Email Alerting like Splunk

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.


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".

Installing the requisites

Install Fluentd if you haven't yet.

Please install fluent-plugin-grepcounter by running:


Next, please install fluent-plugin-mail by running:


Note: If you installed Fluentd using ruby gems, use gem command instead of td-agent-gem.


Full configuration example

Below shows the full configuration example. You can copy the following content and edit it to suit your needs.

@type tail
path /var/log/apache2/access.log # Set the location of your log file
@type apache2
tag apache.access
<match apache.access>
@type grepcounter
count_interval 3 # The time window for counting errors (in secs)
input_key code # The field to apply the regular expression
regexp ^5\d\d$ # The regular expression to be applied
threshold 1 # The minimum number of erros to trigger an alert
add_tag_prefix error_5xx # Generate tags like "error_5xx.apache.access"
<match error_5xx.apache.access>
@type copy
@type stdout # Print to stdout for debugging
@type mail
host # Change this to your SMTP server host
port 587 # Normally 25/587/465 are used for submission
user USERNAME # Use your username to log in
password PASSWORD # Use your login password
enable_starttls_auto true # Use this option to enable STARTTLS
from # Set the sender address
to # Set the recipient address
message_out_keys count # Use the "count" field to replace "%s" above

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.

How this configuration works

The configuration above consists of three main parts:

  1. The first <source> block sets the httpd log file as an event source for the daemon.

  2. The second <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 error_5xx.apache.access.

  3. The third <match> block accepts events with the tag error_5xx.apache.access, and send an email to per event.

In this way, fluentd now works as an email alerting system that monitors the web service for you.

Test the configuration

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".

What's next?


You can learn more about Fluentd and its plugins by
