DevOps Classroomnotes 18/Jun/2022

How Containers Work

Docker Image Layers and Containers

  • Every container created has one R/W Layer added
  • All the changes made by container are in R/W layer
  • When we delete the container the R/W layer will be deleted where as the image (i.e. image layers stay in tact)
  • The data that is generated by the container (i.e. running application) will be lost if we delete the container.
  • Now lets assume we have created mysql in a container. All the db updates will be present in the Thin R/W layer and if we delete mysql contaier we loose all the changes i.e. we loose data
  • We need to have some way to preserve this data created by containers i.e. life time of the data should be independent of the container.
  • Refer Here for image layers & Refer Here for the concept of the impact of image layers
  • To preserve the data loss in the Thin R/W layer of a container docker has a concept of volumes

Docker Volumes

  • Lets create a nginx container
  • Create some files
  • Now Remove the container
  • The data created is lost
  • To persist the data created by container in the Docker Host Docker has volumes
  • For managing volumes docker has docker volume subcommand
  • Now lets create a volume
  • Now lets create nginx container and attach this volume. Add some data
  • Remove the container and inspect volume
  • Even after removing the container the data is not lost
  • Now lets create a new container by attaching the same volume
  • We can use this volume to attach multiple containers and share it across containers
  • Refer Here for the concept of docker volumes.

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