From 1dad9ac47c9564746bde5a473ba054a069f9a513 Mon Sep 17 00:00:00 2001
From: dvarrazzo <dvarrazzo@users.noreply.github.com>
Date: Mon, 8 Apr 2024 19:49:36 +0000
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@@ -2272,6 +2272,58 @@ but if there is anything to point out I'd be happy to know. Best option would be
 &lt;li&gt;&lt;a class="reference external" href="https://pypi.org/packages/source/p/psycopg2/psycopg2-2.3.1.tar.gz.asc"&gt;Psycopg 2.3.1 signature&lt;/a&gt;&lt;/li&gt;
 &lt;/ul&gt;
 &lt;p&gt;Cheers!&lt;/p&gt;
+</content></entry><entry><title>PostgreSQL notifications with Psycopg2 and Eventlet</title><link href="https://www.psycopg.org/articles/2010/12/01/postgresql-notifications-psycopg2-eventlet/" rel="alternate"></link><updated>2010-12-01T00:00:00Z</updated><author><name>Daniele Varrazzo</name></author><id>urn:uuid:85a44537-7b4f-3e10-9273-1a2b12fb4757</id><content type="html">&lt;p&gt;PostgreSQL supports &lt;a class="reference external" href="https://www.postgresql.org/docs/9.0/interactive/sql-notify.html"&gt;asynchronous notifications&lt;/a&gt;, a simple messaging system allowing clients to be notified about events occurred in the database. Notifications can be sent by any session using a &amp;quot;&lt;tt class="docutils literal"&gt;NOTIFY&lt;/tt&gt; &lt;em&gt;channel&lt;/em&gt;&amp;quot; command and will be received by any session which has subscribed with a &lt;tt class="docutils literal"&gt;LISTEN&lt;/tt&gt; &lt;em&gt;channel&lt;/em&gt; to receive updates. The system has been greatly improved in PostgreSQL 9.0 with the addition of a &lt;em&gt;message payload&lt;/em&gt;, making the feature even more useful. Previously a typical use case would have been to notify interested sessions that a certain table was updated: now it is possible to signal for instance &lt;em&gt;which record&lt;/em&gt; was changed. You can put the &lt;tt class="docutils literal"&gt;NOTIFY&lt;/tt&gt; command in a database trigger for automatic notifications on insert or update... the possibilities are actually quite interesting.&lt;/p&gt;
+&lt;!-- CUT-HERE --&gt;
+&lt;p&gt;Psycopg2 allows a client program to &lt;a class="reference external" href="/docs/advanced.html#asynchronous-notifications"&gt;receive notifications from the database backend&lt;/a&gt;. Starting from version 2.3 the notification received will also include the payload, when connected to a PostgreSQL 9.0 server. The notification are received automatically after each query and are made available in the &lt;a class="reference external" href=":/docs/connection.html#connection.notifies"&gt;&lt;tt class="docutils literal"&gt;notifies&lt;/tt&gt;&lt;/a&gt; attribute of the connection. They are also received using the &lt;a class="reference external" href="/docs/connection.html#connection.poll"&gt;&lt;tt class="docutils literal"&gt;poll()&lt;/tt&gt;&lt;/a&gt; method, without the need of issuing queries only to receive notifications.&lt;/p&gt;
+&lt;p&gt;But polling a connection is not the most efficient nor timely way to receive notifications: a better approach is to ask the OS to wake us up when there is something to read on the connection socket. This approach is particularly appealing when using an event-driven framework such as &lt;a class="reference external" href="https://twistedmatrix.com/"&gt;Twisted&lt;/a&gt; or &lt;a class="reference external" href="https://eventlet.net/"&gt;Eventlet&lt;/a&gt;, with which is easy to perform other tasks while there is nothing to read on the connection. In &amp;quot;regular&amp;quot; Python programs it is possible to use &lt;tt class="docutils literal"&gt;select()&lt;/tt&gt; to block until a notification arrives (see the documentation for an example): this would block the entire program. If some concurrency is required it is possible to use the blocking syscall in a thread to allow the rest of the program to carry on.&lt;/p&gt;
+&lt;div class="section" id="eventlet-example"&gt;
+&lt;h2&gt;Eventlet example&lt;/h2&gt;
+&lt;p&gt;This example shows how hook the notifications system into Eventlet: using it with a &lt;a class="reference external" href="https://en.wikipedia.org/wiki/WebSockets"&gt;WebSocket&lt;/a&gt; it is possible to send an event from the database to the application and up to a web browser, all in push mode! The example requires PostgreSQL 9.0 and Psycopg 2.3 as it uses notification payloads.&lt;/p&gt;
