Docker registry, repository, and index

The first component in Docker's distribution system is the registry. Docker utilizes a hierarchical system for storing images, shown in the following screenshot:

Images which you build can be stored in a remote registry for others to use. The Docker registry is a service (an application, in fact) that is storing your Docker images. The Docker Hub is an example of the publicly available registry; it's free and serves a huge, constantly growing collection of existing images. The repository, on the other hand, is a collection (namespace) of related images, usually providing different versions of the same application or service. It's a collection of different Docker images with the same name and different tags.

If your app is named hello-world-java and your username (or namespace) for the Registry is dockerJavaDeveloper then your image will be placed in the dockerJavaDeveloper/hello-world-java repository. You can tag an image and store multiple versions of that image with different IDs in a single named repository and access different tagged versions of an image with a special syntax such as username/image_name:tag. The Docker repository is quite similar to a Git repository. For example, Git, a Docker repository is identified by a URI and can either be public or private. The URI looks the same as the following:

{registryAddress}/{namespace}/{repositoryName}:{tag}  

The Docker Hub is the default registry and Docker will pull images from the Docker Hub if you do not specify a registry address. To search an image in the registry, execute the docker search command; for example:

$ docker search hello-java-world  

Without specifying the remote registry, Docker will conduct a search on the Docker Hub and output the list of images matching your search criteria:

The difference between the registry and repository can be confusing at the beginning, so let's describe what will happen if you execute the following command:

$ docker pull ubuntu:16.04  

The command downloads the image tagged 16.04 within the ubuntu repository from the Docker Hub registry. The official ubuntu repository doesn't use a username, so the namespace part is omitted in this example.

Although the Docker Hub is public, you get one private repository for free with your Docker Hub user account. Last, but not least, the component you should be aware of is an index. An index manages searching and tagging and also user accounts and permissions. In fact, the registry delegates authentication to the index. When executing remote commands, such as push or pull, the index first will look at the name of the image and then check to see if it has a corresponding repository. If so, the index verifies if you are allowed to access or modify the image. If you are, the operation is approved and the registry takes or sends the image.

Let's summarize what we have learned so far:

• The Dockerfile is the recipe to build an image. It's a text file containing ordered instructions. Each Dockerfile has a base image you build upon
• An image is a specific state of a filesystem: a read-only, frozen immutable snapshot of a live container
• An image is composed of layers representing changes in the filesystem at various points in time; layers are a bit same as the commit history of a Git repository. Docker uses the layers cache
• Containers are runtime instances of an image. They can be running or stopped. You can have multiple containers of the same image running
• You can make changes to the filesystem on a container and commit them to make them persisted. Commit always creates a new image
• Only the filesystem changes can be committed, memory changes will be lost
• A registry holds a collection of named repositories, which themselves are a collection of images tracked by their IDs. The registry is same as a Git repository: you can push and pull images

You should now have an understanding of the nature of images with their layers and containers. But Docker is not just a Dockerfile processor and the runtime engine. Let's look at what else is available.