Introduction

High availability is a function of system design that allows an application to automatically restart or reroute work to another capable system in the event of a failure. In terms of servers, there are a few different technologies needed to set up a highly available system. There must be a component that can redirect the work and there must be a mechanism to monitor for failure and transition the system if an interruption is detected.

The keepalived daemon can be used to monitor services or systems and to automatically failover to a standby if problems occur. In this guide, we will demonstrate how to use keepalived to set up a highly available web service. We will configure a floating IP address that can be moved between two capable web servers. If the primary server goes down, the floating IP will be moved to the second server automatically, allowing service to resume.

Prerequisites

In order to complete this guide, you will need to create two Ubuntu 14.04 servers on your DigitalOcean account. Both servers must be located within the same datacenter and should have private networking enabled.

On each of these servers, you will need a non-root user configured with sudo access. You can follow our Ubuntu 14.04 initial server setup guide to learn how to set up these users.

When you are ready to get started, log into both of your servers with your non-root user.

Install and Configure Nginx

While keepalived is often used to monitor and failover load balancers, in order to reduce our operational complexity, we will be using Nginx as a simple web server in this guide.

Start off by updating the local package index on each of your servers. We can then install Nginx:

sudo apt-get update
sudo apt-get install nginx

In most cases, for a highly available setup, you would want both servers to serve exactly the same content. However, for the sake of clarity, in this guide we will use Nginx to indicate which of the two servers is serving our requests at any given time. To do this, we will change the default index.html page on each of our hosts. Open the file now:

sudo nano /usr/share/nginx/html/index.html

On your first server, replace the contents of the file with this:

<h1>Primary</h1>

On your second server, replace the contents of the file with this:

<h1>Secondary</h1>

Save and close the files when you are finished.

Build and Install Keepalived

Next, we will install the keepalived daemon on our servers. There is a version of keepalived in Ubuntu's default repositories, but it is outdated and suffers from a few bugs that prevent our configuration from working. Instead, we will install the latest version of keepalived from source.

Before we begin, we should grab the dependencies we will need to build the software. The build-essential meta-package will provide the compilation tools we need, while the libssl-dev package contains the SSL libraries that keepalived needs to build against:

sudo apt-get install build-essential libssl-dev

Once the dependencies are in place, we can download the tarball for keepalived. Visit this page to find the latest version of the software. Right-click on the latest version and copy the link address. Back on your servers, move to your home directory and use wget to grab the link you copied:

cd ~
wget http://www.keepalived.org/software/keepalived-1.2.19.tar.gz

Use the tar command to expand the archive and then move into the resulting directory:

tar xzvf keepalived*
cd keepalived*

Build and install the daemon by typing:

./configure
make
sudo make install

The daemon should now be installed on the system.

Create a Keepalived Upstart Script

The keepalived installation moved all of the binaries and supporting files into place on our system. However, one piece that was not included was an Upstart script for our Ubuntu 14.04 systems.

We can create a very simple Upstart script that can handle our keepalived service. Open a file called keepalived.conf within the /etc/init directory to get started:

sudo nano /etc/init/keepalived.conf

Inside, we can start with a simple description of the functionality keepalived provides. We'll use the description from the included man page. Next we will specify the runlevels in which the service should be started and stopped. We want this service to be active in all normal conditions (runlevels 2-5) and stopped for all other runlevels (when reboot, poweroff, or single-user mode is initiated, for instance):

description "load-balancing and high-availability service"

start on runlevel [2345]
stop on runlevel [!2345]

Because this service is integral to ensuring our web service remains available, we want to restart this service in the event of a failure. We can then specify the actual exec line that will start the service. We need to add the --dont-fork option so that Upstart can track the pid correctly:

description "load-balancing and high-availability service"

start on runlevel [2345]
stop on runlevel [!2345]

respawn

exec /usr/local/sbin/keepalived --dont-fork

Save and close the files when you are finished.

Create the Keepalived Configuration File

With our Upstart file in place, we can now move on to configuring keepalived.

The service looks for its configuration files in the /etc/keepalived directory. Create that directory now on both of your servers:

sudo mkdir -p /etc/keepalived

Collecting the Private IP addresses of your Servers

Before we create the configuration file, we need to find the private IP addresses of both of our servers. On DigitalOcean servers, you can get our private IP address through the metadata service by typing:

curl http://169.254.169.254/metadata/v1/interfaces/private/0/ipv4/address && echo

Output
10.132.7.107

This can also be found with the iproute2 tools by typing:

ip -4 addr show dev eth1

The value you are looking for will be found here:

Output
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    inet 10.132.7.107/16 brd 10.132.255.255 scope global eth1
       valid_lft forever preferred_lft forever

Copy this value from both of your systems. We will need to reference these addresses inside of our configuration files below.

Creating the Primary Server's Configuration

Next, on your primary server, create the main keepalived configuration file. The daemon looks for a file called keepalived.conf inside of the /etc/keepalived directory:

sudo nano /etc/keepalived/keepalived.conf

Inside, we will start by defining a health check for our Nginx service by opening up a vrrp_script block. This will allow keepalived to monitor our web server for failures so that it can signal that the process is down and begin recover measures.