+&lt;p&gt;This function handles a WebSocket connection in a green thread: it creates a &lt;tt class="docutils literal"&gt;Queue&lt;/tt&gt; and opens a database connection (calling the &lt;tt class="docutils literal"&gt;dblisten()&lt;/tt&gt; function), then blocks on the queue waiting to receive notifications: as soon as any is received, its payload is forwarded to the browser. This is obviously just an example: a database connection for every browser wouldn't scale very much...&lt;/p&gt;
+&lt;pre class="code python literal-block"&gt;
+&lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="nn"&gt;eventlet&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;&lt;span class="kn"&gt;from&lt;/span&gt; &lt;span class="nn"&gt;eventlet&lt;/span&gt; &lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="n"&gt;wsgi&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;&lt;span class="kn"&gt;from&lt;/span&gt; &lt;span class="nn"&gt;eventlet&lt;/span&gt; &lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="n"&gt;websocket&lt;/span&gt;&lt;span class="w"&gt;
+
+&lt;/span&gt;&lt;span class="nd"&gt;&amp;#64;websocket&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;WebSocketWSGI&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;&lt;span class="k"&gt;def&lt;/span&gt; &lt;span class="nf"&gt;handle&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;ws&lt;/span&gt;&lt;span class="p"&gt;):&lt;/span&gt;&lt;span class="w"&gt;
+    &lt;/span&gt;&lt;span class="sd"&gt;&amp;quot;&amp;quot;&amp;quot;
+    Receive a connection and send it database notifications.
+    &amp;quot;&amp;quot;&amp;quot;&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;    &lt;span class="n"&gt;q&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;eventlet&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;Queue&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;    &lt;span class="n"&gt;eventlet&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;spawn&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;dblisten&lt;/span&gt;&lt;span class="p"&gt;,&lt;/span&gt; &lt;span class="n"&gt;q&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;    &lt;span class="k"&gt;while&lt;/span&gt; &lt;span class="mi"&gt;1&lt;/span&gt;&lt;span class="p"&gt;:&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;        &lt;span class="n"&gt;n&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;q&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;get&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;        &lt;span class="nb"&gt;print&lt;/span&gt; &lt;span class="n"&gt;n&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;        &lt;span class="n"&gt;ws&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;send&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;n&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;payload&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;
+&lt;/pre&gt;
+&lt;p&gt;When &lt;tt class="docutils literal"&gt;handle()&lt;/tt&gt; blocks on the queue the control of the program switches to a different green thread, allowing the rest of the program to keep on running even if single-threaded.&lt;/p&gt;
+&lt;p&gt;The &lt;tt class="docutils literal"&gt;dblisten&lt;/tt&gt; function sets up the listening database connection:&lt;/p&gt;
+&lt;pre class="code python literal-block"&gt;
+&lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="nn"&gt;psycopg2&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;&lt;span class="kn"&gt;from&lt;/span&gt; &lt;span class="nn"&gt;psycopg2.extensions&lt;/span&gt; &lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="n"&gt;ISOLATION_LEVEL_AUTOCOMMIT&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;&lt;span class="kn"&gt;from&lt;/span&gt; &lt;span class="nn"&gt;eventlet.hubs&lt;/span&gt; &lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="n"&gt;trampoline&lt;/span&gt;&lt;span class="w"&gt;
+
+&lt;/span&gt;&lt;span class="k"&gt;def&lt;/span&gt; &lt;span class="nf"&gt;dblisten&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;q&lt;/span&gt;&lt;span class="p"&gt;):&lt;/span&gt;&lt;span class="w"&gt;
+    &lt;/span&gt;&lt;span class="sd"&gt;&amp;quot;&amp;quot;&amp;quot;
+    Open a db connection and add notifications to *q*.
