CentOS: Docker – Quick Install and Admin Guide, example PHP site using Swarm

by Kliment Andreev
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In this blog post, I’ll describe how to install Docker, Docker Swarm, play with some common commands and create a simple PHP site running on Apache and nginx as a load balancer. I’ll use two servers, one manager and one worker. The shared storage for the Apache web servers will be on a third server running as a NFS server. For the first part, I’ll work on the manager server only and when we create our first service in the Swarm, we’ll use the worker server. The installation for a manager and a worker is the same.


Each OS has a different way of installing Docker, so it’s better to check the official documentation before doing the install. In my case I’ll use CentOS 7. I am logged as the root user, but you can do this as a regular user if using sudo as prefix. I’ll also create a user called docker and add it to the docker group, which Docker creates it when installed.

yum install -y yum-utils device-mapper-persistent-data lvm2
yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo
yum -y install docker-ce
systemctl enable docker
systemctl start docker
useradd docker -g docker

Change the password for the docker user and log in as docker. In order to see if everything works OK, list the containers. You shouldn’t get any, but you’ll see something like this.

docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

Docker runs containers based on images, so let’s pull an image.

docker pull ubuntu:latest

You can list the images using:

docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
ubuntu              latest              93fd78260bd1        2 weeks ago         86.2MB

Let’s run a container based on that image. We’ll run the container in an interactive mode (-it).

docker container run -it ubuntu:latest /bin/bash
root@250a109cc857:/# hostname

As you can see, when we typed hostname, we got the hostname from the container, not the underlying host. You can exit from the interactive shell, by typing CTRL-P then CTRL-Q.
We can list the running containers, using:

docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED              STATUS              PORTS               NAMES
250a109cc857        ubuntu:latest       "/bin/bash"         About a minute ago   Up About a minute                       nifty_mahavira

As you can see, the running container has a random name, in my case nifty_mahavira. When you interact with the containers, you can use their names or their IDs. If you want, you can name your container too.

docker container run --name myfirstcontainer -it ubuntu:latest /bin/bash

Now, type exit this time to exit out of the container. Once out, type docker ps -a. This command will show both running and stopped containers (-a = all).

docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                     PORTS               NAMES
7947da7460e5        ubuntu:latest       "/bin/bash"         7 minutes ago       Exited (0) 7 minutes ago                       myfirstcontainer
250a109cc857        ubuntu:latest       "/bin/bash"         2 hours ago         Up 2 hours                                     nifty_mahavira

As you can see, there are two containers. One of them is running, the other one is stopped. When we type exit, the container stops. If we type CTRL-P, CTRL-Q, the container runs in the background. Some containers (e.g. nginx, Apache) can be started with -d switch, which makes them run in a daemon mode, meaning they run in the background.
Let’s connect to the runnning container again. As you can see, we are back at the initial prompt.

docker attach nifty_mahavira

Do CTRL-P, CTRL-Q again.
We can see the logs for the container.

docker logs nifty_mahavira
root@250a109cc857:/# hostname

If we make a change into the container and then stop it, our changes will be lost. So, we can create an image of a running container with our changes.
Attach to the first container again and type touch myfile.txt. This command will create an empty file in the container file system.

docker attach nifty_mahavira
root@250a109cc857:/# touch myfile.txt

Exit with CTRL-P, CTRL-Q and create the image.

docker commit -m "Created an empty file" -a "kliment@andreev.it" nifty_mahavira klimenta/myfirstimage

If you list the images now, you’ll see our first image there.

docker images
REPOSITORY              TAG                 IMAGE ID            CREATED             SIZE
klimenta/myfirstimage   latest              0e00e0694e59        53 seconds ago      86.2MB
ubuntu                  latest              93fd78260bd1        2 weeks ago         86.2MB

If we create a container from our image, we can see that the file is there.

docker container run -it klimenta/myfirstimage /bin/bash
root@bd33bb413ada:/# ls -l myfile.txt
-rw-r--r-- 1 root root 0 Dec  5 19:36 myfile.txt

You can exit the container now, exit or CTRL-PQ. Doesn’t matter.
You can get more info about the image, if you type:

docker inspect klimenta/myfirstimage

Let’s say that you don’t need the image anymore. You can delete an image with:

docker rmi klimenta/myfirstimage
Error response from daemon: conflict: unable to remove repository reference "klimenta/myfirstimage" (must force) - container 2223cba9c2da is using its referenced image 0e00e0694e59

