Docker Swarm is a container orchestration and clustering tool to manage Docker hosts, and is a part of Docker Engine. It's a native clustering tool provided by Docker which provides high-availability and high-performance for your application.

The primary objective of Docker Swarm is to group multiple Docker hosts into a single logical virtual server—this ensures availability and high performance for your application by distributing it over a number of Docker hosts instead of just one.

In this tutorial you will learn:
  • What is Docker Swarm
  • How to Configure Hosts
  • How to Install and Run Docker Service
  • How to Configure the Manager Node for Swarm Cluster Initialization
  • How to Configure Worker Nodes to join the Swarm Cluster
  • How to Verify the Swarm Cluster
  • How to Deploy new Service on Swarm Cluster
Docker Swarm Services
Docker Swarm Services.

Software Requirements and Conventions Used

Software Requirements and Linux Command Line Conventions
Category Requirements, Conventions or Software Version Used
System Ubuntu 18.04
Software Docker-CE 18.09
Other Privileged access to your Linux system as root or via the sudo command.
Conventions # - requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command
$ - requires given linux commands to be executed as a regular non-privileged user

Swarm Concept in Detail

The cluster management and orchestration features embedded in the Docker Engine are built using swarmkit.

A swarm consists of multiple Docker hosts which run in swarm mode and act as managers (which manage membership and delegation) and workers (which run swarm services). A given Docker host can be a manager, a worker, or perform both roles. When you create a service, you define its optimal state like number of replicas, network and storage resources available to it, ports the service exposes to the outside world etc. If a worker node becomes unavailable, Docker schedules that node’s tasks on other nodes. A task is a running container which is part of a swarm service and managed by a swarm manager.



One of the key advantages of swarm services over standalone containers is that you can modify a service’s configuration, including the networks and volumes it is connected to, without the need to manually restart the service. Docker will update the configuration, stop the service tasks with the out of date configuration, and create new ones matching the desired configuration.

When Docker is running in swarm mode, you can still run standalone containers on any of the Docker hosts participating in the swarm, as well as swarm services. A key difference between standalone containers and swarm services is that only swarm managers can manage a swarm, while standalone containers can be started on any daemon. Docker daemons can participate in a swarm as managers, workers, or both.

Configure the Docker hosts

Before installing the necessary Docker packages for the swarm cluster, we will configure the hosts file on all the Ubuntu nodes.
Manager Node – 192.168.1.103 (hostname - dockermanager)
Worker Node1 – 192.168.1.107 (hostname – dockerworker1)
Worker Node2 – 192.168.1.108 (hostname - dockerworker2)
Edit the /etc/hosts file across all three nodes via gedit or vim and do the following changes:

192.168.1.103  dockermanager
192.168.1.107  dockerworker1
192.168.1.108  dockerworker2
After modifying with the above details in the hosts file, check the connectivity with ping between all the nodes.

From Docker Manager Host

# ping dockerworker1
# ping 192.168.1.107
# ping dockerworker2
# ping 192.168.1.108

From Docker Worker Node 1

# ping dockermanager
# ping 192.168.1.103

From Docker Worker Node 2

# ping dockermanager
# ping 192.168.1.103

Install and Run Docker Service

To create the swarm cluster, we need to install docker on all server nodes. We will install docker-ce i.e. Docker Community Edition on all three Ubuntu machines.

Before you install Docker CE for the first time on a new host machine, you need to set up the Docker repository. Afterward, you can install and update Docker from the repository. Perform all the below steps across all three Ubuntu Nodes.

Update the apt package index:

# apt-get update

Install packages to allow apt to use a repository over HTTPS:

# apt-get install apt-transport-https ca-certificates curl software-properties-common -y


Add Docker’s official GPG key:

curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

Use the following command to set up the stable repository:

# add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

Again update the apt package:

# apt-get update

Install the latest version of Docker CE:

apt-get install docker-ce

After the installation is complete, start the docker service and enable it to launch every time at system boot.