+    &amp;quot;&amp;quot;&amp;quot;&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;    &lt;span class="n"&gt;cnn&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;psycopg2&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;connect&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;dsn&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;    &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;set_isolation_level&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;ISOLATION_LEVEL_AUTOCOMMIT&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;    &lt;span class="n"&gt;cur&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;cursor&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;    &lt;span class="n"&gt;cur&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;execute&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="s2"&gt;&amp;quot;LISTEN data;&amp;quot;&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;    &lt;span class="k"&gt;while&lt;/span&gt; &lt;span class="mi"&gt;1&lt;/span&gt;&lt;span class="p"&gt;:&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;        &lt;span class="n"&gt;trampoline&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="p"&gt;,&lt;/span&gt; &lt;span class="n"&gt;read&lt;/span&gt;&lt;span class="o"&gt;=&lt;/span&gt;&lt;span class="kc"&gt;True&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;        &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;poll&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;        &lt;span class="k"&gt;while&lt;/span&gt; &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;notifies&lt;/span&gt;&lt;span class="p"&gt;:&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;            &lt;span class="n"&gt;n&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;notifies&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;pop&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
+&lt;/span&gt;            &lt;span class="n"&gt;q&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;put&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;n&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;
+&lt;/pre&gt;
+&lt;p&gt;The connection is set in &lt;a class="reference external" href="/docs/usage.html#transactions-control"&gt;autocommit mode&lt;/a&gt;, otherwise the notification would be postponed until the end of the transaction. The &lt;em&gt;trampoline&lt;/em&gt; is the Eventlet function that schedules a green thread switch: we ask it to return us the control when the file descriptor underlying the connection will have something to read. Once the system wakes up again we use &lt;tt class="docutils literal"&gt;poll()&lt;/tt&gt; to parse eventual notifications received, that will be added to the queue and thus passed to the client.&lt;/p&gt;
+&lt;p&gt;The rest of the example is not shown here: it only consists in a static page and the web server setup. You can &lt;a class="reference external" href="pushdemo.py"&gt;download the complete example&lt;/a&gt;.&lt;/p&gt;
+&lt;p&gt;Running the program and opening the page &lt;a class="reference external" href="http://localhost:7000/"&gt;http://localhost:7000/&lt;/a&gt; in a WebSocket-enabled browser you will see a page with three coloured bars: using a command such as &lt;tt class="docutils literal"&gt;NOTIFY data, 'green'&lt;/tt&gt; in a psql shell, one of the bars in the page will be updated accordingly. Here is a screenshot:&lt;/p&gt;
+&lt;img alt="psycopg eventlet push demo" class="align-center" src="pushdemo.png" style="width: 477px; height: 327px;" /&gt;
+&lt;/div&gt;
 </content></entry><entry><title>Psycopg 2.3.0 released</title><link href="https://www.psycopg.org/articles/2010/12/01/psycopg-230-released/" rel="alternate"></link><updated>2010-12-01T00:00:00Z</updated><author><name>Federico Di Gregorio</name></author><id>urn:uuid:12f1b527-a80a-34f4-9c06-c281d38f0559</id><content type="html">&lt;p&gt;Hi *,&lt;/p&gt;
 &lt;p&gt;lately my involvement on psycopg has been quite small (work and other
 projects are taking their toll) but thanks to Daniele (with some help
@@ -2330,58 +2382,6 @@ versions (&lt;a class="reference external" href="https://psycopg.lighthouseapp.c
