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