Introduction

Now that we have our production application server setup, a recovery plan, and backups, let’s look into adding monitoring to improve our awareness of the state of our servers and services. Monitoring software, such as Nagios, Icinga, and Zabbix, enables you to create dashboards and alerts that will show you which components of your application setup need attention. The goal of this is to help you detect issues with your setup, and start fixing them, before your users encounter them.

In this tutorial, we will set up Nagios 4 to as the monitoring software, and install the NRPE agent on the servers that comprise our application.

For each server in our setup, we will set up monitoring to check if the server is up and if its primary process (i.e. apache, mysql, or haproxy) is running. While this monitoring guide isn’t comprehensive—you will probably want to set up additional checks that we won’t cover here—it is a great place to start.

Prerequisites

If you want to access your logging dashboard via a domain name, create an A Record under your domain, like “monitoring.example.com”, that points to your monitoring server’s public IP address. Alternatively, you can access the monitoring dashboard via the public IP address. It is advisable that you set up the monitoring web server to use HTTPS, and limit access to it by placing it behind a VPN.

Install Nagios on Monitoring Server

Set up Nagios on your monitoring server by following this tutorial: How To Install Nagios 4 and Monitor Your Servers on Ubuntu 14.04. If you prefer, you may also use Icinga, which is a fork of Nagios.

Stop when you reach the Monitor an Ubuntu Host with NRPE section.

Add Servers to Nagios

On each server in your setup (db1, app1, app2, and lb1), go through the Monitor an Ubuntu Host with NRPE section of the Nagios tutorial.

Be sure to add the private hostname or IP address of your monitoring server to the allowed_hosts setting in NRPE configuration file.

When you are done adding each host, you should have a separate file for each server that you want to monitor: db1.cfg, app1.cfg, app2.cfg, and lb1.cfg. Each file should contain the host definition that refers to the respective hostname and address (which can be the server’s hostname or IP address).

Set Up Host and Service Monitoring

Let’s make a list of common things that we want to monitor on every server. For each server, let’s monitor the following services:

• Ping
• SSH
• Current Load
• Current Users
• Disk Utilization

Let’s set that up now.

Define Common Services

In the Nagios setup tutorial, we configured Nagios to look for .cfg files in /usr/local/nagios/etc/servers. To keep things organized, we will create a new Nagios configuration file for the common services we want to monitor called “common.cfg”.

First, open the host configuration file for editing

• sudo vi /usr/local/nagios/etc/servers/common.cfg

Add the following service definitions, with the host_name of each of your servers (defined in the host definitions, earlier):

define service {
        use                             generic-service
        host_name                       db1,app1,app2,lb1
        service_description             PING
        check_command                   check_ping!100.0,20%!500.0,60%
}

define service {
        use                             generic-service
        host_name                       db1,app1,app2,lb1
        service_description             SSH
        check_command                   check_ssh
        notifications_enabled           0
}

define service {
        use                             generic-service
        host_name                       db1,app1,app2,lb1
        service_description             Current Load
        check_command                   check_nrpe!check_load
}

define service {
        use                             generic-service
        host_name                       db1,app1,app2,lb1
        service_description             Current Users
        check_command                   check_nrpe!check_users
}

define service{
        use                             generic-service
        host_name                       db1,app1,app2,lb1
        service_description             Disk Utilization
        check_command                   check_nrpe!check_hda1
}

Save and exit.

Now we’re ready to define the services that are specific to each server. We’ll start with our database server.

Define MySQL Process

Create NRPE Command (on Client)

On your database server, db1, we’ll configure a new NRPE command. Open a new NRPE configuration file, “commands.cfg”:

• sudo vi /etc/nagios/nrpe.d/commands.cfg

Add the following command definition:

command[check_mysqld]=/usr/lib/nagios/plugins/check_procs -c 1: -C mysqld

Save and exit. This allows NRPE to check for a process named “mysqld”, and report a critical status if there is less than 1 processes with that name running.

Reload the NRPE configuration:

• sudo service nagios-nrpe-server reload

Create Service Definition (on Server)

On your Nagios server, monitoring, we need define a new service that uses NRPE to run the check_mysqld command.