 &lt;li&gt;Fixed notices order (&lt;a class="reference external" href="https://psycopg.lighthouseapp.com/projects/62710/tickets/9"&gt;ticket #9&lt;/a&gt;).&lt;/li&gt;
 &lt;/ul&gt;
 &lt;/div&gt;
-</content></entry><entry><title>PostgreSQL notifications with Psycopg2 and Eventlet</title><link href="https://www.psycopg.org/articles/2010/12/01/postgresql-notifications-psycopg2-eventlet/" rel="alternate"></link><updated>2010-12-01T00:00:00Z</updated><author><name>Daniele Varrazzo</name></author><id>urn:uuid:85a44537-7b4f-3e10-9273-1a2b12fb4757</id><content type="html">&lt;p&gt;PostgreSQL supports &lt;a class="reference external" href="https://www.postgresql.org/docs/9.0/interactive/sql-notify.html"&gt;asynchronous notifications&lt;/a&gt;, a simple messaging system allowing clients to be notified about events occurred in the database. Notifications can be sent by any session using a &amp;quot;&lt;tt class="docutils literal"&gt;NOTIFY&lt;/tt&gt; &lt;em&gt;channel&lt;/em&gt;&amp;quot; command and will be received by any session which has subscribed with a &lt;tt class="docutils literal"&gt;LISTEN&lt;/tt&gt; &lt;em&gt;channel&lt;/em&gt; to receive updates. The system has been greatly improved in PostgreSQL 9.0 with the addition of a &lt;em&gt;message payload&lt;/em&gt;, making the feature even more useful. Previously a typical use case would have been to notify interested sessions that a certain table was updated: now it is possible to signal for instance &lt;em&gt;which record&lt;/em&gt; was changed. You can put the &lt;tt class="docutils literal"&gt;NOTIFY&lt;/tt&gt; command in a database trigger for automatic notifications on insert or update... the possibilities are actually quite interesting.&lt;/p&gt;
-&lt;!-- CUT-HERE --&gt;
-&lt;p&gt;Psycopg2 allows a client program to &lt;a class="reference external" href="/docs/advanced.html#asynchronous-notifications"&gt;receive notifications from the database backend&lt;/a&gt;. Starting from version 2.3 the notification received will also include the payload, when connected to a PostgreSQL 9.0 server. The notification are received automatically after each query and are made available in the &lt;a class="reference external" href=":/docs/connection.html#connection.notifies"&gt;&lt;tt class="docutils literal"&gt;notifies&lt;/tt&gt;&lt;/a&gt; attribute of the connection. They are also received using the &lt;a class="reference external" href="/docs/connection.html#connection.poll"&gt;&lt;tt class="docutils literal"&gt;poll()&lt;/tt&gt;&lt;/a&gt; method, without the need of issuing queries only to receive notifications.&lt;/p&gt;
-&lt;p&gt;But polling a connection is not the most efficient nor timely way to receive notifications: a better approach is to ask the OS to wake us up when there is something to read on the connection socket. This approach is particularly appealing when using an event-driven framework such as &lt;a class="reference external" href="https://twistedmatrix.com/"&gt;Twisted&lt;/a&gt; or &lt;a class="reference external" href="https://eventlet.net/"&gt;Eventlet&lt;/a&gt;, with which is easy to perform other tasks while there is nothing to read on the connection. In &amp;quot;regular&amp;quot; Python programs it is possible to use &lt;tt class="docutils literal"&gt;select()&lt;/tt&gt; to block until a notification arrives (see the documentation for an example): this would block the entire program. If some concurrency is required it is possible to use the blocking syscall in a thread to allow the rest of the program to carry on.&lt;/p&gt;
-&lt;div class="section" id="eventlet-example"&gt;
-&lt;h2&gt;Eventlet example&lt;/h2&gt;
-&lt;p&gt;This example shows how hook the notifications system into Eventlet: using it with a &lt;a class="reference external" href="https://en.wikipedia.org/wiki/WebSockets"&gt;WebSocket&lt;/a&gt; it is possible to send an event from the database to the application and up to a web browser, all in push mode! The example requires PostgreSQL 9.0 and Psycopg 2.3 as it uses notification payloads.&lt;/p&gt;
