so we cannot guarantee it.

Quick Start

Installing

Assuming a supported working Python environment:

python3 -m pip install frequenz-channels

[!TIP] For more details please read the Installation Guide.

Examples

Hello World

import asyncio

from frequenz.channels import Anycast


async def main() -> None:
    hello_channel = Anycast[str](name="hello-world-channel")
    sender = hello_channel.new_sender()
    receiver = hello_channel.new_receiver()

    await sender.send("Hello World!")
    message = await receiver.receive()
    print(message)


asyncio.run(main())

Showcase

This is a comprehensive example that shows most of the main features of the library:

import asyncio
from dataclasses import dataclass
from datetime import timedelta
from enum import Enum, auto
from typing import assert_never

from frequenz.channels import (
    Anycast,
    Broadcast,
    Receiver,
    Sender,
    merge,
    select,
    selected_from,
)
from frequenz.channels.timer import SkipMissedAndDrift, Timer, TriggerAllMissed


class Command(Enum):
    PING = auto()
    STOP_SENDER = auto()


class ReplyCommand(Enum):
    PONG = auto()


@dataclass(frozen=True)
class Reply:
    reply: ReplyCommand
    source: str


async def send(
    sender: Sender[str],
    control_command: Receiver[Command],
    control_reply: Sender[Reply],
) -> None:
    """Send a counter value every second, until a stop command is received."""
    print(f"{sender}: Starting")
    timer = Timer(timedelta(seconds=1.0), TriggerAllMissed())
    counter = 0
    async for selected in select(timer, control_command):
        if selected_from(selected, timer):
            print(f"{sender}: Sending {counter}")
            await sender.send(f"{sender}: {counter}")
            counter += 1
        elif selected_from(selected, control_command):
            print(f"{sender}: Received command: {selected.message.name}")
            match selected.message:
                case Command.STOP_SENDER:
                    print(f"{sender}: Stopping")
                    break
                case Command.PING:
                    print(f"{sender}: Ping received, reply with pong")
                    await control_reply.send(Reply(ReplyCommand.PONG, str(sender)))
                case _ as unknown:
                    assert_never(unknown)
    print(f"{sender}: Finished")


async def receive(
    receivers: list[Receiver[str]],
    control_command: Receiver[Command],
    control_reply: Sender[Reply],
) -> None:
    """Receive data from multiple channels, until no more data is received for 2 seconds."""
    print("receive: Starting")
    timer = Timer(timedelta(seconds=2.0), SkipMissedAndDrift())
    print(f"{timer=}")
    merged = merge(*receivers)
    async for selected in select(merged, timer, control_command):
        if selected_from(selected, merged):
            message = selected.message
            print(f"receive: Received {message=}")
            timer.reset()
            print(f"{timer=}")
        elif selected_from(selected, control_command):
            print(f"receive: received command: {selected.message.name}")
            match selected.message:
                case Command.PING:
                    print("receive: Ping received, reply with pong")
                    await control_reply.send(Reply(ReplyCommand.PONG, "receive"))
                case Command.STOP_SENDER:
                    pass  # Ignore
                case _ as unknown:
                    assert_never(unknown)
        elif selected_from(selected, timer):
            drift = selected.message
            print(
                f"receive: No data received for {timer.interval + drift} seconds, "
                "giving up"
            )
            break
    print("receive: Finished")


async def main() -> None:
    data_channel_1 = Anycast[str](name="data-channel-1")
    data_channel_2 = Anycast[str](name="data-channel-2")
    command_channel = Broadcast[Command](name="control-channel")  # (1)!
    reply_channel = Anycast[Reply](name="reply-channel")

    async with asyncio.TaskGroup() as tasks:
        tasks.create_task(
            send(
                data_channel_1.new_sender(),
                command_channel.new_receiver(),
                reply_channel.new_sender(),
            ),
            name="send-channel-1",
        )
        tasks.create_task(
            send(
                data_channel_2.new_sender(),
                command_channel.new_receiver(),
                reply_channel.new_sender(),
            ),
            name="send-channel-2",
        )
        tasks.create_task(
            receive(
                [data_channel_1.new_receiver(), data_channel_2.new_receiver()],
                command_channel.new_receiver(),
                reply_channel.new_sender(),
            ),
            name="receive",
        )

        control_sender = command_channel.new_sender()
        reply_receiver = reply_channel.new_receiver()

        # Send a ping command to all tasks and wait for the replies
        await control_sender.send(Command.PING)
        print(f"main: {await reply_receiver.receive()}")
        print(f"main: {await reply_receiver.receive()}")
        print(f"main: {await reply_receiver.receive()}")

        await asyncio.sleep(5.0)

        # Stop senders, after 2 seconds not receiving any data,
        # the receiver will stop too
        await control_sender.send(Command.STOP_SENDER)


asyncio.run(main())

Documentation

For more information, please read the documentation website.

Contributing

If you want to know how to build this project and contribute to it, please check out the Contributing Guide.