Step-by-Step Guide to Migrate Docker Containers

Docker makes it simple to deploy container images that contain a fully functional operating system and make use of the host kernel. We have already written an article covering the installation and use of Docker containers. In this post, we will go through how to migrate Docker containers from one server to another server.

Docker containers are relatively easy to set up and manage. We can find hundreds of containers on docker hub and create a full-fledged virtual operating system with just a few commands from the console.

In the last post in the Docker category, we learned how to host several websites or applications on a single server by using docker containers. The following commands will allow you to move your Docker containers to a new server. But first, let’s take a quick look at how we’re going to go about accomplishing this.

Typically, we deploy a docker image by first downloading it from a source like Docker Hub. The command docker create automatically downloads and deploys the specified container image.

After successfully deploying an image and publishing our application. During the production time, the application creates data, and we also install packages that modify the base docker image. When migrating docker containers, we must save the operating container as an image, transmit it to the new server, then load the docker image as a brand new container.

Another method of migrating docker containers is to export and import docker containers. Exporting a docker container using docker export command is a little different from saving a docker image using docker save command. docker export command creates a snapshot of a running container whereas docker save commands creates an image that can be used to create container. We can use both the commands to migrate our containers to a new server.

Things to remember before migrating docker container

When we use docker export command, and import the resulting tar file to a new server, it will not transfer the ports and variables from the old system. So you will need to manually open ports and create environment variables. Secondly, docker export and docker save commands do not backup container volumes.

Transfer volumes separately before migrate docker container

With both the commands, we need to transfer container volumes separately to the new server. Do not worry, we can do that easily.

How to migrate docker containers to a new server

In this article, we will use docker save command to migrate an apache server running inside a docker container. As I mentioned above, docker save works with docker images. First we need to create image of the running container. It is preferred to stop the container before starting the process. If your server is in production and generating lots of data, you can put it on maintenance mode to avoid any data loss during the image creation.

To create image, we will use docker commit command.

sudo docker commit container_id image_name

Replace container_id with the actual id of the migrating container. Find the container id using docker ls command.

Replace image_name with any name that you want. For example, I am migrating an apache container, so I have decided to name the container image to be apache-image.

sandy@LinuxAndMint:~$ sudo docker ps
CONTAINER ID   IMAGE                                          COMMAND              CREATED              STATUS              PORTS                                                                                            NAMES
434ccd9f15a4   httpd:latest                                   "httpd-foreground"   10 seconds ago       Up 9 seconds        0.0.0.0:49153->80/tcp, :::49153->80/tcp                                                          apache

sudo docker commit 434ccd9f15a4 apache-image

sandy@LinuxAndMint:~$ sudo docker commit 434ccd9f15a4 apache-image
sha256:22d7b415fe967b3ff67bfde21846a8dcb267d161370699e5c42784547966f73f

Now that the docker commit command has created our container’s image, we can save it in tar file using docker save command.

sudo docker save image_name > image_name.tar

Repace image_name with the name of the image created using docker commit command. Replace image_name.tar with any name you want to give to the saved image archive. For example, in my case, I am migrating an apache server running in the docker, I have decided to name the archive as apache_image.tar.

sudo docker save apache-image > apache-image.tar

sandy@LinuxAndMint:~/images$ sudo docker save apache-image > apache-image.tar
sandy@LinuxAndMint:~/images$ ls
apache-image.tar

Once the image has been created, we can transfer this image to the new server using ftp or scp.

Transferring the image using scp

scp apache-image.tar username@192.168.1.29

For more information on transferring data from local to remote server, follow this article.

Load the image on remote server

Once the image has been transferred to the remote server, ssh into the server and use the docker load command to load the transferred image.

sudo docker load < image-name.tar

Replace the image-name with the transferred image. Make sure you’re SSH into the directory where the was image was stored, or provide the full path to the image.

Now use docker image command to make sure the image is available. Now we are ready to deploy this image using the usual docker run command. Remember to include ports, variables within docker run command.

docker run -d --name container_name -p 80:80 image-name

The above docker run deploy the snapshot of our docker container. Now we are almost ready to proceed further to make DNS changes to point domain name to the new server. One last thing we need to make sure is that our container does not depend on the volume. If yes, we need to manually migrate volumes from old server to the new server.

Docker persistence data is stored inside /var/lib/docker. We need to transfer /var/lib/docker or the specific container directory (volume) to the new server. When it’s transferred successfully to the new server, we can use docker run command and attach the volume to container on creation.

docker run --rm --name container_name -p 80:80 -v /usr/local/bin:/target image-name

-v parameter allows to attach the existing directory (Volume) to the container. If your old container does not require volume, then you can simply create container from the image without -v parameter.