start.py

"""Over-engineered Python 3.10+ version of bash script with netcat (nc) just for fun.

    #!/bin/bash

    check_reachability() {
    while ! nc -z "$1" "${!2}"
    do
      echo "Waiting for $3 to be reachable on port ${!2}"
      sleep 1
    done
    echo "Connection to $3 on port ${!2} verified"
    return 0
    }


    wait_for_services_to_be_reachable() {
      check_reachability rabbitmq RABBITMQ_PORT RabbitMQ
      check_reachability postgres POSTGRES_PORT PostgreSQL
    }

    wait_for_services_to_be_reachable
    exit 0
"""

import asyncio
import logging
import os
import sys
from dataclasses import dataclass, field
from typing import Generator, Type

SOCK_CONNECTED = 0
DEFAULT_PORT = 0
DEFAULT_SLEEP_TIME = 1


class ServiceRegistry(type):
    REGISTRY: dict[str, type['BaseService']] = {}

    def __new__(
        mcs: Type['ServiceRegistry'],
        name: str,
        bases: tuple[type['BaseService']],
        attrs: dict,
    ) -> type['BaseService']:
        service_cls: type['BaseService'] = type.__new__(mcs, name, bases, attrs)
        mcs.REGISTRY[service_cls.__name__] = service_cls
        return service_cls

    @classmethod
    def get_registry(mcs) -> dict[str, type['BaseService']]:
        return mcs.REGISTRY.copy()

    @classmethod
    def get_instances(mcs) -> Generator['BaseService', None, None]:
        return (service_cls() for service_cls in mcs.REGISTRY.values())


@dataclass
class BaseService:
    name: str = field(default='', init=False)
    host: str = field(default='', init=False)
    port: int = field(default=DEFAULT_PORT, init=False)

    def __post_init__(self) -> None:
        if self.__class__ is BaseService:
            raise TypeError('Cannot instantiate abstract class.')


@dataclass
class RabbitMQService(BaseService, metaclass=ServiceRegistry):
    name: str = field(default='RabbitMQ')
    host: str = field(default=os.getenv('RABBITMQ_HOST'))
    port: int = field(default=int(os.getenv('RABBITMQ_PORT', DEFAULT_PORT)))


@dataclass
class PostgreSQLService(BaseService, metaclass=ServiceRegistry):
    name: str = field(default='PostgreSQL')
    host: str = field(default=os.getenv('POSTGRES_HOST'))
    port: int = field(default=int(os.getenv('POSTGRES_PORT', DEFAULT_PORT)))


async def is_port_open(host: str, port: int) -> bool:
    try:
        reader, writer = await asyncio.open_connection(host, port)
        writer.close()
        await writer.wait_closed()
        return True
    except Exception:
        return False


async def check_reachability(service: BaseService) -> None:
    while True:
        print(f'[{service.name}] Waiting to be reachable on port {service.port}')
        if await is_port_open(host=service.host, port=service.port):
            break
        await asyncio.sleep(DEFAULT_SLEEP_TIME)
    print(f'[{service.name}] Connection on port {service.port} verified')


async def main() -> None:
    logging.getLogger('asyncio').setLevel(logging.ERROR)
    coros = [check_reachability(service) for service in ServiceRegistry.get_instances()]
    await asyncio.gather(*coros)


if __name__ == '__main__':
    sys.exit(asyncio.run(main()))