Data Persistence in Docker for Stateful Applications

• Kindly refer the following articles if you are not aware of layer
  • Image Layers
  • Impact of Layers on Container
• By default, all the files created/modified by the containers reside on the writable layer of the container.
• The problem with the writable layer of the container is "Writable layer will be deleted once the container is deleted"
• To persist the data into the Docker Host (Machine on which Docker is installed) Docker provides the following options
  • Volumes:
    • Stored in the hostfile system managed by Docker (/var/lib/docker/volumes/ on Linux).
    • Non Docker Processes should not modify the file system
  • Bind Mounts:
    • Can be stored any where on the host system.
    • Non-Docker Processes can modify
  • tmpfs Mounts:
    • Store on the host systems memory, never written to file system

Docker Volumes

• Docker Volumes are preferred mechanisms for persisting data generated by and used by Docker Containers.
• Advantages
  • Easier to backup or migrate that bind mounts
  • Can be managed by Docker CLI or the API
  • Work in both Linux & Windows Containers
  • Volume Drivers lets to store volumes on remote hosts or cloud providers, encryption can be added.
  • Volumes donot increase the size of the docker containers writable layer as the volumes’s contents exist outside of container lifecycle.

Creating and Managing Docker Volumes

• Two Options Exist for the Creation of Docker Vlume
• -v:
  • Consists of three fields separated by :.
  • fields order should be correnct
  • syntax: __-v <name of volume>:<path to mount in container>:<options>
• Lets Experiment with volumes using -v and docker volume command
• Create a docker volume called as my-tools. Execute the following commands on docker host

docker volume create my-tools

docker volume ls 
##Output##
DRIVER              VOLUME NAME
local               my-tools

docker volume inspect my-tools
##output##
[
    {
        "CreatedAt": "2019-10-03T13:21:12Z",
        "Driver": "local",
        "Labels": {},
        "Mountpoint": "/var/lib/docker/volumes/my-tools/_data",
        "Name": "my-tools",
        "Options": {},
        "Scope": "local"
    }
]

• Mount the docker volume to alpine container using -v command

docker container run --name cont1 -d -v my-tools:/tools alpine sleep 1d

• Execute inspect command & navigate to mount on container cont1 & execute other commands as shown below

docker container inspect cont1

##Output##
"Mounts": [
            {
                "Type": "volume",
                "Name": "my-tools",
                "Source": "/var/lib/docker/volumes/my-tools/_data",
                "Destination": "/tools",
                "Driver": "local",
                "Mode": "z",
                "RW": true,
                "Propagation": ""
            }
        ]

docker container exec cont1 touch /tools/1.txt
docker container exec cont1 touch /tools/2.txt
docker container exec cont1 touch /tools/3.txt
docker container exec cont1 ls /tools
##Output##
1.txt
2.txt
3.txt

• Lets mount the same volume on different container, in different path and see the contents of volume

docker container run --name cont2 -d -v my-tools:/mytools tomcat:8

docker container exec cont2 ls /mytools
##Output##
1.txt
2.txt
3.txt

• –mount:
  • Initially was used only for docker services, now can be used for standalone containers as well
  • Syntax: –mount ‘type=<bind/volume/tmpfs>,source/src=<nameofvolume>,destination/dst/target=<mountpath in container>,
  • Refer Here for complete syntax
• Lets use the same my-tools volume using –mount
• Mount the docker volume to alpine container using –mount command

docker container run --name cont3 -d --mount 'type=volume,src=my-tools,dst=/tools' alpine sleep 1d

• Execute the rest of commands as mentioned above

Docker Volume Drivers

• Docker volume driver can be specified while creating a volume or when to start the container
• Refer Here for Docker Volume Drivers
• Refer Here for azure persistent volumes
• Refer Here for aws persistent data volume

References

• Refer Here for Bind Mounts
• Refer Here for tmpfs mounts

Other Way of Creating Docker Volume

• In the Dockerfile use the VOLUME instruction.