Обсуждение: How to get a signal from the database when a INSERT INTO is done?

Hi all,

I'm designing an application in Python and using PostgreSQL.

This is a mixed question Python/PostgreSQL... I need to get a signal
in my python application when a new insert is done. How can this be
done, any clues?


Best Regards,
André

On Tue, Feb 28, 2012 at 11:07:47AM +0000, Andre Lopes wrote:
> Hi all,
>
> I'm designing an application in Python and using PostgreSQL.
>
> This is a mixed question Python/PostgreSQL... I need to get a signal
> in my python application when a new insert is done. How can this be
> done, any clues?

listen/notify
http://www.postgresql.org/docs/current/interactive/sql-listen.html
http://www.postgresql.org/docs/current/interactive/sql-notify.html

Best regards,

depesz

--
The best thing about modern society is how easy it is to avoid contact with it.
                                                             http://depesz.com/

On Tue, Feb 28, 2012 at 13:07, Andre Lopes <lopes80andre@gmail.com> wrote:
> This is a mixed question Python/PostgreSQL... I need to get a signal
> in my python application when a new insert is done. How can this be
> done, any clues?

As depesz mentioned, you can use the LISTEN and NOTIFY commands for
this asynchronous signalling. You can add an after-insert trigger to
send the NOTIFY.

On the Python end, you have to call psycopg2 connection.poll() method
periodically or in response to select() activation, and check the
connection.notifies list. Note that this list can grow even when you
run other queries -- not just on poll.

There's an example here:
http://initd.org/psycopg/docs/advanced.html#async-notify

Regards,
Marti

On Tue, Feb 28, 2012 at 2:15 PM, Marti Raudsepp <marti@juffo.org> wrote:
> On Tue, Feb 28, 2012 at 13:07, Andre Lopes <lopes80andre@gmail.com> wrote:
>> This is a mixed question Python/PostgreSQL... I need to get a signal
>> in my python application when a new insert is done. How can this be
>> done, any clues?
>
> As depesz mentioned, you can use the LISTEN and NOTIFY commands for
> this asynchronous signalling. You can add an after-insert trigger to
> send the NOTIFY.
>
> On the Python end, you have to call psycopg2 connection.poll() method
> periodically or in response to select() activation, and check the
> connection.notifies list. Note that this list can grow even when you
> run other queries -- not just on poll.
>
> There's an example here:
> http://initd.org/psycopg/docs/advanced.html#async-notify

Using an asynchronous IO-driven framework such as eventlet or gevent
you don't even need to poll the connection to look for events: you
just get notified as soon as there is something to read. Here is an
example:

http://initd.org/psycopg/articles/2010/12/01/postgresql-notifications-psycopg2-eventlet/

It's enough to have a NOTIFY executed in a trigger on insert, maybe
putting the id in the notify payload, and you get timely notifications
about inserts in Python world.

You may be able to do something similar in Twisted using txPostgres:
when I asked Jan about that, IIRC he said they weren't ready yet, but
it was several months ago, it may have changed since then.

-- Daniele

On Tue, Feb 28, 2012 at 17:41, Daniele Varrazzo
<daniele.varrazzo@gmail.com> wrote:
> On Tue, Feb 28, 2012 at 2:15 PM, Marti Raudsepp <marti@juffo.org> wrote:
>> On the Python end, you have to call psycopg2 connection.poll() method
>> periodically or in response to select() activation
>> There's an example here:
>> http://initd.org/psycopg/docs/advanced.html#async-notify

> Using an asynchronous IO-driven framework such as eventlet or gevent
> you don't even need to poll the connection to look for events

As mentioned above and as demonstrated in the example, select() also
does the job. Using such a fancy framework is usually an overkill.

Regards,
Marti

On Tue, Feb 28, 2012 at 3:45 PM, Marti Raudsepp <marti@juffo.org> wrote:
> On Tue, Feb 28, 2012 at 17:41, Daniele Varrazzo
> <daniele.varrazzo@gmail.com> wrote:
>> On Tue, Feb 28, 2012 at 2:15 PM, Marti Raudsepp <marti@juffo.org> wrote:
>>> On the Python end, you have to call psycopg2 connection.poll() method
>>> periodically or in response to select() activation
>>> There's an example here:
>>> http://initd.org/psycopg/docs/advanced.html#async-notify
>
>> Using an asynchronous IO-driven framework such as eventlet or gevent
>> you don't even need to poll the connection to look for events
>
> As mentioned above and as demonstrated in the example, select() also
> does the job. Using such a fancy framework is usually an overkill.

Yeah, the problem is usually if you have to do something else apart
from listening from the notification. select() will block the entire
application, so you would put it into a separate thread to have the
app running on. From here to start benefiting from eventlet the step
is very short. If a blocking behaviour is ok, then no problem using
bare select().

-- Daniele

Daniele Varrazzo <daniele.varrazzo@gmail.com> writes:

>> As mentioned above and as demonstrated in the example, select() also
>> does the job. Using such a fancy framework is usually an overkill.

> Yeah, the problem is usually if you have to do something else apart
> from listening from the notification. select() will block the entire
> application, so you would put it into a separate thread to have the
> app running on.

I don't know Python, but the C library function of select() has a
timeout parameter that can be set to 0.