This error means that we have a container based on that image and the container’s ID is 2223cba9c2da.
You can find that container by typing:

docker ps -a | grep 2223cba9c2da
2223cba9c2da        klimenta/myfirstimage   "/bin/bash"         4 minutes ago       Exited (2) 4 minutes ago                        condescending_goldstine

First, remove the container.

docker rm 2223cba9c2da

Then, delete the image.

docker rmi klimenta/myfirstimage
Untagged: klimenta/myfirstimage:latest
Deleted: sha256:0e00e0694e596a5c85aa57d9a89797ac6872626207b315df62c0a0142704e57d
Deleted: sha256:a216cafc1ec085642f38b89c4bf6cf63fec65bc9097fe141f69d5f81797f9892

If you want to remove all containers, do:

docker rm `docker ps -a -q`

Make sure that all containers are stopped. If you want to remove all running container regardless of their running state, add the –force suffix to the command above.
We mentioned how to create our own image by using commit command. Another way of doing it is by using the Dockerfile. There is a lot to read about the Dockerfile. Here I’ll present a small Dockerfile to build an image based on ubuntu. So, create a file called Dockerfile and add these lines.

#This is a custom image
FROM ubuntu:latest
LABEL maintainer "kliment@andreev.it"
RUN apt-get update
RUN apt-get install -y openssh-server

Once this file is saved, create the image. Don’t forget the dot at the end.

docker build -t klimenta/mysecondimage:v1 .

It will take some time, but you’ll have the image ready. You can verify with docker images.

Docker Swarm

Docker Swarm is a cluster manager and an orchestrator. We’ll install it on the manager node and on the worker node. First, we have to initialize the swarm.

docker swarm init
Swarm initialized: current node (qgtrimmoxuhdk639kgqiuevsy) is now a manager.

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

    docker swarm join --token SWMTKN-1-5qna1cjc7p8xeom7u1xdzr5kjzkdgrt87wilsd8c93b4py75vy-03ll63wt9c8nk0vvs7ckgrerv

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

If we want to see the nodes in the cluster, we can do:

docker node ls
ID                            HOSTNAME            STATUS              AVAILABILITY        MANAGER STATUS      ENGINE VERSION
qgtrimmoxuhdk639kgqiuevsy *   manager             Ready               Active              Leader              18.09.0

To join a node as a worker, we’ll have to execute the command that was given above. In case, you cleared the screen, you can always get the worker token back by doing:

docker swarm join-token worker
To add a worker to this swarm, run the following command:

    docker swarm join --token SWMTKN-1-5qna1cjc7p8xeom7u1xdzr5kjzkdgrt87wilsd8c93b4py75vy-03ll63wt9c8nk0vvs7ckgrerv

So, log in on the worker node and execute the command from the above. You should get confirmation that This node joined a swarm as a worker. If you try to see the nodes from the worker, you’ll get an error saying “Error response from daemon: This node is not a swarm manager. Worker nodes can’t be used to view or modify cluster state. Please run this command on a manager node or promote the current node to a manager.” So, you’ll have to do docker node ls from the manager, or on the worker you can type:

docker info | grep Swarm
Swarm: active
docker info | grep Manager
 Is Manager: false
 Manager Addresses:

and at least see that the swarm is active and that this node is not a manager.
If you want to remove the node from the swarm, you can use the command:

docker node rm <nodeID>

If you get an error that the worker is still up and running, you can add the –force suffix. Do this only when the worker node is not available, let’s say there is a network issue preventing it to login to it, or the node crashed and it’s not coming back.
Preferably, you would like to leave the swarm first. So, on the worker node, do:

docker swarm leave
Node left the swarm.

