Difference between Docker registry and repository

Question

I'm confused as to the difference between docker registries and repositories. It seems like the Docker documentation uses the two words interchangeably. Also, repositories are sometimes referred to as images, such as this from their docs:

In order to push a repository to its registry, you need to have named an image or committed your container to a named image as we saw here.

Now you can push this repository to the registry designated by its name or tag.

How can you push a repository to a registry? Aren't you pushing the image to the repository?

Answer 1

From the book Using Docker, Developing and deploying Software with Containers

Registries, Repositories, Images, and Tags

There is a hierarchical system for storing images. The following terminology is used:

Registry

A service responsible for hosting and distributing images. The default registry is the Docker Hub.

Repository

A collection of related images (usually providing different versions of the same application or service).

Tag

An alphanumeric identifier attached to images within a repository (e.g., 14.04 or stable ).

So the command docker pull amouat/revealjs:latest will download the image tagged latest within the amouat/revealjs repository from the Docker Hub registry.

Answer 2

Complementing the information:

• You usually push a repository to a registry (and all images that are part of it). But you can push a single image to a registry. In all cases, you use docker push.
• An image has a 12-hex-digit Image ID, but is also identified by: namespace/repo-name:tag
• The image full name can be optionally prefixed by the registry host name and port: myregistryhost:5000/namespace/repo-name:tag
• A common naming convention is to use your registry user-name as what I called "namespace".

Answer 3

A docker repository is a cute combination of registry and image.

docker tag foo <registry>/<image>:<tag>

is the same as

docker tag foo <repository>:<tag>

Answer 4

Docker Registry is a service, which you can either host yourself (Trusted and Private) or you can let docker hub be the host for this service. Usually, if your software is commercial, you will have hosted this as a "Private and Trusted" registry. For Java Developers, this is somewhat analogous to Maven Artifactory setup.

Docker Repository is a set of "Tagged" images. An example is that you might have tagged 5 of ubuntu:latest images:

a) Nano editor (image1_tag:v1)

b) A specific software 1 (image1_tag:v2)

c) Sudo (image1_tag:v3)

d) apache http daemon (image1_tag:v4)

e) tomcat (image1_tag:v5)

You can use docker push command to push each of the above images to your repository. As long as the repository names match, they will be pushed successfully, and appear under your chosen repository and correctly tagged.

Now, your question is, "So where is this repository hosted/who is managing the service"? That is where Docker Registry comes into picture. By default you will get a docker hub registry (Open Source) which you can use to keep your private/public repository. So without any modification, your images will be pushed to your private repository in docker hub. An example output when you pushing your image tags are the following:

docker@my-docker-vm:/$ docker push mydockerhub/my-helloworld-repo:my_tag
The push refers to repository [docker.io/mydockerhub/my-helloworld-repo]
bf41e934d39d: Pushed
70d93396f87f: Pushed
6ec525dfd060: Pushed
705419d10b13: Pushed
a4aaef726d02: Pushed
04964fddc946: Pushed
latest: digest: sha256:eb93c92351bce785aa3ec0de489cfeeaafd55b7d90adf95ecea02629b376e577 size: 1571
docker@my-docker-vm:/$

And if you type immediately docker images --digests -a you can confirm that your pushed image tags are now showing new signature against the private repository managed by docker hub registry.

Answer 5

A Docker image registry is the place to store all your Docker images. The image registry allows you to push and pull the container images as needed.

Registries can be private or public. When the registry is public, the images are shared with the whole world whereas in the private registry the images are shared only amongst the members of an enterprise or a team.

A registry makes it possible for the Docker daemon to easily pull and run your Docker images.

Answer 6

Docker Hub and other third party repository hosting services are called “registries”. A registry stores a collection of repositories.

As a registry can have many repositories and a repository can have many different versions of the same image which are individually versioned with tags.

Answer 7

The confusion starts with this definition of a tag: "An alphanumeric identifier attached to images in a repository"

I'd rather call that alphanumeric identifier that you append with a ':' a tag-suffix for now. When somebody says "'latest' is the default tag", then this kind of tag-suffix is meant.

In reality, the :latest' suffix is technically part of the tag. The entire name is a tag. All these are tags (possibly referring to the same image):

myimagename myimagename:latest username/theirimagename:1.0 myrepo:5000/username/imagename:1.0

(I say imagename here, just to illustrate the other main source of confusion. That's the repositoryname, of course. Sorry.)

Examples:

a) When you want to name your image while building, you use docker build -t thisname ... -- that is -t for tag, (not -n for name).

b) When you want to push that image to a registry, you need to have the full URL (starting with registryname and ending with a tag-suffix) as a tag:

docker tag thisname mylocalregistry:5000/username/repoimagething:1.0

Now you push the image known as thisname by saying:

docker push mylocalregistry:5000/username/repoimagething:1.0

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Naming things is hard. Alas! A repository is not a "container" (aaargh...) where you put things in, that is what muggles think...