Introduction
Docker is a tool for packaging and shipping apps. Based on the idea of a shipping container, it provides a standardized way for developers or system administrators to create lightweight images, or collections of images, for each element of an application, and then easily and quickly deploy the image. Since the image is standardized, it can be uniformly deployed on development or production, leading to a much simpler workflow, faster development time for the dev team, and lower management overhead for the ops team.
First, a quick overview of a few things Docker is:
- An open source tool that places a layer on top of Linux containers (LXC) to make it simple to package and ship complex apps
- A tool for creating a layered filesystem; each layer is versioned and can be shared across running instances, making for much more lightweight deployments
- A company behind the project, as well as a site called the "Docker Hub" for sharing containers
And, a few things Docker isn't:
- A virtual machine. Unlike a true VM, a docker container does not also require a host OS, meaning it's much slimmer than a real VM
- An infrastructure automation tool, like Puppet or Chef. Like those other tools, Docker is a major player in the DevOps space, but its focus is around running apps in a container environment, as opposed to representing a machine state.
This guide introduces the key ideas you'll use again and again in Docker, such as images, layers, containers, commits, tags, and so forth. The main things to understand include:
- an image is a specific state of a filesystem
- an image is composed of layers representing changes in the filesystem at various points in time; layers are a bit like the commit history of a git repository
- a container is a running process that is started based on an image
- you can change the state of the filesystem on a container and commit it to create a new image
- changes in memory / state are not committed -- only changes on the filesystem
Docker uses the git-style command format:
$ docker [OPTIONS] COMMAND [arg...]The following table, taken from the "docker help" command, provides a quick summary of the commands.
| Command | Description |
|---|---|
| attach | Attach to a running container |
| build | Build an image from a Dockerfile |
| commit | Create a new image from a container's changes |
| cp | Copy files/folders from a container's filesystem to the host path |
| diff | Inspect changes on a container's filesystem |
| events | Get real time events from the server |
| export | Stream the contents of a container as a tar archive |
| history | Show the history of an image |
| images | List images |
| import | Create a new filesystem image from the contents of a tarball |
| info | Display system-wide information |
| inspect | Return low-level information on a container |
| kill | Kill a running container |
| load | Load an image from a tar archive |
| login | Register or log in to the Docker registry server |
| logs | Fetch the logs of a container |
| port | Lookup the public-facing port that is NAT-ed to PRIVATE_PORT |
| pause | Pause all processes within a container |
| ps | List containers |
| pull | Pull an image or a repository from a Docker registry server |
| push | Push an image or a repository to a Docker registry server |
| restart | Restart a running container |
| rm | Remove one or more containers |
| rmi | Remove one or more images |
| run | Run a command in a new container |
| save | Save an image to a tar archive |
| search | Search for an image on the Docker Hub |
| start | Start a stopped container |
| stop | Stop a running container |
| tag | Tag an image into a repository |
| top | Lookup the running processes of a container |
| unpause | Unpause a paused container |
| version | Show the Docker version information |
| wait | Block until a container stops, then print its exit code |
As you scan the list (which I've left in alphabetical order), you'll notice 3 key groups of commands:
- Commands related to managing images, such as images, build, save, rmi, and tag.
- Commands related to containers, such as run, ps, kill, restart, top, and pause.
- Commands related to the Docker hub, such as login, search, pull, and push. (More on this in the next chapter.)
This guide is organized along these same basic lines. First, we'll cover how to get Docker up and running on a Mac or Windows machine using a tool called boot2docker. (If you're using a Linux-based machine, you can skip this chapter.) Next, we'll talk about images and layers, and then talk about containers, which are the "running" instances of an image. Next, we'll talk about the Docker Hub, which is a place where you can post your images, as well as find images maintained by other Docker users.
Although the examples might be a bit contrived, the goal of the guide is to give you a quick introduction to the ideas you'll use again and again as you explore Docker in depth.