In this tutorial, we’ll describe the simplest HA Jenkins setup, which creates a baseline when we later discuss other modes of deployment. Here, we deploy HA Jenkins in the following configuration:

First, install Jenkins Enterprise packages to alpha and bravo. You need to install both the jenkins package (for the Jenkins Enterprise by CloudBees itself), and the jenkins-ha-monitor package, which is the Jenkins Enterprise by CloudBees HA monitor tool.

Choose the appropriate debian/redhat/openSUSE package format depending on the type of your distribution. Upon installation, both instances of Jenkins will start running. Stop them by issuing /etc/init.d/jenkins stop, while we work on the HA setup.

Let’s say sierra exports the /jenkins directory that hosts $JENKINS_HOME. Log on to alpha and mount this directory.

$ mount -t nfs -o rw,hard,intr sierra:/jenkins /var/lib/jenkins

/var/lib/jenkins is chosen to match what the Jenkins Enterprise packages use as $JENKINS_HOME. If you change them, update /etc/default/jenkins (on Debian) or /etc/sysconfig/jenkins (on RedHat and SUSE) to have $JENKINS_HOME point to the correct directory.

To make this mount automatically happen, update your /etc/fstab by adding the following entry:

sierra:/jenkins nfs rw,hard,intr 0 2

Repeat this mount setup on bravo, and ensure that both alpha and bravo see the same data. (For example, touch a from alpha, and make sure ls from bravo will see it. Make sure the uid and the gid appear the same on alpha and bravo.)

Boot Jenkins on alpha and bravo by issuing /etc/init.d/jenkins start. Now Jenkins Enterprise boots up in a two-node HA cluster. Access http://alpha:8080/ and http://bravo:8080/. One will serve the familiar Jenkins UI, and the other will tell you that it’s acting as a stand-by node. In the Jenkins UI, go to "Manage Jenkins" then click "High Availability Status" and make sure two nodes are listed as members. You can kill the primary JVM (for example by kill -9 PID) while watching the log file via tail -f /var/log/jenkins/jenkins.log, and you’ll see the stand-by node take over the primary role.

Finally, we set up a monitoring service to ensure that the floating IP address gets assigned to the system that’s hosting the primary. To do this, log on to alpha and install the jenkins-ha-monitor package.

This monitoring program watches Jenkins as root, and when the role transition occurs, it’ll execute the promotion script or the demotion script. In this tutorial, we’ll make these scripts assign/release the floating IP address.

In /etc/jenkins-ha-monitor/promotion.sh, write the following script:

#!/bin/sh
# assign the floating IP address 1.2.3.4 as an alias of eth1
ifconfig eth1:100 1.2.3.4

Similarly, in /etc/jenkins-ha-monitor/demotion.sh, write the following script:

#!/bin/sh
# release the floating IP address
ifconfig eth1:100 down

eth1:100 needs to be your network interface name followed by an unique alias ID (see "Linux IP aliasing" for more details.) Now that the configuration file is updated, restart the JA monitoring service by running /etc/init.d/jenkins-ha-monitor restart. Access the "High Availability Status" in the "Manage Jenkins" section from the web UI to verify that the monitoring service is recognized as a part of the cluster. Run ifconfig to verify that the virtual IP address is assigned to the system that’s hosting the primary JVM.

Congratulations, now you have a highly-available Jenkins!

# this section is a stock setting
global
        log 127.0.0.1   local0
        log 127.0.0.1   local1 notice
        maxconn 4096
        user haproxy
        group haproxy

defaults
        log     global
        mode    http
        option  httplog
        option  dontlognull
        retries 3
        redispatch
        maxconn 2000
        contimeout      1500
        clitimeout      500000
        srvtimeout      5000

# this block specifies a Jenkins Enterprise HA cluster
listen application 0.0.0.0:80
  balance roundrobin
  option httpchk /ha/health-check
  server alpha alpha:8080 check
  server bravo bravo:8080 check

# monitor port
listen status 0.0.0.0:8081
  stats enable
  stats uri /
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