Managing application Data in Containers

• Every Container gets a read write layer extra on top of image layers
• Two concerns to be addressed are
  • How does docker container mount image layers + R/W layer as one file system ?
    • Storage Drivers
  • How to preserve the data which is present in R/W Layer
    • Docker Volumes

Storage Drivers

• Copy-on-write Strategy:
• We need a file system which understands image layers and R/W Layers and to the continer it should be file system much like any other linux/Windows file system
• Storage Driver understands read only image layers, Read/Write Thin layer of container and combines (union) the layers to give one filesystem to container
• Docker has the following storage drivers
  • overlay2: This is preferred storage driver for all linux distributions
  • aufs: This was preferred storage driver for Docker 18.06 and previous versions
  • devicemapper
  • btrfs and zfs
• Refer Here for article on storage drivers

Docker Volumes

• Refer Here for article on Docker Volumes
• By default all the files created by container are stored on thin R/w layer. This layer will be existing on a system as long as container exists. Once the container is deleted this data will be lost
• Docker has two options for storing the files on the host machine, so that the changes done in the container are persisted even after contianer is deleted.
  • Volumes
  • Bind Mount
• Create a Docker Image from the file without Volume instruction and with volume instruction Refer Here for changeset
• Now lets create a docker container from image without volume instruction and other with volume instruction
• Even after the container is terminated data will still exist on the docker host
• What if the VOLUME instruction is not used in Docker Image?
  • Refer Here
• If your organization is running some kind of third party storage like netapp, vsphere storage etc Refer Here to install volume drivers
• Refer Here for cloud related storage drivers

Host Network and Guest Network in Networking

Docker Networking

• Launch a virtual machine with plain os (ubuntu /centos).
• Execute the command ifconfig
• Now lets install docker and then execute ifconfig
• Now just run the following command

docker container run -d --name my-cont-1 tomcat:jdk8-openjdk
docker container inspect my-cont-1

• Now execute ifconfig
• What is happening to make containers accesible via host network?