Our check will be very simple. Every two seconds, we will check that a process called nginx is still claiming a pid:

vrrp_script chk_nginx {
    script "pidof nginx"
    interval 2
}

Next, we will open a block called vrrp_instance. This is the main configuration section that defines the way that keepalived will implement high availability.

We will start off by telling keepalived to communicate with its peers over eth1, our private interface. Since we are configuring our primary server, we will set the state configuration to "MASTER". This is the initial value that keepalived will use until the daemon can contact its peer and hold an election.

During the election, the priority option is used to decide which member is elected. The decision is simply based on which server has the highest number for this setting. We will use "200" for our primary server:

vrrp_script chk_nginx {
    script "pidof nginx"
    interval 2
}

vrrp_instance VI_1 {
    interface eth1
    state MASTER
    priority 200


}

Next, we will assign an ID for this cluster group that will be shared by both nodes. We will use "33" for this example. We need to set unicast_src_ip to our primary server's private IP address that we retrieved earlier. We will set unicast_peer to our secondary server's private IP address:

vrrp_script chk_nginx {
    script "pidof nginx"
    interval 2
}

vrrp_instance VI_1 {
    interface eth1
    state MASTER
    priority 200

    virtual_router_id 33
    unicast_src_ip primary_private_IP
    unicast_peer {
        secondary_private_IP
    }


}

Next, we can set up some simple authentication for our keepalived daemons to communicate with one another. This is just a basic measure to ensure that the servers in question are legitimate. Create an authentication sub-block. Inside, specify password authentication by setting the auth_type. For the auth_pass parameter, set a shared secret that will be used by both nodes. Unfortunately, only the first eight characters are significant:

vrrp_script chk_nginx {
    script "pidof nginx"
    interval 2
}

vrrp_instance VI_1 {
    interface eth1
    state MASTER
    priority 200

    virtual_router_id 33
    unicast_src_ip primary_private_IP
    unicast_peer {
        secondary_private_IP
    }

    authentication {
        auth_type PASS
        auth_pass password
    }


}

Next, we will tell keepalived to use the routine we created at the top of the file, labeled chk_nginx, to determine the health of the local system. Finally, we will set a notify_master script, which is executed whenever this node becomes the "master" of the pair. This script will be responsible for triggering the floating IP address reassignment. We will create this script momentarily:

vrrp_script chk_nginx {
    script "pidof nginx"
    interval 2
}

vrrp_instance VI_1 {
    interface eth1
    state MASTER
    priority 200

    virtual_router_id 33
    unicast_src_ip primary_private_IP
    unicast_peer {
        secondary_private_IP
    }

    authentication {
        auth_type PASS
        auth_pass password
    }

    track_script {
        chk_nginx
    }

    notify_master /etc/keepalived/master.sh
}

Once you've set up the information above, save and close the file.

Creating the Secondary Server's Configuration

Next, we will create the companion script on our secondary server. Open a file at /etc/keepalived/keepalived.conf on your secondary server:

sudo nano /etc/keepalived/keepalived.conf

Inside, the script that we will use will be largely equivalent to the primary server's script. The items that we need to change are:

• state: This should be changed to "BACKUP" on the secondary server so that the node initializes to the backup state before elections occur.
• priority: This should be set to a lower value than the primary server. We will use the value "100" in this guide.
• unicast_src_ip: This should be the private IP address of the secondary server.
• unicast_peer: This should contain the private IP address of the primary server.

When you change those values, the script for the secondary server should look like this:

vrrp_script chk_nginx {
    script "pidof nginx"
    interval 2
}

vrrp_instance VI_1 {
    interface eth1
    state BACKUP
    priority 100

    virtual_router_id 33
    unicast_src_ip secondary_private_IP
    unicast_peer {
        primary_private_IP
    }

    authentication {
        auth_type PASS
        auth_pass password
    }

    track_script {
        chk_nginx
    }

    notify_master /etc/keepalived/master.sh
}

Once you've entered the script and changed the appropriate values, save and close the file.

Create the Floating IP Transition Scripts

Next, we will need to create a pair of scripts that we can use to reassign the floating IP address to the current Droplet whenever the local keepalived instance becomes the master server.

Download the Floating IP Assignment Script

First, we will download a generic Python script (written by a DigitalOcean community manager) that can be used to reassign a floating IP address to a Droplet using the DigitalOcean API. We should download this file to the /usr/local/bin directory:

cd /usr/local/bin
sudo curl -LO http://do.co/assign-ip

This script allows you to re-assign an existing floating IP by running:

python /usr/local/bin/assign-ip floating_ip droplet_ID

This will only work if you have an environmental variable called DO_TOKEN set to a valid DigitalOcean API token for your account.

Create a DigitalOcean API Token

In order to use the script above, we will need to create a DigitalOcean API token in our account.