Open the file that defines that defines your database host. In our example, it’s called “db1.cfg”:

• sudo vi /usr/local/nagios/etc/servers/db1.cfg

At the end of the file, add this service definition (make sure the host_name value matches the name of the host definition):

define service {
        use                             generic-service
        host_name                       db1
        service_description             Check MySQL Process
        check_command                   check_nrpe!check_mysqld
}

Save and exit. This configures Nagios to use NRPE to run the check_mysqld command on the database server.

For this change to take effect, we must reload Nagios. However, we will move on to monitoring the Apache process first.

Define Apache Process

Create NRPE Command (on Client)

On your application servers, app1 and app2, we’ll configure a new NRPE command. Open a new NRPE configuration file, “commands.cfg”:

• sudo vi /etc/nagios/nrpe.d/commands.cfg

Add the following command definition:

command[check_apache2]=/usr/lib/nagios/plugins/check_procs -c 1: -w 3: -C apache2

Save and exit. This allows NRPE to check for a process named “apache2”, and report a critical status if there are no matching processes running or report a warning status if there are less than three matching processes.

Reload the NRPE configuration:

• sudo service nagios-nrpe-server reload

Be sure to repeat this on any additional application servers.

Create Service Definition (on Server)

On your Nagios server, monitoring, we need define a new service that uses NRPE to run the check_apache2 command.

Open the file that defines that defines your database host. In our example, they’re called “app1.cfg” and “app2.cfg”:

• sudo vi /usr/local/nagios/etc/servers/app1.cfg

At the end of the file, add this service definition (make sure the host_name value matches the name of the host definition):

define service {
        use                             generic-service
        host_name                       app1
        service_description             Check Apache2 Process
        check_command                   check_nrpe!check_apache2
}

Save and exit. This configures Nagios to use NRPE to run the check_apache2 command on the application servers. Be sure to repeat this for each of your application servers.

For this change to take effect, we must reload Nagios. However, we will move on to monitoring the HAProxy process before that.

Define HAProxy Process

Create NRPE Command (on Client)

On your load balancer server, lb1, we’ll configure a new NRPE command. Open a new NRPE configuration file, “commands.cfg”:

• sudo vi /etc/nagios/nrpe.d/commands.cfg

Add the following command definition:

command[check_haproxy]=/usr/lib/nagios/plugins/check_procs -c 1: -C haproxy

Save and exit. This allows NRPE to check for a process named “haproxy”, and report a critical status if there is less than 1 processes with that name running.

Reload the NRPE configuration:

• sudo service nagios-nrpe-server reload

Be sure to repeat this on any additional application servers.

Create Service Definition (on Server)

On your Nagios server, monitoring, we need define a new service that uses NRPE to run the check_haproxy command.

Open the file that defines that defines your database host. In our example, it’s called “lb1.cfg”:

• sudo vi /usr/local/nagios/etc/servers/lb1.cfg

At the end of the file, add this service definition (make sure the host_name value matches the name of the host definition):

define service {
        use                             generic-service
        host_name                       lb1
        service_description             Check HAProxy Process
        check_command                   check_nrpe!check_haproxy
}

This configures Nagios to use NRPE to run the check_haproxy command on your load balancer server.

For this change to take effect, we must reload Nagios.

Reload Nagios Configuration

To reload Nagios, and put all of our changes into effect, enter this command:

sudo service nagios reload

If there are no syntactical errors in the configuration, you should be set.

Check Nagios Services

Before moving on, you will want to verify that Nagios is monitoring all of the hosts and services that you defined. Access your Nagios server via its public hostname or IP address, e.g. https://monitoring.example.com/nagios/. Enter the login you set up during the Nagios server installation.

In the side menu, click on the Services link. You should be taken to a page that looks like this:

Ideally, you will see each host and all services will be in an “OK” status. In the screenshot, we can see that there is a problem with the app2 server because it was powered off during most recent status checks. If any of your services are not “OK”, fix them or, if the services are fine, review your Nagios configuration for errors.

Other Considerations

You will most likely want to create a recovery plan for your monitoring server, and back up your Nagios configuration files (/usr/local/nagios/etc). Once backups are set up, you will probably want to configure monitoring, for additional services, and email notifications.

Conclusion

Now you should be able to see the status of your servers and services by simply glancing at your monitoring dashboard. In the event of an outage, your monitoring system will help you identify which server(s) and service(s) are not running properly which should help reduce the downtime of your application.

Continue to the next tutorial to start setting up the centralized logging for your production server setup: Building for Production: Web Applications — Centralized Logging.