1<!--
2doc/src/sgml/ref/notify.sgml
3PostgreSQL documentation
4-->
5
6<refentry id="sql-notify">
7 <indexterm zone="sql-notify">
8  <primary>NOTIFY</primary>
9 </indexterm>
10
11 <refmeta>
12  <refentrytitle>NOTIFY</refentrytitle>
13  <manvolnum>7</manvolnum>
14  <refmiscinfo>SQL - Language Statements</refmiscinfo>
15 </refmeta>
16
17 <refnamediv>
18  <refname>NOTIFY</refname>
19  <refpurpose>generate a notification</refpurpose>
20 </refnamediv>
21
22 <refsynopsisdiv>
23<synopsis>
24NOTIFY <replaceable class="parameter">channel</replaceable> [ , <replaceable class="parameter">payload</replaceable> ]
25</synopsis>
26 </refsynopsisdiv>
27
28 <refsect1>
29  <title>Description</title>
30
31  <para>
32   The <command>NOTIFY</command> command sends a notification event together
33   with an optional <quote>payload</quote> string to each client application that
34   has previously executed
35   <command>LISTEN <replaceable class="parameter">channel</replaceable></command>
36   for the specified channel name in the current database.
37   Notifications are visible to all users.
38  </para>
39
40  <para>
41   <command>NOTIFY</command> provides a simple
42   interprocess communication mechanism for a collection of processes
43   accessing the same <productname>PostgreSQL</productname> database.
44   A payload string can be sent along with the notification, and
45   higher-level mechanisms for passing structured data can be built by using
46   tables in the database to pass additional data from notifier to listener(s).
47  </para>
48
49  <para>
50   The information passed to the client for a notification event includes the
51   notification channel
52   name, the notifying session's server process <acronym>PID</acronym>, and the
53   payload string, which is an empty string if it has not been specified.
54  </para>
55
56  <para>
57   It is up to the database designer to define the channel names that will
58   be used in a given database and what each one means.
59   Commonly, the channel name is the same as the name of some table in
60   the database, and the notify event essentially means, <quote>I changed this table,
61   take a look at it to see what's new</quote>.  But no such association is enforced by
62   the <command>NOTIFY</command> and <command>LISTEN</command> commands.  For
63   example, a database designer could use several different channel names
64   to signal different sorts of changes to a single table.  Alternatively,
65   the payload string could be used to differentiate various cases.
66  </para>
67
68  <para>
69   When <command>NOTIFY</command> is used to signal the occurrence of changes
70   to a particular table, a useful programming technique is to put the
71   <command>NOTIFY</command> in a statement trigger that is triggered by table updates.
72   In this way, notification happens automatically when the table is changed,
73   and the application programmer cannot accidentally forget to do it.
74  </para>
75
76  <para>
77   <command>NOTIFY</command> interacts with SQL transactions in some important
78   ways.  Firstly, if a <command>NOTIFY</command> is executed inside a
79   transaction, the notify events are not delivered until and unless the
80   transaction is committed.  This is appropriate, since if the transaction
81   is aborted, all the commands within it have had no
82   effect, including <command>NOTIFY</command>.  But it can be disconcerting if one
83   is expecting the notification events to be delivered immediately.  Secondly, if
84   a listening session receives a notification signal while it is within a transaction,
85   the notification event will not be delivered to its connected client until just
86   after the transaction is completed (either committed or aborted).  Again, the
87   reasoning is that if a notification were delivered within a transaction that was
88   later aborted, one would want the notification to be undone somehow &mdash;
89   but
90   the server cannot <quote>take back</quote> a notification once it has sent it to the client.
91   So notification events are only delivered between transactions.  The upshot of this
92   is that applications using <command>NOTIFY</command> for real-time signaling
93   should try to keep their transactions short.
94  </para>
95
96  <para>
97   If the same channel name is signaled multiple times with identical
98   payload strings within the same transaction, only one instance of the
99   notification event is delivered to listeners.
100   On the other hand, notifications with distinct payload strings will
101   always be delivered as distinct notifications. Similarly, notifications from
102   different transactions will never get folded into one notification.
103   Except for dropping later instances of duplicate notifications,
104   <command>NOTIFY</command> guarantees that notifications from the same
105   transaction get delivered in the order they were sent.  It is also
106   guaranteed that messages from different transactions are delivered in
107   the order in which the transactions committed.