In the control panel, click on the "API" link at the top. On the right-hand side of the API page, click "Generate new token":

On the next page, select a name for your token and click on the "Generate Token" button:

On the API page, your new token will be displayed:

Copy the token now. For security purposes, there is no way to display this token again later. If you lose this token, you will have to destroy it and create another one.

Configure a Floating IP for your Infrastructure

Next, we will create and assign a floating IP address to use for our servers.

In the DigitalOcean control panel, click on the "Networking" tab and select the "Floating IPs" navigation item. Select the Droplet from the list that you assigned as your "primary" server:

A new floating IP address will be created in your account and assigned to the Droplet specified:

If you visit the floating IP in your web browser, you should see the "primary" server index.html page:

Copy the floating IP address down. You will need this value in the script below.

Create the Wrapper Script

Now, we have the items we need to create the wrapper script that will call our /usr/local/bin/assign-ip script with the correct credentials.

Create the file now on both servers by typing:

sudo nano /etc/keepalived/master.sh

Inside, start by assigning and exporting a variable called DO_TOKEN that holds the API token you just created. Below that, we can assign a variable called IP that holds your floating IP address:

export DO_TOKEN='digitalocean_api_token'
IP='floating_ip_addr'

Next, we will use curl to ask the metadata service for the Droplet ID of the server we're currently on. This will be assigned to a variable called ID. We will also ask whether this Droplet currently has the floating IP address assigned to it. We will store the results of that request in a variable called HAS_FLOATING_IP:

export DO_TOKEN='digitalocean_api_token'
IP='floating_ip_addr'
ID=$(curl -s http://169.254.169.254/metadata/v1/id)
HAS_FLOATING_IP=$(curl -s http://169.254.169.254/metadata/v1/floating_ip/ipv4/active)

Now, we can use the variables above to call the assign-ip script. We will only call the script if the floating IP is not already associated with our Droplet. This will help minimize API calls and will help prevent conflicting requests to the API in cases where the master status switches between your servers rapidly.

To handle cases where the floating IP already has an event in progress, we will retry the assign-ip script a few times. Below, we attempt to run the script 10 times, with a 3 second interval between each call. The loop will end immediately if the floating IP move is successful:

export DO_TOKEN='digitalocean_api_token'
IP='floating_ip_addr'
ID=$(curl -s http://169.254.169.254/metadata/v1/id)
HAS_FLOATING_IP=$(curl -s http://169.254.169.254/metadata/v1/floating_ip/ipv4/active)

if [ $HAS_FLOATING_IP = "false" ]; then
    n=0
    while [ $n -lt 10 ]
    do
        python /usr/local/bin/assign-ip $IP $ID && break
        n=$((n+1))
        sleep 3
    done
fi

Save and close the file when you are finished.

Now, we just need to make the script executable so that keepalived can call it:

sudo chmod +x /etc/keepalived/master.sh

Start Up the Keepalived Service and Test Failover

The keepalived daemon and all of its companion scripts should now be completely configured. We can start the service on both of our machines by typing:

sudo start keepalived

The service should start up on each server and contact its peer, authenticating with the shared secret we configured. Each daemon will monitor the local Nginx process, and will listen to signals from the remote keepalived process.

When both servers are healthy, if you visit your floating IP in your web browser, you should be taken to the primary server's Nginx page:

Now, we are ready to test the failover capabilities of our configuration.

Failover should occur when either of the following conditions occur:

• When the Nginx health check on the primary server indicates that Nginx is no longer running. In this case, the primary server's keepalived daemon will enter the "fault" state. It will notify the secondary server that it should transition to the master state and claim the floating IP.
• When the secondary server loses its keepalived connection to the primary server. If the secondary server cannot reach the primary server for any reason, it will transition to the "master" state and attempt to claim the floating IP.

If the primary server later recovers, it will transition back to the master state and reclaim the floating IP because it will initiate a new election (it will still have the highest priority number).

Testing Nginx Failure

We can test the first condition by stopping the Nginx service on the primary server:

sudo service nginx stop

If you refresh your web browser, you might initially get a response indicating that the page is not available:

However, after just a few seconds, if you refresh the page a few times, you will see that the secondary server has claimed the floating IP address:

We can recover from the failure by restarting the Nginx daemon on the primary server:

sudo service nginx start

After a few seconds, if you refresh the page, you will find that the primary server has reclaimed ownership of the floating IP again:

Testing Server Failure

The other scenario we should test is whether the secondary correctly transitions to the master state if it cannot connect to the primary server. We can reboot the master server to test this:

sudo reboot

Again, we should at first see a service interruption at the floating IP address:

A few seconds later, the secondary server will pick up the requests:

A moment later, when the primary server finishes rebooting, it will reclaim the IP address:

This verifies our second failure scenario.

Conclusion

In this guide, we configured a highly available web server environment using keepalived, the DigitalOcean API, and a floating IP address. The actual infrastructure was rather simple, but the concepts can be applied to any type of infrastructure where service availability and uptime is important.