Splunk is a great tool for searching logs. One of its key features is the ability to "grep" logs and send alert emails when certain conditions are met.
In this little "how to" article, we will show you how to build a similar system using Fluentd. More specifically, we will create a system that sends an alert email when it detects a 5xx HTTP status code in an Apache access log.
If you want a more general introduction to use Fluentd as a free alternative to Splunk, see the article "Free Alternative to Splunk Using Fluentd".
Install Fluentd if you haven't yet.
fluent-plugin-grepcounter by running:
$ sudo /usr/sbin/td-agent-gem install fluent-plugin-grepcounter
Next, please install
fluent-plugin-mail by running:
$ sudo /usr/sbin/td-agent-gem install fluent-plugin-mail
Note: If you installed Fluentd using ruby gems, use
gem command instead of
Below shows the full configuration example. You can copy the following content and edit it to suit your needs.
<source>@type tailpath /var/log/apache2/access.log # Set the location of your log file<parse>@type apache2</parse>tag apache.access</source><match apache.access>@type grepcountercount_interval 3 # The time window for counting errors (in secs)input_key code # The field to apply the regular expressionregexp ^5\d\d$ # The regular expression to be appliedthreshold 1 # The minimum number of erros to trigger an alertadd_tag_prefix error_5xx # Generate tags like "error_5xx.apache.access"</match><match error_5xx.apache.access>@type copy<store>@type stdout # Print to stdout for debugging</store><store>@type mailhost smtp.gmail.com # Change this to your SMTP server hostport 587 # Normally 25/587/465 are used for submissionuser USERNAME # Use your username to log inpassword PASSWORD # Use your login passwordenable_starttls_auto true # Use this option to enable STARTTLSfrom firstname.lastname@example.org # Set the sender addressto email@example.com # Set the recipient addresssubject 'HTTP SERVER ERROR'message Total 5xx error count: %s\n\nPlease check your Apache webserver ASAPmessage_out_keys count # Use the "count" field to replace "%s" above</store></match>
Save your settings to
/etc/td-agent/td-agent.conf (If you installed Fluentd without td-agent, save the content as 'alert-email.conf' instead).
Before proceeding, please confirm:
The SMTP configuration is correct. You need a working mail server
and a proper recipient address to run this example.
The access log file has a proper file permission. You need to make
the file readable to the td-agent/Fluentd daemon.
The configuration above consists of three main parts:
<source> block sets the httpd log file as an event source for the daemon.
<match> block tells Fluentd to count the number of 5xx responses per time window (3 seconds). If the number exceeds (or is equal to) the given threshold, Fluentd will emit an event with the tag
<match> block accepts events with the tag
error_5xx.apache.access, and send an email to
firstname.lastname@example.org per event.
In this way, fluentd now works as an email alerting system that monitors the web service for you.
After saving the configuration, restart the td-agent process:
# for init.d users$ sudo /etc/init.d/td-agent restart# for systemd users$ sudo systemctl restart td-agent
If you installed the standalone version of Fluentd, launch the fluentd process manually:
$ fluentd -c alert-email.conf
Then generate some 5xx errors in the web server. If you do not have a convenient way to accomplish this, appending 5xx lines to the log file manually will produce the same result.
Now you will receive an alert email titled "HTTP SERVER ERROR".
Admittedly, this is a contrived example. In reality, you would set the threshold higher. Also, you might be interested in tracking 4xx pages as well. In addition to Apache logs, Fluentd can handle Nginx logs, syslogs, or any single- or multi-lined logs.
You can learn more about Fluentd and its plugins by
exploring other plugins
asking questions on the [mailing
If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is a open source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.