108  </para>
109
110  <para>
111   It is common for a client that executes <command>NOTIFY</command>
112   to be listening on the same notification channel itself.  In that case
113   it will get back a notification event, just like all the other
114   listening sessions.  Depending on the application logic, this could
115   result in useless work, for example, reading a database table to
116   find the same updates that that session just wrote out.  It is
117   possible to avoid such extra work by noticing whether the notifying
118   session's server process <acronym>PID</acronym> (supplied in the
119   notification event message) is the same as one's own session's
120   <acronym>PID</acronym> (available from <application>libpq</application>).  When they
121   are the same, the notification event is one's own work bouncing
122   back, and can be ignored.
123  </para>
124 </refsect1>
125
126 <refsect1>
127  <title>Parameters</title>
128
129  <variablelist>
130   <varlistentry>
131    <term><replaceable class="parameter">channel</replaceable></term>
132    <listitem>
133     <para>
134      Name of the notification channel to be signaled (any identifier).
135     </para>
136    </listitem>
137   </varlistentry>
138   <varlistentry>
139    <term><replaceable class="parameter">payload</replaceable></term>
140    <listitem>
141     <para>
142      The <quote>payload</quote> string to be communicated along with the
143      notification.  This must be specified as a simple string literal.
144      In the default configuration it must be shorter than 8000 bytes.
145      (If binary data or large amounts of information need to be communicated,
146      it's best to put it in a database table and send the key of the record.)
147     </para>
148    </listitem>
149   </varlistentry>
150  </variablelist>
151 </refsect1>
152
153 <refsect1>
154  <title>Notes</title>
155
156  <para>
157   There is a queue that holds notifications that have been sent but not
158   yet processed by all listening sessions.  If this queue becomes full,
159   transactions calling <command>NOTIFY</command> will fail at commit.
160   The queue is quite large (8GB in a standard installation) and should be
161   sufficiently sized for almost every use case. However, no cleanup can take
162   place if a session executes <command>LISTEN</command> and then enters a
163   transaction for a very long time. Once the queue is half full you will see
164   warnings in the log file pointing you to the session that is preventing
165   cleanup. In this case you should make sure that this session ends its
166   current transaction so that cleanup can proceed.
167  </para>
168  <para>
169   The function <function>pg_notification_queue_usage</function> returns the
170   fraction of the queue that is currently occupied by pending notifications.
171   See <xref linkend="functions-info"/> for more information.
172  </para>
173  <para>
174   A transaction that has executed <command>NOTIFY</command> cannot be
175   prepared for two-phase commit.
176  </para>
177
178  <refsect2>
179   <title>pg_notify</title>
180
181   <indexterm>
182    <primary>pg_notify</primary>
183   </indexterm>
184
185   <para>
186    To send a notification you can also use the function
187    <literal><function>pg_notify</function>(<type>text</type>,
188    <type>text</type>)</literal>. The function takes the channel name as the
189    first argument and the payload as the second. The function is much easier
190    to use than the <command>NOTIFY</command> command if you need to work with
191    non-constant channel names and payloads.
192   </para>
193  </refsect2>
194 </refsect1>
195
196 <refsect1>
197  <title>Examples</title>
198
199  <para>
200   Configure and execute a listen/notify sequence from
201   <application>psql</application>:
202
203<programlisting>
204LISTEN virtual;
205NOTIFY virtual;
206Asynchronous notification "virtual" received from server process with PID 8448.
207NOTIFY virtual, 'This is the payload';
208Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448.
209
210LISTEN foo;
211SELECT pg_notify('fo' || 'o', 'pay' || 'load');
212Asynchronous notification "foo" with payload "payload" received from server process with PID 14728.
213</programlisting></para>
214 </refsect1>
215
216 <refsect1>
217  <title>Compatibility</title>
218
219  <para>
220   There is no <command>NOTIFY</command> statement in the SQL
221   standard.
222  </para>
223 </refsect1>
224
225 <refsect1>
226  <title>See Also</title>
227
228  <simplelist type="inline">
229   <member><xref linkend="sql-listen"/></member>
230   <member><xref linkend="sql-unlisten"/></member>
231  </simplelist>
232 </refsect1>
233</refentry>
234