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How to install Docker Swarm in CentOS 7

Docker Swarm Cluster Gonkar IT Security

How to install Docker Swarm in CentOS 7

About Docker

Docker is a tool designed to make it easier to create, deploy, and run applications by using containers. Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and ship it all out as one package. By doing so, thanks to the container, the developer can rest assured that the application will run on any other Linux machine regardless of any customized settings that machine might have that could differ from the machine used for writing and testing the code.

In a way, Docker is a bit like a virtual machine. But unlike a virtual machine, rather than creating a whole virtual operating system, Docker allows applications to use the same Linux kernel as the system that they’re running on and only requires applications be shipped with things not already running on the host computer. This gives a significant performance boost and reduces the size of the application.

Docker Swarm Highlights:

• Cluster management integrated with Docker Engine
• Decentralized design
• Declarative service model
• Scaling
• Multi-host networking
• Service discovery
• Load balancing
• Secure by default
• Rolling updates

About this Article

We will be talking about the installation of Docker Swarm Cluster in CentOS 7 and how to deploy our first service and replicated along the cluster. We will perform a simple deployment with 1 Manager node and 2 Worker(Compute) nodes. We recommend to run nodes with at least 4GB of RAM, here you can check our VPS options Because of Docker Swarm nature it will Load Balance all applications within all cluster nodes. The service we are deploying is a Apache Server.


1.- Uninstall older versions of Docker.

yum remove docker docker-common docker-selinux docker-engine

Don’t worry if you have images, volumes or any data in your existing docker installation it will be preserve anyway.

2.- Install Docker-CE

First we need to install yum-utils in order to enable Docker-CE repository and devicemapper packages cause it is use by Docker storage driver.

yum install -y yum-utils device-mapper-persistent-data lvm2

Next, we install Docker-CE repositories

yum-config-manager --add-repo

Now we install Docker-CE packages

yum install docker-ce

After that, we will have the latest stable packages from docker-ce repository.

At this point Docker is installed and we just need to check that it is working properly

systemctl start docker
systemctl enable docker
systemctl status docker

And to check Docker engine we could execute this command

docker run hello-world

3.- Setup Docker Swarm

With Docker up and running we can move forward to configure our Docker Swarm Cluster

We need to remember that we are deploying in three machines for this example, we will call them:

• master01
• compute01
• compute02

First we have to setup the /etc/hosts file in each machine of our cluster to looks like: master01 compute01 compute02

It is important to set up this information in all nodes in order to be reachable between each other.

Next we need to configure machines hostname with this command

hostnamectl set-hostname master01

hostnamectl set-hostname compute01

hostnamectl set-hostname compute02

In order to allow our Swarm Cluster to communicate we have to open the ports in our firewall, in the case of this article we focus on firewalld which comes with CentOS by default

firewall-cmd --permanent --add-port=2376/tcp
firewall-cmd --permanent --add-port=2377/tcp
firewall-cmd --permanent --add-port=7946/tcp
firewall-cmd --permanent --add-port=7946/udp
firewall-cmd --permanent --add-port=4789/tcp
firewall-cmd --permanent --add-port=80/tcp
firewall-cmd –reload

Now we are ready to initialized our cluster, first we have to go to our master01 node and run this command

docker swarm init --advertise-addr

It should output something similar to this

Swarm initialized: current node (CJKVemqwq3by7WJ80FWegLsxc) is now a manager.

To add a worker to this swarm, run the following command:

docker swarm join --token SWMTKN-1-1yn39o5d0aeiuvdiufp45rwbdbg5gxhrvbp3v38s5q6kcjh0q0-CJKVemqwq3by7WJ80FWegLsxc

To add a manager to this swarm, run 'docker swarm join-token manager' and follow the instructions.

With our Docker Swarm Master configured we just need to copy the docker swarm join command from the output and copy it in our compute nodes.

Connect to the compute nodes and execute the command we got from the master

docker swarm join --token SWMTKN-1-1yn39o5d0aeiuvdiufp45rwbdbg5gxhrvbp3v38s5q6kcjh0q0-CJKVemqwq3by7WJ80FWegLsxc

We should get a message confirming that our node is part of the Swarm Cluster

This node joined a swarm as a worker.

To check the status of the cluster we can run

docker node ls

ID                            HOSTNAME                        STATUS              AVAILABILITY        MANAGER STATUS      ENGINE VERSION
ac6orrj3xchl4xn956krvbwvn *             Ready               Active              Leader              18.09.6
pibw78gx1w89eckyxe8imtsd1            Ready               Active                                  18.09.6
tifegjhj3dxhfk0qbddu2nof9            Ready               Active                                  18.09.6


4.- Deploy our first Service in our Swarm Cluster

It is important to perform the deployment from the master node

docker service create -p 80:80 --name webapp --replicas 3 httpd

With that we will have our WebApp running in all the machines within the cluster and we can access it just pointing any IP address of our machines in the browser. For example

Useful commands:

docker service ls #To list all swarm services
docker services ps webapp #list all containers of our service
docker service scale webapp=6 #scale up our servie to 6 instances

Now we have it, Docker Swam Cluster running our first service replicated in all Swarm nodes.

Alejandro Betancor

Author Alejandro Betancor

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