wait for a server to respond in python

When starting a python application, our application may wait for a server to respond.

This article introduce a common technique to wait for a server to be available using socket from python stdlib. A script may wait for a database to start or for an API to become available.

I have written wait_port, the following function, for the open source library fixtup. fixtup starts containers so that a test can run. There is a need to wait before returning control or executing migration code until the container is operational. The wait_port helper allows you to wait in a hook before returning control to pytest.

# wait postgresql to start until 5 seconds or raise Timeout exception
wait_port(5432, timeout=5000) 

here is the snippet you can reuse in your project

def wait_port(port: int, host: str = 'localhost', timeout: Optional[int] = None, attempt_every: int = 100) -> None:
    """
    wait until a port would be open, for example the port 5432 for postgresql
    before going further

    >>> fixtup.helper.wait_port(5432, timeout=5000)

    :param port: port that has to be open
    :param remote_ip: host on which the port has to be open. It will be localhost by default
    :param timeout: timeout in ms before raising TimeoutError.
    :param attempt_every: time in ms between each attempt to check if the port is responding
    """
    start = monotonic()
    connected = False
    while not connected:
        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
            try:
                s.connect((host, port))
                connected = True
            except ConnectionRefusedError:
                if timeout is not None and monotonic() - start > (timeout / 1000):
                    raise TimeoutError()

        sleep(attempt_every / 1000)

We can go further with the principle of this snippet. It can be used to verify that a database is ready for use by checking for the existence of a table or by executing a SELECT 1 + 1 query instead of connecting with a socket.

If you had to write resilient code to wait for a server, share your experience with us in the comments.