1999-07-22 17:09:15 +02:00
|
|
|
<!--
|
2001-09-13 17:55:24 +02:00
|
|
|
$Header: /cvsroot/pgsql/doc/src/sgml/ref/notify.sgml,v 1.15 2001/09/13 15:55:24 petere Exp $
|
1999-07-22 17:09:15 +02:00
|
|
|
Postgres documentation
|
|
|
|
-->
|
|
|
|
|
1999-07-06 19:16:42 +02:00
|
|
|
<refentry id="SQL-NOTIFY">
|
|
|
|
<refmeta>
|
1999-07-22 17:09:15 +02:00
|
|
|
<refentrytitle id="sql-notify-title">
|
1999-07-06 19:16:42 +02:00
|
|
|
NOTIFY
|
|
|
|
</refentrytitle>
|
|
|
|
<refmiscinfo>SQL - Language Statements</refmiscinfo>
|
|
|
|
</refmeta>
|
|
|
|
<refnamediv>
|
|
|
|
<refname>
|
|
|
|
NOTIFY
|
|
|
|
</refname>
|
|
|
|
<refpurpose>
|
2001-09-03 14:57:50 +02:00
|
|
|
generate a notification
|
1999-07-06 19:16:42 +02:00
|
|
|
</refpurpose>
|
|
|
|
</refnamediv>
|
|
|
|
<refsynopsisdiv>
|
|
|
|
<refsynopsisdivinfo>
|
1999-07-22 17:09:15 +02:00
|
|
|
<date>1999-07-20</date>
|
1999-07-06 19:16:42 +02:00
|
|
|
</refsynopsisdivinfo>
|
|
|
|
<synopsis>
|
|
|
|
NOTIFY <replaceable class="PARAMETER">name</replaceable>
|
|
|
|
</synopsis>
|
|
|
|
|
|
|
|
<refsect2 id="R2-SQL-NOTIFY-1">
|
|
|
|
<refsect2info>
|
|
|
|
<date>1998-10-07</date>
|
|
|
|
</refsect2info>
|
|
|
|
<title>
|
|
|
|
Inputs
|
|
|
|
</title>
|
|
|
|
<para>
|
|
|
|
|
|
|
|
<variablelist>
|
|
|
|
<varlistentry>
|
|
|
|
<term><replaceable class="PARAMETER">notifyname</replaceable></term>
|
|
|
|
<listitem>
|
|
|
|
<para>
|
|
|
|
Notify condition to be signaled.
|
|
|
|
</para>
|
|
|
|
</listitem>
|
|
|
|
</varlistentry>
|
|
|
|
</variablelist>
|
|
|
|
</para>
|
|
|
|
</refsect2>
|
|
|
|
|
|
|
|
<refsect2 id="R2-SQL-NOTIFY-2">
|
|
|
|
<refsect2info>
|
|
|
|
<date>1998-10-07</date>
|
|
|
|
</refsect2info>
|
|
|
|
<title>
|
|
|
|
Outputs
|
|
|
|
</title>
|
|
|
|
<para>
|
|
|
|
|
|
|
|
<variablelist>
|
|
|
|
<varlistentry>
|
|
|
|
<term><computeroutput>
|
1998-09-02 05:01:02 +02:00
|
|
|
NOTIFY
|
1999-07-06 19:16:42 +02:00
|
|
|
</computeroutput></term>
|
|
|
|
<listitem>
|
|
|
|
<para>
|
|
|
|
Acknowledgement that notify command has executed.
|
|
|
|
</para>
|
|
|
|
</listitem>
|
|
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
|
|
<term><replaceable>Notify events</replaceable></term>
|
|
|
|
<listitem>
|
|
|
|
<para>
|
|
|
|
Events are delivered to listening frontends; whether and how each frontend
|
|
|
|
application reacts depends on its programming.
|
|
|
|
</para>
|
|
|
|
</listitem>
|
|
|
|
</varlistentry>
|
|
|
|
</variablelist>
|
|
|
|
</para>
|
|
|
|
</refsect2>
|
|
|
|
</refsynopsisdiv>
|
|
|
|
|
|
|
|
<refsect1 id="R1-SQL-NOTIFY-1">
|
|
|
|
<refsect1info>
|
|
|
|
<date>1998-10-07</date>
|
|
|
|
</refsect1info>
|
|
|
|
<title>
|
|
|
|
Description
|
|
|
|
</title>
|
|
|
|
<para>
|
|
|
|
The <command>NOTIFY</command> command sends a notify event to each
|
|
|
|
frontend application that has previously executed
|
|
|
|
<command>LISTEN <replaceable class="parameter">notifyname</replaceable></command>
|
|
|
|
for the specified notify condition in the current database.