-&lt;p&gt;This function handles a WebSocket connection in a green thread: it creates a &lt;tt class="docutils literal"&gt;Queue&lt;/tt&gt; and opens a database connection (calling the &lt;tt class="docutils literal"&gt;dblisten()&lt;/tt&gt; function), then blocks on the queue waiting to receive notifications: as soon as any is received, its payload is forwarded to the browser. This is obviously just an example: a database connection for every browser wouldn't scale very much...&lt;/p&gt;
-&lt;pre class="code python literal-block"&gt;
-&lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="nn"&gt;eventlet&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;&lt;span class="kn"&gt;from&lt;/span&gt; &lt;span class="nn"&gt;eventlet&lt;/span&gt; &lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="n"&gt;wsgi&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;&lt;span class="kn"&gt;from&lt;/span&gt; &lt;span class="nn"&gt;eventlet&lt;/span&gt; &lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="n"&gt;websocket&lt;/span&gt;&lt;span class="w"&gt;
-
-&lt;/span&gt;&lt;span class="nd"&gt;&amp;#64;websocket&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;WebSocketWSGI&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;&lt;span class="k"&gt;def&lt;/span&gt; &lt;span class="nf"&gt;handle&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;ws&lt;/span&gt;&lt;span class="p"&gt;):&lt;/span&gt;&lt;span class="w"&gt;
-    &lt;/span&gt;&lt;span class="sd"&gt;&amp;quot;&amp;quot;&amp;quot;
-    Receive a connection and send it database notifications.
-    &amp;quot;&amp;quot;&amp;quot;&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;    &lt;span class="n"&gt;q&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;eventlet&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;Queue&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;    &lt;span class="n"&gt;eventlet&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;spawn&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;dblisten&lt;/span&gt;&lt;span class="p"&gt;,&lt;/span&gt; &lt;span class="n"&gt;q&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;    &lt;span class="k"&gt;while&lt;/span&gt; &lt;span class="mi"&gt;1&lt;/span&gt;&lt;span class="p"&gt;:&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;        &lt;span class="n"&gt;n&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;q&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;get&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;        &lt;span class="nb"&gt;print&lt;/span&gt; &lt;span class="n"&gt;n&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;        &lt;span class="n"&gt;ws&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;send&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;n&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;payload&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;
-&lt;/pre&gt;
-&lt;p&gt;When &lt;tt class="docutils literal"&gt;handle()&lt;/tt&gt; blocks on the queue the control of the program switches to a different green thread, allowing the rest of the program to keep on running even if single-threaded.&lt;/p&gt;
-&lt;p&gt;The &lt;tt class="docutils literal"&gt;dblisten&lt;/tt&gt; function sets up the listening database connection:&lt;/p&gt;
-&lt;pre class="code python literal-block"&gt;
-&lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="nn"&gt;psycopg2&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;&lt;span class="kn"&gt;from&lt;/span&gt; &lt;span class="nn"&gt;psycopg2.extensions&lt;/span&gt; &lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="n"&gt;ISOLATION_LEVEL_AUTOCOMMIT&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;&lt;span class="kn"&gt;from&lt;/span&gt; &lt;span class="nn"&gt;eventlet.hubs&lt;/span&gt; &lt;span class="kn"&gt;import&lt;/span&gt; &lt;span class="n"&gt;trampoline&lt;/span&gt;&lt;span class="w"&gt;
-
-&lt;/span&gt;&lt;span class="k"&gt;def&lt;/span&gt; &lt;span class="nf"&gt;dblisten&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;q&lt;/span&gt;&lt;span class="p"&gt;):&lt;/span&gt;&lt;span class="w"&gt;
-    &lt;/span&gt;&lt;span class="sd"&gt;&amp;quot;&amp;quot;&amp;quot;
-    Open a db connection and add notifications to *q*.
