How to Install and Configure Docker Swarm on CentOS 7 / Ubuntu 16.04 / Fedora 26/25
Adding Nodes to a Swarm Cluster
If you remember the output of “swarm init” command we ran on the manager, the output has the token to add nodes to the cluster. Just go back to that output and copy it, then paste that on the worker nodes.
Node 1:
Node 2:
On Docker manager, run the “docker node ls” command to list down the worker nodes.
[root@dockerm ~]# docker node ls
Output:
ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS azaocjsw83llg4n08sqx8rg05 node2.itzgeek.local Ready Active n6eal0ysgzhz3mug3tx3ssf3u node1.itzgeek.local Ready Active uhh38rpazd5tnzjph2g5rhgxy * dockerm.itzgeek.local Ready Active Leader
Troubleshooting Tip
Retrieve Token
If you had lost the token of any swarm, run the following command on the manager node to retrieve it.
[root@dockerm ~]# docker swarm join-token worker
Output:
To add a worker to this swarm, run the following command: docker swarm join \ --token SWMTKN-1-5y1u36a0osykywxeme2akpjp4jgx2l67mbqbc4fnazs39bp314-e6djx8ma2qnevolztpplkn26j \ 192.168.12.10:2377
Now you can use the above command to attach the nodes to swarm.
Certificate Error:
If you get a certificate error, ensure all the nodes are in time sync.
Error response from daemon: x509: certificate has expired or is not yet valid
Launching service in Docker Swarm mode
Now, the Docker Swarm is up and running. Let’s test a swarm by deploying service on it. Here, we will run a service called “web” with the image of “httpd“.
Run the following command on the Docker manager.
[root@dockerm ~]# docker service create --name web -p 80:80 httpd 4vpj4xihx7i3bim00f60zfbwc [root@dockerm ~]#
If you specify –replicas <unit>, docker swarm will create “unit” number of tasks (or containers) of the same image for the specified service across the Docker worker nodes.
To know which services are running on a cluster and how many replicas are there.
[root@dockerm ~]# docker service ls ID NAME MODE REPLICAS IMAGE 4vpj4xihx7i3 web replicated 1/1 httpd:latest [root@dockerm ~]#
To find on which node the service is running on by using the following command.
[root@dockerm ~]# docker service ps web ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS vz5wp563rok2 web.1 httpd:latest node2.itzgeek.local Running Running about a minute ago [root@dockerm ~]#
In our example, the service “web” is running on “node2.itzgeek.local“. Since we are running a web service, you can verify the service by pointing your browser to http://node2.itzgeek.local or http://192.168.12.30. You should get the following page.
Mesh Networking
Let’s see, what is mesh networking?. Try to point your browser to any another node. (Ex. http://node1.itzgeek.local or http://192.168.12.20) on your Docker Swarm.
Viola! You got the same page.
Scale up and Scale down service in the swarm
In Docker swarm, we can scale up or down a service by creating additional instances. As you know, currently we have only one container for service “web” running on Docker manager. To test the scalability, let’s scale the service “web” to “five” instance.
[root@dockerm ~]# docker service scale web=5 web scaled to 5 [root@dockerm ~]#
Check where your instances are started.
[root@dockerm ~]# docker service ps web ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS 0gu4lkbjq12x web.1 httpd:latest node1.itzgeek.local Running Running 5 hours ago 5jgd6308ntp1 web.2 httpd:latest node2.itzgeek.local Running Running 5 hours ago 3wpqwalt3urj web.3 httpd:latest dockerm.itzgeek.local Running Running 6 seconds ago lxn71f7766xt web.4 httpd:latest dockerm.itzgeek.local Running Running 5 seconds ago nfonpcu7ecol web.5 httpd:latest node1.itzgeek.local Running Running 5 hours ago
In some cases, we may likely to scale down.
[root@dockerm ~]# docker service scale web=3 web scaled to 3 [root@dockerm ~]#
Verify the instances by running following command. Only three containers are running right now.
[root@dockerm ~]# docker service ps web ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS 0gu4lkbjq12x web.1 httpd:latest node1.itzgeek.local Running Running 5 hours ago 5jgd6308ntp1 web.2 httpd:latest node2.itzgeek.local Running Running 5 hours ago 3wpqwalt3urj web.3 httpd:latest dockerm.itzgeek.local Running Running about a minute ago
You can also verify the “web” service by going to “http://192.168.12.10“, “http://192.168.12.20” and “http://192.168.12.30“. You should get the following page.
High Availability or Container Self-Healing
Container self-healing feature in the swarm mode provides high availability for your service. If anything goes wrong with a container or a worker node, the manager will make sure atleast 3 containers must be running for service “web“.
To test the self-healing, just stop the Docker service running on any one of the worker nodes. For Ex, I’m stopping the Docker service on “node2.itzgeek.local”
[root@node2 ~]# service docker stop Redirecting to /bin/systemctl stop docker.service [root@node2 ~]#
Now we can see that a new container is launched on “node1.itzgeek.local” to make the service “web” highly available.
[root@dockerm ~]# docker service ps web
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
0gu4lkbjq12x web.1 httpd:latest node1.itzgeek.local Running Running 5 hours ago
twr2u1k3jlg3 web.2 httpd:latest node1.itzgeek.local Running Running 5 hours ago
5jgd6308ntp1 \_ web.2 httpd:latest node2.itzgeek.local Shutdown Running 5 hours ago
3wpqwalt3urj web.3 httpd:latest dockerm.itzgeek.local Running Running 8 minutes ago
That’s All.
