There is Safety in Numbers: How to Build Fail-Safe Clusters

Very often, websites can't cope with any extra load, whether from high traffic or performance reducing tasks. As a result, companies lose money, as well as taking a reputation hit. Programmers blame system administrators. System administrators blame managers. Managers blame service providers. And vice versa. In the end, in this day and age, you can't afford down time: it will hurt your bottom line. So, how do you solve this issue?

One of the best solutions for such issues is using a cluster architecture for your applications. We'll show you how to create fail-safe clusters within GlassFish and within Jelastic.

1. Background

You need two hardware nodes, n1 and n2, for GlassFish servers and then machine lb1 for load balancing and machine db1 for the database. Make sure, that all machines can "see" each other (ping).

All software will work under Linux control. Also we'll use a preset image of a virtual machine with CentOS installed. GlassFish uses SSH to control remote nodes. That's why we have to install SSH-server on each machine of the cluster. Moreover we need separate accounts in every OS (specify their names, for example: glassfish).

GlassFish uses nonstandard ports, so you have to specify required rules  in the firewall or just switch it off. Let's choose the easiest variant:

And of course, install JVM.

Unfortunately, there are a lot of configurations and settings, when you create a cluster manually. So, be patient!

2. Configure the cluster

We configure  the cluster via command line with the asadmin utility.

1. Download GlassFish distribution

and unzip the package.

2. Start the administrative domain instance (domain1):

GlassFish disables remote connections to the admin domain. That's why all cluster nodes, except DAS nodes, can't cooperate with it.

Typical error:

To fix it type the next string:

and restart administrative domain.

Create your cluster:

Create two application servers on this hardware node:

3. Start the cluster:

4. Add new instances on the second  hardware node :

5. Go to GlassFish admin page, find the instance you have just created, change its type to SSH and change Node Host to n2. Also you have to specify the login and password for SSH access to machine n2. As you can see below, all the clusters are working now:

3. Load balancing and HA

1. Install NGINX:

2. Set round-robbin balancing for 4 servers on NGINX with sticky session support:

 3. Hosts n1 and n2 should have their common parent domain, for example: cluster.com. Edit the files /etc/hosts on both machines:

n1: 127.0.0.1 localhost 127.0.1.1 n1.cluster.com 127.0.0.1 n1.cluster.com 192.168.0.2 n2.cluster.com n2: 127.0.0.1 localhost 127.0.1.1 n2.cluster.com 127.0.0.1 n2.cluster.com 192.168.0.1 n1.cluster.com

4. Check multicast on each hardware node:

5. Change n2 to n2.cluster.com in DAS admin console. 6. Web-descriptor of your application should contain a suitable instruction (<distributable>) for providing replication of http-sessions. Don't forget to set the Availability flag, when you deploy your app.

4. Configure database

We'll use PostgreSQL as an example.

Install the database, create a new user and database:

Specify allowed subnets in the pg_hba.conf (/var/lib/pgsql/data/pg_hba.conf) file :

host all all 192.168.0.0/24 md5

5. Application deployment

Now you can deploy your application (for example: jforum).

Obviously, this is time consuming!

An easier way is to do this in Jelastic.  

You can create fail-safe clusters with just a few clicks from within the dashboard:

If you found this article useful, please let me know.