-    &amp;quot;&amp;quot;&amp;quot;&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;    &lt;span class="n"&gt;cnn&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;psycopg2&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;connect&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;dsn&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;    &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;set_isolation_level&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;ISOLATION_LEVEL_AUTOCOMMIT&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;    &lt;span class="n"&gt;cur&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;cursor&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;    &lt;span class="n"&gt;cur&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;execute&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="s2"&gt;&amp;quot;LISTEN data;&amp;quot;&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;    &lt;span class="k"&gt;while&lt;/span&gt; &lt;span class="mi"&gt;1&lt;/span&gt;&lt;span class="p"&gt;:&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;        &lt;span class="n"&gt;trampoline&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="p"&gt;,&lt;/span&gt; &lt;span class="n"&gt;read&lt;/span&gt;&lt;span class="o"&gt;=&lt;/span&gt;&lt;span class="kc"&gt;True&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;        &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;poll&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;        &lt;span class="k"&gt;while&lt;/span&gt; &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;notifies&lt;/span&gt;&lt;span class="p"&gt;:&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;            &lt;span class="n"&gt;n&lt;/span&gt; &lt;span class="o"&gt;=&lt;/span&gt; &lt;span class="n"&gt;cnn&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;notifies&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;pop&lt;/span&gt;&lt;span class="p"&gt;()&lt;/span&gt;&lt;span class="w"&gt;
-&lt;/span&gt;            &lt;span class="n"&gt;q&lt;/span&gt;&lt;span class="o"&gt;.&lt;/span&gt;&lt;span class="n"&gt;put&lt;/span&gt;&lt;span class="p"&gt;(&lt;/span&gt;&lt;span class="n"&gt;n&lt;/span&gt;&lt;span class="p"&gt;)&lt;/span&gt;
-&lt;/pre&gt;
-&lt;p&gt;The connection is set in &lt;a class="reference external" href="/docs/usage.html#transactions-control"&gt;autocommit mode&lt;/a&gt;, otherwise the notification would be postponed until the end of the transaction. The &lt;em&gt;trampoline&lt;/em&gt; is the Eventlet function that schedules a green thread switch: we ask it to return us the control when the file descriptor underlying the connection will have something to read. Once the system wakes up again we use &lt;tt class="docutils literal"&gt;poll()&lt;/tt&gt; to parse eventual notifications received, that will be added to the queue and thus passed to the client.&lt;/p&gt;
-&lt;p&gt;The rest of the example is not shown here: it only consists in a static page and the web server setup. You can &lt;a class="reference external" href="pushdemo.py"&gt;download the complete example&lt;/a&gt;.&lt;/p&gt;
-&lt;p&gt;Running the program and opening the page &lt;a class="reference external" href="http://localhost:7000/"&gt;http://localhost:7000/&lt;/a&gt; in a WebSocket-enabled browser you will see a page with three coloured bars: using a command such as &lt;tt class="docutils literal"&gt;NOTIFY data, 'green'&lt;/tt&gt; in a psql shell, one of the bars in the page will be updated accordingly. Here is a screenshot:&lt;/p&gt;
-&lt;img alt="psycopg eventlet push demo" class="align-center" src="pushdemo.png" style="width: 477px; height: 327px;" /&gt;
-&lt;/div&gt;
 </content></entry><entry><title>Psycopg at PGDay Italy 2010</title><link href="https://www.psycopg.org/articles/2010/11/12/psycopg-pgday-italy-2010/" rel="alternate"></link><updated>2010-11-12T00:00:00Z</updated><author><name>Daniele Varrazzo</name></author><id>urn:uuid:07db87bd-f9fc-3840-9855-94198b55b739</id><content type="html">&lt;a class="reference external image-reference" href="https://2010.pgday.it/"&gt;&lt;img alt="PGDay Roma, 10 Dicembre 2010" class="inline-right" src="banner_200x60.gif" style="width: 200px; height: 60px;" /&gt;&lt;/a&gt;
 &lt;p&gt;Psycopg will be at PGDay Italy in Rome, on December 10th.&lt;/p&gt;
 &lt;!-- CUT-HERE --&gt;