Is there a way in Django to execute some python code only once when gunicorn / celery services are getting restarted?

Question

My Django Application contains celery based tasks, which download certain files from a SFTP server every time it executes. Till now i create an SFTP connection every time a job starts and close the connection at the end of job. But some how it is cause file descriptor error in code (too many SFTP connection opened). I am thinking of making SFTP connection via a singleton class so that only one SFTP connection gets established and that to only at the time when i start the gunicorn/celery services.

Is there a way i can execute the python code to create an SFTP connection only once when the application comes up or gets restarted? and its closed the services are stopped?

Answer 1

Singleton class

class Singleton:
    def __init__(self, klass):
        self.klass = klass
        self.instance = None

    def __call__(self, *args, **kwds):
        if self.instance is None:
            self.instance = self.klass(*args, **kwds)
        return self.instance

@Singleton
class SFTP_SINGLETON(object):

    def __init__(self, hostname=None, username=None, password=None, port=22, default_path=None):

        self._hostname = hostname
        self._username = username
        self._Password = password

        self._sftp = None
        self._port = port
        self._default_path = default_path
        self._sftp_live = False
        self._transport = self._start_transport()

    def _start_transport(self):
        try:
            self._transport = paramiko.Transport((self._hostname, self._port))
            self._transport.connect(username=self._username, password=self._Password)
            logger.info(" connected to {} : {}".format(self._hostname, self._port))
            return self._transport
        except (AttributeError, socket.gaierror) as e:
            # couldn't connect
            logger.info(" exception in establish_connection : %s ", e)
            raise ConnectionException(self._hostname, self._port)

    def connect(self):
        """Establish the SFTP connection."""
        if not self._sftp_live:
            self._sftp = paramiko.SFTPClient.from_transport(self._transport)
            # if self._default_path is not None:
            #     self._sftp.chdir(self._default_path)
            self._sftp_live = True

    def close(self):
        """Closes the connection and cleans up."""
        # Close SFTP Connection.
        if self._sftp_live:
            self._sftp.close()
            self._sftp_live = False
        # Close the SSH Transport.
        if self._transport:
            self._transport.close()
            self._transport = None