How to Configure Docker Swarm Cluster on CentOS 7

Configure Docker Swarm Cluster on CentOS 7

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 are configuring a Docker Swarm cluster on CentOS 7 based 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.

Before move on any further, we clarify that this post follows a more practical approach to demostrate how things can be done, without diving into the theoritical nitty gritty. Therefore, we recommend you should read Docker Deep Dive (PAID LINK) for some basic to advance level understanding of the technology.

Configure Docker Swarm Cluster on CentOS 7

Table of Contents:


System Specification:

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

Hostname: docker-manager-01 docker-worker-01 docker-worker-02
IP Address:
CPU: 3.4 Ghz (1 Core) 3.4 Ghz (1 Core) 3.4 Ghz (1 Core)
Memory: 512 MB 512 MB 512 MB
Storage: 40 GB 40 GB 40 GB
Operating System: CentOS 7.6 CentOS 7.6 CentOS 7.6
Docker Version: Docker CE 18.09 Docker CE 18.09 Docker CE 18.09


Installing Docker Engine CE 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
> docker-manager-01
> docker-worker-01
> docker-worker-02

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=
Loaded plugins: fastestmirror
adding repo from:
grabbing file 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
Loaded plugins: fastestmirror
=========================== repo: docker-ce-nightly ============================
async = True
bandwidth = 0
base_persistdir = /var/lib/yum/repos/x86_64/7
baseurl =
cache = 0
cachedir = /var/cache/yum/x86_64/7/docker-ce-nightly
check_config_file_age = True
compare_providers_priority = 80
cost = 1000
deltarpm_metadata_percentage = 100
deltarpm_percentage =
enabled = 1
enablegroups = True
exclude =
failovermethod = priority
ftp_disable_epsv = False
gpgcadir = /var/lib/yum/repos/x86_64/7/docker-ce-nightly/gpgcadir
gpgcakey =
gpgcheck = True
gpgdir = /var/lib/yum/repos/x86_64/7/docker-ce-nightly/gpgdir
gpgkey =
hdrdir = /var/cache/yum/x86_64/7/docker-ce-nightly/headers
http_caching = all
includepkgs =
ip_resolve =
keepalive = True
keepcache = False
mddownloadpolicy = sqlite
mdpolicy = group:small
mediaid =
metadata_expire = 21600
metadata_expire_filter = read-only:present
metalink =
minrate = 0
mirrorlist =
mirrorlist_expire = 86400
name = Docker CE Nightly - x86_64
old_base_cache_dir =
password =
persistdir = /var/lib/yum/repos/x86_64/7/docker-ce-nightly
pkgdir = /var/cache/yum/x86_64/7/docker-ce-nightly/packages
proxy = False
proxy_dict =
proxy_password =
proxy_username =
repo_gpgcheck = False
retries = 10
skip_if_unavailable = False
ssl_check_cert_permissions = True
sslcacert =
sslclientcert =
sslclientkey =
sslverify = True
throttle = 0
timeout = 30.0
ui_id = docker-ce-nightly/x86_64
ui_repoid_vars = releasever,
username =

Build yum cache before using EPEL and Docker yum repositories.

# yum makecache fast
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
epel/x86_64/metalink                                     | 9.1 kB     00:00
 * base:
 * epel:
 * extras:
 * updates:
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
Created symlink from /etc/systemd/system/ to /usr/lib/systemd/system/docker.service.
# systemctl start docker.service

Docker required following service ports to function.

Port Protocol Description
2376, 2377 TCP used for Docker daemon encrypted communication
7946 TCP, UDP used for container network discovery
4789 UDP used for container ingress network

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
 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
  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.


Configuring Docker Swarm Cluster 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
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

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
This node joined a swarm as a worker.

Repeat the same step on docker-worker-02.

# docker swarm join --token SWMTKN-1-1yn39o5d0aeiuvdiufp45rwbdbg5gxhrvbp3v38s5q6kcjh0q0-3m3vysmghac17vt3iz89mse9u
This node joined a swarm as a worker.

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

# docker info
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
 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:
 SubnetSize: 24
  Task History Retention Limit: 5
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 10
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
  Force Rotate: 0
 Autolock Managers: false
 Root Rotation In Progress: false
 Node Address:
 Manager Addresses:
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: bb71b10fd8f58240ca47fbb579b9d1028eea7c84
runc version: 2b18fe1d885ee5083ef9f0838fee39b62d653e30
init version: fec3683
Security Options:
  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
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Experimental: false
Insecure Registries:
Live Restore Enabled: false
Product License: Community Engine

To check the status of nodes in Docker Swarm cluster.

# docker node ls
ID                            HOSTNAME                        STATUS              AVAILABILITY        MANAGER STATUS      ENGINE VERSION
3b9wynaya1wu910nf01m5jeeq *   Ready               Active              Leader              18.09.3
ydgqdyoksx2mb0snhe1hwvco7    Ready               Active                                  18.09.3
vz02oe9e82deh8utiymnfobpk    Ready               Active                                  18.09.3

Our Docker Swarm cluster is configured successfully.


Creating 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
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
ID                  NAME                IMAGE               NODE                            DESIRED STATE       CURRENT STATE           ERROR               PORTS
go4pev4p0v53        web1.1              nginx:latest Running Running 4 minutes ago vevveyhefy4e web1.2 nginx:latest Running Running 4 minutes ago 5xzt23en2ldy web1.3 nginx:latest Running Running 4 minutes ago 96zo7cfq5bmx web1.4 nginx:latest Running Running 4 minutes ago m7fibotbacs5 web1.5 nginx:latest 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
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   612  100   612    0     0   100k      0 --:--:-- --:--:-- --:--:--  119k
<title>Welcome to nginx!</title>
# curl http://docker-worker-01 | grep title
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   612  100   612    0     0  61937      0 --:--:-- --:--:-- --:--:-- 68000
<title>Welcome to nginx!</title>
# curl http://docker-worker-02 | grep title
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   612  100   612    0     0  75958      0 --:--:-- --:--:-- --:--:-- 87428
<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.

Docker Swarm cluster on CentOS 7 has been configured.


  1. Thank you this tutorial makes a lot clear.
    is it possible to cluster the manager. (ex. 2 managers and 3 workers)

    1. Hi,
      Thanks for liking this article.
      Yes, we can configure more than one managers in Docker Swarm.

  2. Can you please show us how to update the nginx default index.html on the docker manager and push to the 2 docker nodes? Thank you.

    1. Hi, you should create a directory with custom index.html file. While running a container you can mount that directory at /var/www. nginx will server your custom index.html.

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