How to install Docker Swarm on CentOS 7

Learn how to install Docker Swarm on CentOS 7 with our comprehensive step-by-step guide. Follow detailed instructions for setting up and managing a robust Docker Swarm environment on your CentOS 7 system. #centlinux #linux #docker

What is Docker Swarm?

Docker Swarm is the native clustering and scheduling tool for Docker containers. Current versions of Docker include Swarm mode for natively managing a cluster of Docker Engines. Docker Swarm clusters can be configured and managed using the same Docker-CLI commands.

In this article, we install Docker Swarm cluster on CentOS 7 based Linux servers. We are using three nodes for our Docker Swarm cluster. One node as the Manager node and the other two as the Worker nodes.

Linux Server Specification

We have provisioned three identical virtual machines with CentOS 7.6 operating system and following specifications.

Read Also: How to install Portainer on CentOS 7

Before move on any further, we clarify that this post follows a more practical approach to demonstrate how things can be done, without diving into the theoretical nitty gritty. Therefore, we recommend you should join:

Recommended Training: Docker Mastery: with Kubernetes +Swarm from a Docker Captain

Install Docker on CentOS 7

To run Docker in Swarm mode, we are required to install Docker Engine CE on each node.

Connect with docker-manager-01 using ssh as root user. Execute following command to configure local DNS resolver.

cat >> /etc/hosts << EOF
192.168.116.150 docker-manager-01.centlinux.com docker-manager-01
192.168.116.151 docker-worker-01.centlinux.com docker-worker-01
192.168.116.152 docker-worker-02.centlinux.com docker-worker-02
EOF

Some of the required packages by Docker Engine CE are available in EPEL (Extra Packages for Enterprise Linux) yum repository. Therefore, we are installing EPEL yum repository before installing Docker Engine CE.

yum install -y epel-release.noarch

Install Docker yum repository for CentOS 7 as follows:

yum-config-manager --add-repo=https://download.docker.com/linux/centos/docker-ce.repo

Output:

Loaded plugins: fastestmirror
adding repo from: https://download.docker.com/linux/centos/docker-ce.repo
grabbing file https://download.docker.com/linux/centos/docker-ce.repo to /etc/yum.repos.d/docker-ce.repo
repo saved to /etc/yum.repos.d/docker-ce.repo

Enable Docker-CE (Nightly) yum repository.

yum-config-manager --enable docker-ce-nightly 

Build yum cache before using EPEL and Docker yum repositories.

yum makecache fast

Output:

Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
epel/x86_64/metalink                          | 9.1 kB     00:00
 * base: repo.inara.pk
 * epel: sg.fedora.ipserverone.com
 * extras: repo.inara.pk
 * updates: repo.inara.pk
base                                          | 3.6 kB     00:00
docker-ce-nightly                             | 3.5 kB     00:00
docker-ce-stable                              | 3.5 kB     00:00
extras                                        | 3.4 kB     00:00
updates                                       | 3.4 kB     00:00
Metadata Cache Created

Install Docker Engine CE using yum command.

yum install -y docker-ce

Start and enable Docker service.

systemctl enable docker.service
systemctl start docker.service

Docker required following service ports to function.

Therefore, allow above service ports in Linux Firewall.

firewall-cmd --permanent --add-port={2376,2377,7946}/tcp
firewall-cmd --permanent --add-port={7946,4789}/udp
firewall-cmd --reload

Verify docker installation by checking its version.

docker version

Output:

Client:
 Version:           18.09.3
 API version:       1.39
 Go version:        go1.10.8
 Git commit:        774a1f4
 Built:             Thu Feb 28 06:33:21 2019
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          18.09.3
  API version:      1.39 (minimum version 1.12)
  Go version:       go1.10.8
  Git commit:       774a1f4
  Built:            Thu Feb 28 06:02:24 2019
  OS/Arch:          linux/amd64
  Experimental:     false

We have installed Docker Engine CE on CentOS 7 server. Repeat the same steps on remaining two nodes (i.e. docker-worker-01 and docker-worker-02) to install Docker Engine CE on them.

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Install Docker Swarm on CentOS 7

Since, we have installed and configured three Docker nodes. Now, its time to use them to form a Docker Swarm cluster.

Initialize Docker Swarm mode on the manager node (i.e. docker-manager-01).

docker swarm init --advertise-addr 192.168.116.150

Output:

Swarm initialized: current node (3b9wynaya1wu910nf01m5jeeq) is now a manager.

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

    docker swarm join --token SWMTKN-1-1yn39o5d0aeiuvdiufp45rwbdbg5gxhrvbp3v38s5q6kcjh0q0-3m3vysmghac17vt3iz89mse9u 192.168.116.150:2377

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

Our Docker Swarm’s manager node has been initialized.

Docker provides us a command to join other workers and managers to our Docker Swarm cluster. Therefore, we use this command on docker-worker-01 node to join it to Docker Swarm as a worker node.

Connect with docker-worker-01 using ssh as root user and execute the command, as provided by Docker in previous step.

docker swarm join --token SWMTKN-1-1yn39o5d0aeiuvdiufp45rwbdbg5gxhrvbp3v38s5q6kcjh0q0-3m3vysmghac17vt3iz89mse9u 192.168.116.150:2377

Output:

This node joined a swarm as a worker.

Repeat the same step on docker-worker-02.

docker swarm join --token SWMTKN-1-1yn39o5d0aeiuvdiufp45rwbdbg5gxhrvbp3v38s5q6kcjh0q0-3m3vysmghac17vt3iz89mse9u 192.168.116.150:2377

Output:

This node joined a swarm as a worker.