# systemctl start docker
# systemctl enable docker

To configure docker to run as a normal user or non-root user, run the following command:

# usermod -aG docker <username>
# usermod -aG docker manager
# usermod -aG docker worker1
# usermod -aG docker worker2
Now, login as designated user and run the docker hello-world to verify.
# su - manager
$ docker run hello-world

Upon successful run it will give the below output

Add New User for Hadoop
Docker Installation check Hello_World.

Configure the Manager Node for Swarm Cluster Initialization

In this step, we will create the swarm cluster of our nodes. To create the swarm cluster, we need to initialize the swarm mode on the 'dockermanager' node and then join the 'dockerworker1' and ‘dockerworker2’ node to the cluster.

Initialize the Docker Swarm mode by running the following docker command on the 'dockermanager' node.



docker swarm init --advertise-addr <manager node IP address>
$ docker swarm init --advertise-addr 192.168.1.103
Swarm Cluster initialization
Swarm Cluster initialization.

'join token' has been generated by the 'dockermanager' which will be required to join the worker nodes to the cluster manager.

Configure Worker Nodes to join the Swarm Cluster

Now, to join the worker nodes to the swarm, we will run the docker swarm join command on all worker nodes which we received in the swarm initialization step:

$ docker swarm join --token SWMTKN-1-4htf3vnzmbhc88vxjyguipo91ihmutrxi2p1si2de4whaqylr6-3oed1hnttwkalur1ey7zkdp9l 192.168.1.103:2377
Worker Node 1 Joining the Swarm Cluster
Worker Node 1 Joining the Swarm Cluster.

Worker Node 2 Joining the Swarm Cluster
Worker Node 2 Joining the Swarm Cluster.

Verify the Swarm Cluster

To see the node status, so that we can determine if the nodes are active/available etc, from the manager node, list all the nodes in the swarm:
$ docker node ls
Docker Swarm Cluster Verification
Docker Swarm Cluster Verification.

If at any time, you lost your join token, it can be retrieved by running the following command on the manager node for the manager token:

$ docker swarm join-token manager -q

The same way to retrieve the worker token run the following command on the manager node:

$ docker swarm join-token worker -q

Deploy new Service on Swarm Cluster

In this step, we will create and deploy our first service to the swarm cluster. The new service nginx web server will run on default http port 80, and then expose it to the port 8081 on the host machine. We will create this nginx service with 2 replicas, which means that there will be 2 containers of nginx running in our swarm. If any of these containers fail, they will be spawned again to have the desired number that we set on the replica option.
$ docker service create --name my-web1 --publish 8081:80 --replicas 2 nginx

After successful deployment of the service you can see the below output :

Deploy Nginx Service on Swarm Cluster
Deploy Nginx Service on Swarm Cluster.


To check the newly created nginx service using below docker service commands.

$ docker service ls
List the newly deployed service on Swarm Cluster
List the newly deployed service on Swarm Cluster.

docker service ps <service name>
$ docker service ps my-web1
Lists the tasks running as part of specified services on Swarm Cluster
Lists the tasks running as part of specified services on Swarm Cluster.

If we need to check whether nginx service is working fine, either we can use the curl command or check in the browser on host machine for the nginx web server welcome page.

$ curl http://dockermanager:8081
Nginx Web service check via CURL
Nginx Web service check via CURL.

In the browser on host machine we can access the Welcome Page of nginx

Nginx service check via browser
Nginx service check via browser.

Now , if we need to scale the nginx service we will make 3 replicas and to do that run the following command on manager node:

$ docker service scale my-web1=3
Service Scaling for the desired number of replicas
Service Scaling for the desired number of replicas.

To check the output after scaling we can use docker service ls or docker service ps command.

We can use docker service inspect command to check the extended details of a deployed service on swarm. By default, this renders all results in a JSON array.

Conclusion

Docker has become an extremely popular way to configure, save, and share server environments using containers. Because of this, installing an application or even a large stack can often be as simple as running docker pull or docker run. Separating application functions into different containers also offers advantages in security and dependency management.
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