|
|
|
|
</para>
|
|
|
|
<para>
|
|
|
|
The information passed to the frontend for a notify event includes the notify
|
|
|
|
condition name and the notifying backend process's PID. It is up to the
|
|
|
|
database designer to define the condition names that will be used in a given
|
|
|
|
database and what each one means.
|
|
|
|
</para>
|
|
|
|
<para>
|
|
|
|
Commonly, the notify condition name is the same as the name of some table in
|
2001-09-13 17:55:24 +02:00
|
|
|
the database, and the notify event essentially means <quote>I changed this table,
|
|
|
|
take a look at it to see what's new</quote>. But no such association is enforced by
|
1999-07-06 19:16:42 +02:00
|
|
|
the <command>NOTIFY</command> and <command>LISTEN</command> commands. For
|
|
|
|
example, a database designer could use several different condition names
|
|
|
|
to signal different sorts of changes to a single table.
|
|
|
|
</para>
|
|
|
|
<para>
|
|
|
|
<command>NOTIFY</command> provides a simple form of signal or
|
|
|
|
IPC (interprocess communication) mechanism for a collection of processes
|
|
|
|
accessing the same <productname>Postgres</productname> database.
|
|
|
|
Higher-level mechanisms can be built by using tables in the database to
|
|
|
|
pass additional data (beyond a mere condition name) from notifier to
|
|
|
|
listener(s).
|
|
|
|
</para>
|
|
|
|
<para>
|
|
|
|
When <command>NOTIFY</command> is used to signal the occurrence of changes
|
|
|
|
to a particular table, a useful programming technique is to put the
|
|
|
|
<command>NOTIFY</command> in a rule that is triggered by table updates.
|
|
|
|
In this way, notification happens automatically when the table is changed,
|
|
|
|
and the application programmer can't accidentally forget to do it.
|
|
|
|
</para>
|
|
|
|
<para>
|
|
|
|
<command>NOTIFY</command> interacts with SQL transactions in some important
|
|
|
|
ways. Firstly, if a <command>NOTIFY</command> is executed inside a
|
|
|
|
transaction, the notify events are not delivered until and unless the
|
|
|
|
transaction is committed. This is appropriate, since if the transaction
|
|
|
|
is aborted we would like all the commands within it to have had no
|
|
|
|
effect, including <command>NOTIFY</command>. But it can be disconcerting if one
|
|
|
|
is expecting the notify events to be delivered immediately. Secondly, if
|
|
|
|
a listening backend receives a notify signal while it is within a transaction,
|
|
|
|
the notify event will not be delivered to its connected frontend until just
|
|
|
|
after the transaction is completed (either committed or aborted). Again, the
|
|
|
|
reasoning is that if a notify were delivered within a transaction that was
|
2000-10-05 21:48:34 +02:00
|
|
|
later aborted, one would want the notification to be undone somehow---but
|
2001-09-13 17:55:24 +02:00
|
|
|
the backend cannot <quote>take back</quote> a notify once it has sent it to the frontend.
|
1999-07-06 19:16:42 +02:00
|
|
|
So notify events are only delivered between transactions. The upshot of this
|
|
|
|
is that applications using <command>NOTIFY</command> for real-time signaling
|
|
|
|
should try to keep their transactions short.
|
|
|
|
</para>
|
|
|
|
<para>
|
|
|
|
<command>NOTIFY</command> behaves like Unix signals in one important
|
|
|
|
respect: if the same condition name is signaled multiple times in quick
|
|
|
|
succession, recipients may get only one notify event for several executions
|
|
|
|
of <command>NOTIFY</command>. So it is a bad idea to depend on the number
|
|
|
|
of notifies received. Instead, use <command>NOTIFY</command> to wake up
|
|
|
|
applications that need to pay attention to something, and use a database
|
|
|
|
object (such as a sequence) to keep track of what happened or how many times
|
|
|
|
it happened.
|
|
|
|
</para>
|
|
|
|
<para>
|
|
|
|
It is common for a frontend that sends <command>NOTIFY</command> to be
|
|
|
|
listening on the same notify name itself. In that case it will get back a
|
|
|
|
notify event, just like all the other listening frontends. Depending on the
|
2000-10-05 21:48:34 +02:00
|
|
|
application logic, this could result in useless work---for example,
|
1999-07-06 19:16:42 +02:00
|
|
|
re-reading a database table to find the same updates that that frontend just
|
|
|
|
wrote out. In <productname>Postgres</productname> 6.4 and later, it is
|
|
|
|
possible to avoid such extra work by noticing whether the notifying backend
|
|
|
|
process's PID (supplied in the notify event message) is the same as one's own
|
|
|
|
backend's PID (available from libpq). When they are the same, the notify
|
|
|
|
event is one's own work bouncing back, and can be ignored. (Despite what was
|
|
|
|
said in the preceding paragraph, this is a safe technique.
|
|
|
|
<productname>Postgres</productname> keeps self-notifies separate from notifies
|
|
|
|
arriving from other backends, so you cannot miss an outside notify by ignoring
|
|
|
|
your own notifies.)
