Thread: Is there any way to listen to NOTIFY in php without polling?

Hi,

Sorry for asking such a newbie-question, I've used a search engine -
however I haven't found what I am searching for.

Is there any way to listen to NOTIFY in php without polling using a
callback or blocking call?
I've only found pg_get_notify(), however it requires polling as far as
I understand.

Thank you in advance, Clemens

In response to Clemens Eisserer <linuxhippy@gmail.com>:
>
> Is there any way to listen to NOTIFY in php without polling using a
> callback or blocking call?

Not at this time.

--
Bill Moran
http://www.potentialtech.com
http://people.collaborativefusion.com/~wmoran/

Hi Bill,

>> Is there any way to listen to NOTIFY in php without polling using a
>> callback or blocking call?
>
> Not at this time.

Too bad ... Thanks for the confirmation.

I'll try to invoke a native libpg binary which stays alive until a
NOTIFY is received, should do the trick in case update-frequency is
low.

Thanks, Clemens

Hi Misa

> What is the main goal?

The main goal is to perform some inter-system communication in the
case some rows in one table are updated (very seldom event).

> even using libpg - you need to call pg notify... Doc says, just using
> libpgtcl would be possible to get Notify event - without checking from time
> to time...

I found example 27-2 at
http://www.postgresql.org/docs/8.0/static/libpq-example.html , that
does exactly what I was looking for using low-level socket functions.
It seems to me this solution works without polling.

Regards, Clemens

PS: Compared to the libpq and php interfaces, the jdbc driver does a
really good job providing a useable interface for listen/notify to the
developers. Thanks :)

Hi Misa

> But Example shows - that C app - actually asks all the time to get notify...
> when gets something more then 4 times - exit... until 4 times loops...
> The same you can achieve with PHP...

As far as I understood, with php I have to query the server again and
again, and pg_get_notify will either return something or not depending
on the query result:
http://php.net/manual/en/function.pg-get-notify.php

The provided sample however blocks until a notify is received (and
does so 4 times just for demonstration puporse), so instead of
frequently asking the server for notifications (polling), it wakes up
when a notification arrives.

> My guess is that main goal is let Web App know - something happened in
> database...
It isn't ;)

Regards, Clemens

On 2013-03-27, Misa Simic <misa.simic@gmail.com> wrote:
> --20cf3074d6a0c370ce04d8ef50c1
> Content-Type: text/plain; charset=UTF-8
>
> Hi Clemens,
>
> Well, I am not sure what you mean by polling...
>
> But Example shows - that C app - actually asks all the time to get
> notify... when gets something more then 4 times - exit... until 4 times
> loops...

that's polling.

I just do a select() on the socket and the select either times out or
lets me know something has come from the server. then I call whatever
it is to check for notifies.

> The same you can achieve with PHP...

except PHP doesn't have select() and hides the socket.

--
⚂⚃ 100% natural

    Misa Simic wrote:

> I am not a C developer - was not aware about select()... I was read it as
> some kind of sleep...

php provides socket_select() as an equivalent to C's select().
See http://php.net/manual/en/function.socket-select.php

But it takes "socket resources" as arguments and the postgres php layer
does not provides a conversion from the file descriptor returned by
PQsocket() to such php socket resources.
Thus socket_select() can't be used to monitor a postgres connection.

However, you can get a behavior that's close enough in practice to
select() with code like this:

pg_query($conn, "LISTEN event_name");
while (!$end) {
  $arr=pg_get_notify($conn);
  if (!$arr) {
    usleep(100000);
  }
  else
    // process the notification
}

This will catch a notification 1/10 second max after it's available and sleep
the rest of the time.

Best regards,
--
Daniel
PostgreSQL-powered mail user agent and storage: http://www.manitou-mail.org