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

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.

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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

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Hi,

What is the main goal?

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...

Kind Regards,

Misa

2013/3/27 Clemens Eisserer <linuxhippy@gmail.com>

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 :)

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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...

The same you can achieve with PHP...

But I am not sure that is the main goal...

My guess is that main goal is let Web App know - something happened in
database...

I am not aware about something else in JDBC then the same principle...

One object (listner) - in background thread - always asks for notify DB -
if gets something - fires event... (then other Java objects - if subscribed
on Listner - do whatever need to doon that event....)

If we want - to let WebPage aware about what happened - still we are in the
problem... (if we will not use - some kind of polling from WebPage)

Potential Solution: websocket

The same solution would be possible and with php...

But better to dont make this mail more complex - if that is not the goal...
:)

Kind Regards,

Misa

2013/3/27 Clemens Eisserer <linuxhippy@gmail.com>

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

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On 2013-03-27, Misa Simic <misa.simic@gmail.com> wrote:

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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.

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Thanks Janes...

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

Than Clemens explained to me what select() does...

However - to me it is just implementation detail... Which is possible in
one language, but not in another...

But technically, is the loop inside select() - or outside is irrelevant to
me (i see the benefit... Though, not sure the benefit - is that
big...nowdays.....)

From my point of view - what is important - is the main goal what should be
achieved...

If with resources what I have (php - though I am not php developer either)
I dont have the same power like in another language - who cares - main goal
must be achived . things work - or dont, but we must make them work :)

I am just curious:

LISTEN foo
Select()

Another client says: NOTIFY bar

Will select() - get something from server?

Thanks,

Misa

On Thursday, March 28, 2013, Jasen Betts wrote:

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,
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