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<refsect2 id="R2-SQL-NOTIFY-3">
|
|
|
|
<refsect2info>
|
|
|
|
<date>1998-10-07</date>
|
|
|
|
</refsect2info>
|
|
|
|
<title>
|
|
|
|
Notes
|
|
|
|
</title>
|
|
|
|
<para>
|
|
|
|
<replaceable class="PARAMETER">name</replaceable>
|
|
|
|
can be any string valid as a name;
|
|
|
|
it need not correspond to the name of any actual table. If
|
|
|
|
<replaceable class="PARAMETER">name</replaceable>
|
|
|
|
is enclosed in double-quotes, it need not even be a syntactically
|
|
|
|
valid name, but can be any string up to 31 characters long.
|
|
|
|
</para>
|
|
|
|
<para>
|
|
|
|
In some previous releases of
|
|
|
|
<productname>Postgres</productname>,
|
|
|
|
<replaceable class="PARAMETER">name</replaceable>
|
|
|
|
had to be enclosed in double-quotes when it did not correspond to any existing
|
|
|
|
table name, even if syntactically valid as a name. That is no longer required.
|
|
|
|
</para>
|
|
|
|
<para>
|
|
|
|
In <productname>Postgres</productname> releases prior to 6.4, the backend
|
|
|
|
PID delivered in a notify message was always the PID of the frontend's own
|
|
|
|
backend. So it was not possible to distinguish one's own notifies from other
|
|
|
|
clients' notifies in those earlier releases.
|
|
|
|
</para>
|
|
|
|
</refsect2>
|
|
|
|
</refsect1>
|
1998-09-02 05:01:02 +02:00
|
|
|
|
1999-07-06 19:16:42 +02:00
|
|
|
<refsect1 id="R1-SQL-NOTIFY-2">
|
|
|
|
<title>
|
|
|
|
Usage
|
|
|
|
</title>
|
|
|
|
<para>
|
|
|
|
Configure and execute a listen/notify sequence from
|
|
|
|
<application>psql</application>:
|
1998-12-29 03:24:47 +01:00
|
|
|
|
2000-03-26 20:32:30 +02:00
|
|
|
<programlisting>
|
2000-07-21 06:42:28 +02:00
|
|
|
LISTEN virtual;
|
|
|
|
NOTIFY virtual;
|
2000-03-26 20:32:30 +02:00
|
|
|
Asynchronous NOTIFY 'virtual' from backend with pid '8448' received.
|
|
|
|
</programlisting>
|
1999-07-06 19:16:42 +02:00
|
|
|
</para>
|
|
|
|
</refsect1>
|
1998-12-29 03:24:47 +01:00
|
|
|
|
1999-07-06 19:16:42 +02:00
|
|
|
<refsect1 id="R1-SQL-NOTIFY-3">
|
|
|
|
<title>
|
|
|
|
Compatibility
|
|
|
|
</title>
|
1998-12-29 03:24:47 +01:00
|
|
|
|
1999-07-06 19:16:42 +02:00
|
|
|
<refsect2 id="R2-SQL-NOTIFY-4">
|
|
|
|
<refsect2info>
|
|
|
|
<date>1998-09-24</date>
|
|
|
|
</refsect2info>
|
|
|
|
<title>
|
|
|
|
SQL92
|
|
|
|
</title>
|
|
|
|
<para>
|
|
|
|
There is no <command>NOTIFY</command> statement in
|
|
|
|
<acronym>SQL92</acronym>.
|
|
|
|
</para>
|
|
|
|
</refsect2>
|
|
|
|
</refsect1>
|
|
|
|
</refentry>
|
1998-12-29 03:24:47 +01:00
|
|
|
|
1999-07-06 19:16:42 +02:00
|
|
|
<!-- Keep this comment at the end of the file
|
|
|
|
Local variables:
|
|
|
|
mode: sgml
|
|
|
|
sgml-omittag:nil
|
|
|
|
sgml-shorttag:t
|
|
|
|
sgml-minimize-attributes:nil
|
|
|
|
sgml-always-quote-attributes:t
|
|
|
|
sgml-indent-step:1
|
|
|
|
sgml-indent-data:t
|
|
|
|
sgml-parent-document:nil
|
|
|
|
sgml-default-dtd-file:"../reference.ced"
|
|
|
|
sgml-exposed-tags:nil
|
|
|
|
sgml-local-catalogs:"/usr/lib/sgml/catalog"
|
|
|
|
sgml-local-ecat-files:nil
|
|
|
|
End:
|
|
|
|
-->
|