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><H1
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><A
NAME="LIBPQ-NOTIFY"
>30.7. Asynchronous Notification</A
></H1
><A
NAME="AEN34881"
></A
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> offers asynchronous notification
via the <TT
CLASS="COMMAND"
>LISTEN</TT
> and <TT
CLASS="COMMAND"
>NOTIFY</TT
>
commands. A client session registers its interest in a particular
notification condition with the <TT
CLASS="COMMAND"
>LISTEN</TT
> command (and
can stop listening with the <TT
CLASS="COMMAND"
>UNLISTEN</TT
> command). All
sessions listening on a particular condition will be notified
asynchronously when a <TT
CLASS="COMMAND"
>NOTIFY</TT
> command with that
condition name is executed by any session. No additional information
is passed from the notifier to the listener. Thus, typically, any
actual data that needs to be communicated is transferred through a
database table. Commonly, the condition name is the same as the
associated table, but it is not necessary for there to be any associated
table.
</P
><P
> <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> applications submit
<TT
CLASS="COMMAND"
>LISTEN</TT
> and <TT
CLASS="COMMAND"
>UNLISTEN</TT
> commands as
ordinary SQL commands. The arrival of <TT
CLASS="COMMAND"
>NOTIFY</TT
>
messages can subsequently be detected by calling
<CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>.<A
NAME="AEN34897"
></A
>
</P
><P
> The function <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>
returns the next notification from a list of unhandled
notification messages received from the server. It returns a null pointer if
there are no pending notifications. Once a notification is
returned from <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>, it is considered handled and will be
removed from the list of notifications.
</P><PRE
CLASS="SYNOPSIS"
> PGnotify *PQnotifies(PGconn *conn);
typedef struct pgNotify {
char *relname; /* notification condition name */
int be_pid; /* process ID of notifying server process */
char *extra; /* notification parameter */
} PGnotify;
</PRE
><P>
After processing a <TT
CLASS="STRUCTNAME"
>PGnotify</TT
> object returned
by <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>, be sure to free it with
<CODE
CLASS="FUNCTION"
>PQfreemem</CODE
>. It is sufficient to free the
<TT
CLASS="STRUCTNAME"
>PGnotify</TT
> pointer; the
<TT
CLASS="STRUCTFIELD"
>relname</TT
> and <TT
CLASS="STRUCTFIELD"
>extra</TT
>
fields do not represent separate allocations. (At present, the
<TT
CLASS="STRUCTFIELD"
>extra</TT
> field is unused and will always point
to an empty string.)
</P
><P
> <A
HREF="libpq-example.html#LIBPQ-EXAMPLE-2"
>Example 30-2</A
> gives a sample program that illustrates
the use of asynchronous notification.
</P
><P
> <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> does not actually read data from the
server; it just returns messages previously absorbed by another
<SPAN
CLASS="APPLICATION"
>libpq</SPAN
> function. In prior releases of
<SPAN
CLASS="APPLICATION"
>libpq</SPAN
>, the only way to ensure timely receipt
of <TT
CLASS="COMMAND"
>NOTIFY</TT
> messages was to constantly submit commands, even
empty ones, and then check <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> after each
<CODE
CLASS="FUNCTION"
>PQexec</CODE
>. While this still works, it is deprecated
as a waste of processing power.
</P
><P
> A better way to check for <TT
CLASS="COMMAND"
>NOTIFY</TT
> messages when you have no
useful commands to execute is to call
<CODE
CLASS="FUNCTION"
>PQconsumeInput</CODE
>, then check
<CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>. You can use
<CODE
CLASS="FUNCTION"
>select()</CODE
> to wait for data to arrive from the
server, thereby using no <ACRONYM
CLASS="ACRONYM"
>CPU</ACRONYM
> power unless there is
something to do. (See <CODE
CLASS="FUNCTION"
>PQsocket</CODE
> to obtain the file
descriptor number to use with <CODE
CLASS="FUNCTION"
>select()</CODE
>.) Note that
this will work OK whether you submit commands with
<CODE
CLASS="FUNCTION"
>PQsendQuery</CODE
>/<CODE
CLASS="FUNCTION"
>PQgetResult</CODE
> or
simply use <CODE
CLASS="FUNCTION"
>PQexec</CODE
>. You should, however, remember
to check <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> after each
<CODE
CLASS="FUNCTION"
>PQgetResult</CODE
> or <CODE
CLASS="FUNCTION"
>PQexec</CODE
>, to
see if any notifications came in during the processing of the command.
</P
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