Listening for postgres NOTIFY with django.db - adding support for connection.fileno()

[Fábio Molinar]

Hi, 

I have a custom method on my model manager that allows me to listen for notifications from modifications on the DB using postgreSQL. A short version of this code looks like this:

def listen_for_notify(self):

        import select

        import psycopg2

        import psycopg2.extensions

        from django.conf import settings

        

        db_data = settings.DATABASES['default']

        listened = None

        returned_empty = None

search_timeout = 15

        conn = psycopg2.connect(dbname=db_data['NAME'], user=db_data['USER'], password=db_data['PASSWORD'], host=db_data['HOST'], port=db_data['PORT'])

        conn.set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT)

        curs = conn.cursor()

        curs.execute("LISTEN default;")

        timeout = timezone.now() + timezone.timedelta(0, search_timeout)

        while timezone.now() < timeout:

            time_diff = timeout - timezone.now()

            if select.select([conn], [], [], float(time_diff.seconds)) == ([], [], []):

                listened = False

                timeout = timezone.now()

            else:

                conn.poll()

                while conn.notifies:

                    notify = conn.notifies.pop(0)

                    if notify.payload == "notified":

                        listened = True

                        returned_empty = False

                        timeout = timezone.now()

                    if notify.payload == 'search request returned empty':

                        listened = True

                        returned_empty = True

                        timeout = timezone.now()

        curs.close()

        conn.close()

        return listened, returned_empty

It would be really nice if instead of using the psycopg2 library, I could use only django.db. Something like this:

def listen_for_notify(self):

        from django.db import connection as conn

        

        listened = None

        returned_empty = None

search_timeout = 15

        with conn.cursor() as curs

timeout = timezone.now() + timezone.timedelta(0, search_timeout)

while timezone.now() < timeout:

time_diff = timeout - timezone.now()

if select.select([conn], [], [], float(time_diff.seconds)) == ([], [], []):

listened = False

timeout = timezone.now()

else:

conn.poll()

while conn.notifies:

notify = conn.notifies.pop(0)

if notify.payload == "notified":

listened = True

returned_empty = False

timeout = timezone.now()

if notify.payload == 'search request returned empty':

listened = True

returned_empty = True

timeout = timezone.now()

        return listened, returned_empty

I tried out the solution above using django.db, but it doesn't work because the django.db.connection object don't have a fileno() method.

Is this currently not supported or am I missing something? I though that the django.db is kind of just a wrapper around the actual psycopg2 library. So I wonder why I can't use the fileno() method on it.

[Florian Apolloner]

Hi,

On Monday, December 31, 2018 at 12:38:31 PM UTC+1, Fábio Molinar wrote:

Is this currently not supported or am I missing something? I though that the django.db is kind of just a wrapper around the actual psycopg2 library. So I wonder why I can't use the fileno() method on it.

django.db.connection provides a common interface for all databases, the underlying actual connection is stored as connection attribute as seen in https://github.com/django/django/blob/master/django/db/backends/base/base.py#L194 -- I hope this helps.

cheers,

Florian

[Fábio Molinar]

Thank you sir, that's what I was hoping to find! :)

Yours sincerely,

Fábio Thomaz Molinar

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