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<h1>Handling a massive syslog database insert rate with Rsyslog</h1>
<P><small><i>Written by
<a href="http://www.gerhards.net/rainer">Rainer
Gerhards</a> (2008-01-31)</i></small></P>
<h2>Abstract</h2>
<p><i><b>In this paper, I describe how log massive amounts of
<a href="http://www.monitorware.com/en/topics/syslog/">syslog</a>
messages to a database. </b>This HOWTO is currently under development and thus a
bit brief. Updates are promised ;).</i></p>
<h2>The Intention</h2>
<p>Database updates are inherently slow when it comes to storing syslog
messages. However, there are a number of applications where it is handy to have
the message inside a database. Rsyslog supports native database writing via
output plugins. As of this writing, there are plugins available for MySQL an
PostgreSQL. Maybe additional plugins have become available by the time you read
this. Be sure to check.</p>
<p>In order to successfully write messages to a database backend, the backend
must be capable to record messages at the expected average arrival rate. This is
the rate if you take all messages that can arrive within a day and divide it by
86400 (the number of seconds per day). Let's say you expect 43,200,000 messages
per day. That's an average rate of 500 messages per second (mps). Your database
server MUST be able to handle that amount of message per second on a sustained
rate. If it doesn't, you either need to add an additional server, lower the
number of message - or forget about it.</p>
<p>However, this is probably not your peak rate. Let's simply assume your
systems work only half a day, that's 12 hours (and, yes, I know this is
unrealistic, but you'll get the point soon). So your average rate is actually
1,000 mps during work hours and 0 mps during non-work hours. To make matters
worse, workload is not divided evenly during the day. So you may have peaks of
up to 10,000mps while at other times the load may go down to maybe just 100mps.
Peaks may stay well above 2,000mps for a few minutes.</p>
<p>So how the hack you will be able to handle all of this traffic (including the
peaks) with a database server that is just capable of inserting a maximum of
500mps?</p>
<p>The key here is buffering. Messages that the database server is not capable
to handle will be buffered until it is. Of course, that means database insert
are NOT real-time. If you need real-time inserts, you need to make sure your
database server can handle traffic at the actual peak rate. But lets assume you
are OK with some delay.</p>
<p>Buffering is fine. But how about these massive amounts of data? That can't be
hold in memory, so don't we run out of luck with buffering? The key here is that
rsyslog can not only buffer in memory but also buffer to disk (this may remind
you of "spooling" which gets you the right idea). There are several queuing
modes available, offering differnent throughput. In general, the idea is to
buffer in memory until the memory buffer is exhausted and switch to
disk-buffering when needed (and only as long as needed). All of this is handled
automatically and transparently by rsyslog.</p>
<p>With our above scenario, the disk buffer would build up during the day and
rsyslog would use the night to drain it. Obviously, this is an extreme example,
but it shows what can be done. Please note that queue content survies rsyslogd
restarts, so even a reboot of the system will not cause any message loss.</p>
<h2>How To Setup</h2>
<p>Frankly, it's quite easy. You just need to do is instruct rsyslog to use a
disk queue and then configure your action. There is nothing else to do. With the
following simple config file, you log anything you receive to a MySQL database
and have buffering applied automatically.</p>
<textarea rows="11" cols="80">
$ModLoad ommysql # load the output driver (use ompgsql for PostgreSQL)
$ModLoad imudp # network reception
$UDPServerRun 514 # start a udp server at port 514
$ModLoad imuxsock # local message reception
$WorkDirectory /rsyslog/work # default location for work (spool) files
$MainMsgQueueFileName mainq # set file name, also enables disk mode
$ActionResumeRetryCount -1 # infinite retries on insert failure
# for PostgreSQL replace :ommysql: by :ompgsql: below:
*.* :ommysql:hostname,dbname,userid,password;
</textarea>
<p>The simple setup above has one drawback: the write database action is
executed together with all other actions. Typically, local files are also
written. These local file writes are now bound to the speed of the database
action. So if the database is down, or threre is a large backlog, local files
are also not (or late) written.</p>
<p><b>There is an easy way to avoid this with rsyslog.</b> It involves a
slightly more complicated setup. In rsyslog, each action can utilize its own
queue. If so, messages are simply pulled over from the main queue and then the
action queue handles action processing on its own. This way, main processing and
the action are de-coupled. In the above example, this means that local file
writes will happen immediately while the database writes are queued. As a
side-note, each action can have its own queue, so if you would like to more than
a single database or send messages reliably to another host, you can do all of
this on their own queues, de-coupling their processing speeds.</p>
<p>The configuration for the de-coupled database write involves just a few more
commands:</p>
<textarea rows="11" cols="80">
$ModLoad ommysql # load the output driver (use ompgsql for PostgreSQL)
$ModLoad imudp # network reception
$UDPServerRun 514 # start a udp server at port 514
$ModLoad imuxsock # local message reception
$WorkDirectory /rsyslog/work # default location for work (spool) files
$ActionQueueType LinkedList # use asynchronous processing
$ActionQueueFileName dbq # set file name, also enables disk mode
$ActionResumeRetryCount -1 # infinite retries on insert failure
# for PostgreSQL replace :ommysql: by :ompgsql: below:
*.* :ommysql:hostname,dbname,userid,password;
</textarea>
<p><b>This is the recommended configuration for this use case.</b> It requires
rsyslog 3.11.0 or above.</p>
<p>In this example, the main message queue is NOT disk-assisted (there is no
$MainMsgQueueFileName directive). We still could do that, but have not done it
because there seems to be no need. The only slow running action is the database
writer and it has its own queue. So there is no real reason to use a large main
message queue (except, of course, if you expect *really* heavy traffic bursts).</p>
<p>Note that you can modify a lot of queue performance parameters, but the above
config will get you going with default values. If you consider using this on a real
busy server, it is strongly recommended to invest some time in setting the tuning
parameters to appropriate values.</p>
<h3>Feedback requested</h3>
<P>I would appreciate feedback on this tutorial. If you have additional ideas,
comments or find bugs (I *do* bugs - no way... ;)), please
<a href="mailto:rgerhards@adiscon.com">let me know</a>.</P>
<h2>Revision History</h2>
<ul>
<li>2008-01-28 *
<a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer Gerhards</a> * Initial Version created</li>
<li>2008-01-28 *
<a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer Gerhards</a>
* Updated to new v3.11.0 capabilities</li>
</ul>
<h2>Copyright</h2>
<p>Copyright (c) 2008
<a href="http://www.adiscon.com/en/people/rainer-gerhards.php">Rainer Gerhards</a> and
<a href="http://www.adiscon.com/en/">Adiscon</a>.</p>
<p> Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license can be viewed at
<a href="http://www.gnu.org/copyleft/fdl.html">
http://www.gnu.org/copyleft/fdl.html</a>.</p>
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<p><font size="2">This documentation is part of the
<a href="http://www.rsyslog.com/">rsyslog</a> project.<br>
Copyright © 2008 by <a href="http://www.gerhards.net/rainer">Rainer Gerhards</a> and
<a href="http://www.adiscon.com/">Adiscon</a>. Released under the GNU GPL
version 2 or higher.</font></p>
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