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Today I Learned: Docker

docker-mind-map.png

Dockerfile

  • Series of command-line instructions to assemble the docker image

Docker Images

  • A template/model that setups the environment and dependencies of the application as an isolated package
  • It consists of a stack of layers; a layer is equivalent to a command-line instruction in Dockerfile
  • Layers can be compared to commits in git (personal observation)
  • Your machine downloads these layers in docker cache
  • Only one copy of commit hash is downloaded and stored in docker cache. This saves up space in our host machine

Docker tags

  • Image tag is like a pointer to a specific layer in an image
  • Tags are greatly used for versioning

Tips when Building a Dockerfile

Sequence of commands is important as each command represents a layer in docker cache; place on top of the Dockerfile the commands that change the least, and at the bottom of the file the commands that change the most

Combine related and multiple commands in one layer. We use the && to combine or chain one command after another. This saves time and space

Docker Containers

  • A running instance of an image
  • Containers are immutable and ephemeral
  • They don't change; thus if change is needed, dispose the current and just deploy a new one

Volumes

  • how about persistent data? We use Volumes and Bind Mounts
  • volumes can outlive a container, this is good for persistent data such as databases

Bind Mounting

  • File sharing between a host and a Docker container
  • Bind mount when you want a file or directory on the host machine to be mounted into a container
  • As observed, we mount the source code of the application so the changes are immediately visible

Docker Compose [WIP]

  • Configure relationships between containers
  • Configure multiple containers in one file