Execute following command on any node to see the detail information about that node.

docker info

Output:

Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 0
Server Version: 18.09.3
Storage Driver: overlay2
 Backing Filesystem: xfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
Swarm: active
 NodeID: 3b9wynaya1wu910nf01m5jeeq
 Is Manager: true
 ClusterID: 45d0haajzr0gcwy09jglqbyc9
 Managers: 1
 Nodes: 4
 Default Address Pool: 10.0.0.0/8
 SubnetSize: 24
 Orchestration:
  Task History Retention Limit: 5
 Raft:
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 10
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
  Force Rotate: 0
 Autolock Managers: false
 Root Rotation In Progress: false
 Node Address: 192.168.116.150
 Manager Addresses:
  192.168.116.150:2377
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: bb71b10fd8f58240ca47fbb579b9d1028eea7c84
runc version: 2b18fe1d885ee5083ef9f0838fee39b62d653e30
init version: fec3683
Security Options:
 seccomp
  Profile: default
Kernel Version: 3.10.0-957.el7.x86_64
Operating System: CentOS Linux 7 (Core)
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 468.6MiB
Name: docker-manager-01.centlinux.com
ID: YC7A:KE73:DL34:YG3W:OVYF:IRVA:ACFV:HB6O:RJHU:B54I:J4BK:SWHX
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false
Product License: Community Engine

To check the status of nodes in Docker Swarm cluster.

docker node ls

Output:

ID                            HOSTNAME                        STATUS              AVAILABILITY        MANAGER STATUS      ENGINE VERSION
3b9wynaya1wu910nf01m5jeeq *   docker-manager-01.centlinux.com   Ready               Active              Leader              18.09.3
ydgqdyoksx2mb0snhe1hwvco7     docker-worker-01.centlinux.com    Ready               Active                                  18.09.3
vz02oe9e82deh8utiymnfobpk     docker-worker-02.centlinux.com    Ready               Active                                  18.09.3

Our Docker Swarm cluster is configured successfully.

Create a Replicated Service on Docker Swarm

To demonstrate use of our Docker Swarm cluster, we are creating a replicated service on it.

docker service create --name web1 -p 80:80 --replicas 5 nginx

Output:

vus0grc7koogpwipbmzga94k6
overall progress: 5 out of 5 tasks
1/5: running
2/5: running
3/5: running
4/5: running
5/5: running
verify: Service converged

A service with 5 replicas has been created and respective containers are converged across the Docker Swarm cluster.

To check where the containers are created and running, use the following command on docker-manager-01 node.

docker service ps web1     

Output:

ID                  NAME                IMAGE               NODE                            DESIRED STATE       CURRENT STATE           ERROR               PORTS
go4pev4p0v53        web1.1              nginx:latest  

docker-worker-01.centlinux.com Running Running 4 minutes ago
vevveyhefy4e web1.2 nginx:latest

docker-worker-02.centlinux.com Running Running 4 minutes ago
5xzt23en2ldy web1.3 nginx:latest

docker-manager-01.centlinux.com Running Running 4 minutes ago
96zo7cfq5bmx web1.4 nginx:latest

docker-worker-01.centlinux.com Running Running 4 minutes ago
m7fibotbacs5 web1.5 nginx:latest

docker-manager-01.centlinux.com Running Running 4 minutes ago 

Here, we have created a service using nginx image and publised the port 80 of web1 containers with port 80 of host machines. Therefore, we are also required to allow service port 80 in host machine firewall to access it through the network.

Execute following command on all nodes to allow http service in Linux firewall.

firewall-cmd --permanent --add-service=http
firewall-cmd --reload

Browse any Docker Swarm node and you will be routed to the default webpage of nginx web server.

curl http://docker-manager-01 | grep title
curl http://docker-worker-01 | grep title
curl http://docker-worker-02 | grep title

Output:

<title>Welcome to nginx!</title>
<title>Welcome to nginx!</title>
<title>Welcome to nginx!</title>

Our Docker service is successfully configured. Currently, we are using three node address to browse it. However, we can also configure a HTTP load balancer to create a common address to access services on any node.

Frequently Asked Questions (FAQs)

What is Docker Swarm?
Docker Swarm is a native clustering and orchestration tool for Docker, allowing you to manage multiple Docker hosts as a single virtual system.

Do I need multiple servers to set up Docker Swarm?
No, you can set up a basic Swarm on a single CentOS 7 machine for testing, but production environments require multiple nodes for high availability.

What are the prerequisites for installing Docker Swarm on CentOS 7?
You need a CentOS 7 system with Docker installed, proper network connectivity, and open firewall ports for Swarm communication.

How do I initialize a Docker Swarm?
After installing Docker, you initialize Swarm by running a command that designates the current node as a manager, which then generates a join token for additional nodes.

Can I add worker nodes later after initializing the Swarm?
Yes, you can easily add worker or manager nodes later using the join token provided during Swarm initialization.

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Final Thoughts

By installing Docker Swarm on CentOS 7, you have set up a powerful and scalable platform for orchestrating containerized applications. Docker Swarm’s native clustering capabilities allow you to manage a group of Docker nodes as a single virtual system, making deployment, scaling, and management much more efficient.

To maximize the reliability and security of your Swarm cluster, consider implementing proper node monitoring, backup strategies, and secure communication between nodes. With your Docker Swarm environment ready, you can now focus on deploying highly available and resilient containerized services across your infrastructure.

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Thank you for reading, and best of luck with your Docker Swarm Cluster setup on CentOS 7!