And then remove the node from the manager with docker node rm .

docker node rm 5g96otbl7lbbffc2l29s5g6lr

Let’s go with our service example now, but before doing that add the worker node back to the swarm. We’ll create a new service that runs on all nodes (including the manager, which is not recommended). But, first thing first. Let’s create a shared volume for the Apache servers. Follow this link to create the server. On the manager and the worker, install the NFS package so Docker can mount an NFS share.

yum -y install nfs-utils

On the NFS server, create a file where you created the NFS share. In my case I have a directory /nfs.

echo '<?php echo gethostname(), "\n";?>' > /nfs/index.php

Define the variable that we’ll pass to the Docker command when creating the service. Replace with your values and the IP of the NFS server.

export NFS_VOL_NAME=nfs NFS_LOCAL_MNT=/var/www/html NFS_SERVER= NFS_SHARE=/nfs NFS_OPTS=vers=4,soft

Now, we can create the service from an image from Docker Hub that comes with Apache and PHP.

docker service create -p 8080:80 --mount "src=$NFS_VOL_NAME,dst=$NFS_LOCAL_MNT,volume-opt=device=:$NFS_SHARE,\"volume-opt=o=addr=$NFS_SERVER,$NFS_OPTS\",type=volume,volume-driver=local,volume-opt=type=nfs" -d --name webserver --replicas 3 php:7.2-apache

So, I named my service webserver and I have 3 replicas. I am exposing port 80 on the containers as port 8080 on the Docker host. The reason is because we’ll have our nginx listening on port 80 and acting as a load balancer.
You can list the services.

docker service ls
ID                  NAME                MODE                REPLICAS            IMAGE               PORTS
id7362udkbj0        webserver           replicated          3/3                 php:7.2-apache      *:8080->80/tcp

You can see the details for this particular service.

docker service ps webserver --no-trunc
ID                          NAME                IMAGE                                                                                    NODE                DESIRED STATE       CURRENT STATE                ERROR               PORTS
xjhzzm8e9ai3h33qdq5rnhsvm   webserver.1         php:7.2-apache@sha256:e3488a95726dd01a29056348b18e3461bc89ea7512742cb2c05ca9cb5f445c24   worker              Running             Running about a minute ago
09apro4je3xnsn1zlmfvw83xg   webserver.2         php:7.2-apache@sha256:e3488a95726dd01a29056348b18e3461bc89ea7512742cb2c05ca9cb5f445c24   manager             Running             Running about a minute ago
43uzdyo23vrl5p1057wrcibpw   webserver.3         php:7.2-apache@sha256:e3488a95726dd01a29056348b18e3461bc89ea7512742cb2c05ca9cb5f445c24   manager             Running             Running about a minute ago

You can check the individual containers on each node.

docker ps -a

You can attach to a single container. Replace with your ID.

docker ps -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS               NAMES
1833bab6a41e        php:7.2-apache      "docker-php-entrypoi…"   3 minutes ago       Up 3 minutes        80/tcp              webserver.3.43uzdyo23vrl5p1057wrcibpw
7d4a8fd7cad7        php:7.2-apache      "docker-php-entrypoi…"   3 minutes ago       Up 3 minutes        80/tcp              webserver.2.09apro4je3xnsn1zlmfvw83xg
docker exec -it 1833bab6a41e /bin/bash
root@1833bab6a41e:/var/www/html# cat index.php
<?php echo gethostname(), "\n";?>
root@1833bab6a41e:/var/www/html# exit

If you open up a browser on a machine outside of the swarm, you can check our first webserver. It will display the hostname of the Apache server that serves the content. Docker Swarm also acts as a load balancer but it’s only a TCP load balancer, not an HTTP LB.

If you go on the NFS server and create a small script loop.sh, you can see how the swarm load balances the load.

cat loop.sh
while :
        sleep 1

Now we need an nginx service to balance the load. In order to do that, we’ll create our own image with a specific configuration file.
Create a new Dockerfile and add the following:

# A custom config nginx load balancer
FROM nginx:latest
LABEL maintainer "kliment@andreev.it"
COPY nginx.conf /etc/nginx/nginx.conf

In the same directory where you create the Dockerfile, create a file called nginx.conf and add the following lines.

events { worker_connections 1024; }

http {

 upstream localhost {
 server {
    listen 80;
    server_name localhost;
    location / {
       proxy_pass http://localhost;

Where and are the manager’s and the worker’s IPs. Now, you can create the custom image.

docker built -t lbtemplate .

And provision a service from that image template.

docker service create --name loadbalancer -d -p 80:80 lbtemplate:latest

If you check the services with docker service ls and see that all replicas are up, you can access the load balancer with and you’ll get a response from one of the web servers.

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