How Containers Work

• Refer Here
• Cgroups
  
  

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.