What is a container registry?

A key advantage of container registries is how they easily connect to container orchestration systems or platforms such as Kubernetes and Docker.

During container-based application development, teams can also use container registries in a DevOps capacity, enabling optimized integration with continuous integration (CI) workflows and continuous delivery (CD) workflows. Both workflows are core activities within software development. CI and CD are both automation-based and built to accommodate frequent code changes. They differ primarily in that CI prepares code for eventual release, while CD concerns the actual release of code. Container registries support both workflows.

There are two types of container registries:

• Public registries feature ways to optimize data transfer and relaxed access control but lack advanced security protocols.
• Private repositories are typically used in situations requiring safer operation and more rigorous protection of workloads.

A container registry stores container images, allowing users to upload images to the registry (a process called "pushing") or download them to a different system (a process called "pulling").

A container registry can be used alone or in conjunction with other container registries. If multiple containers are combined into a single operational unit for the provision of cloud services, its functionality mimics an ecosystem whose inhabitants work in close conjunction to deliver a shared output.

Container registries use object storage to manifest metadata about container images. Although this provides a means for successfully transferring such metadata, it is, by nature, limited. For example, in the case of data that might involve several images, there are limits to the number of available listing tags possible.

Dependency containers are another storage method. They enable the management, registration and resolution of dependencies in an application. Within the context of programming, dependencies describe situations where one object or process must occur before another object can function as ordered.

Such dependencies are managed by dependency containers, which help simplify testing, enhance scalability and minimize the need for code coupling (an overly strong interdependence between objects being programmed), which can lead to the introduction of errors and version-control issues.

Given the pressing and ongoing need for online security, the authentication of individuals granted access to repositories is vitally important. That depends upon a series of permissions specific to that container registry. Permissions define who are authorized to use computer resources from that container registry.

The concept of runtimes is vital. Container runtimes are the software that lets containers function within a host system. Runtimes use a series of steps to create containers. These steps encompass the entire process of forming containers and initializing their environment per the guidance of a container image that holds the app and its dependencies.

After the containers are formed, the runtime remains actively involved—administering container lifecycles, tracking their ongoing viability and removing their resources when they are no longer needed.

Container registries also support load balancing. In container-native load balancing, balancer traffic is directly routed to the areas intended to receive that traffic. In this way, container registries work hand in hand with load balancers, eliminating the need for traffic to be routed through an extra network loop.

Container registries provide numerous benefits, including the following:

The following activities make extensive use of container registries:

Software development is the primary use case for container registries, which is why they were created in the first place—to assist software developers by giving them a workspace to warehouse, access and share images.

Private container registries are typically seen as a necessary infrastructure for organizations that are designing and delivering software apps internally. Private registries give users a higher level of control over software supply chains.

Many container registries assist microservices users by clarifying the process of locating and connecting to specific microservices in a container cluster. Such registries offer Domain Name System services that give images names and virtual IPs.

Container registries help foster rapid iteration of apps because of the registries’ modular and lightweight nature. Other benefits to deployment are registries’ portability and consistency.

Container registries are being customized for specialized scientific purposes, such as supporting genetics data and empowering genomic research. Research results can easily be reproduced in other formats, allowing key code and artifacts to be shared.

There are two main providers of container orchestration that dominate container registry use:

Docker is a container orchestration system developed and introduced in 2013 to help reduce the complexity of creating, deploying and running apps. Docker has won popularity most recently for being seen as an easy way to build lightweight, portable and self-contained containers that can run on any platform despite the infrastructure. That reputation for ease of use has helped Docker become a go-to option for those deploying applications devoted to artificial intelligence (AI) and machine learning (ML).

By the same token, Docker containers offer supreme portability, letting them be moved from one machine to another, regardless of its infrastructure. Again, we see a benefit in that, with Docker, it’s relatively simple to introduce AI and ML apps in alternative environments, like local machines, edge devices or cloud servers.

Kubernetes is an open source platform for container orchestration. Also started in 2013 and named after a Greek term for “pilot,” Kubernetes is used to automate various software processes such as management, deployment and scaling.

In a Kubernetes service, one or more computers (which can either be virtual machines or bare metal servers) are linked in a Kubernetes cluster, where container workloads of varying sizes and types can be run. Kubernetes is often deployed with Helm charts, which are file collections that work to describe a Kubernetes cluster’s various resources and then package those combined resources as an app.

Kubernetes enables the use of containerized ML apps and simple scaling, and it supports ML frameworks and tools. Kubernetes can also be used as a platform for AI training workloads.

Docker and Kubernetes are similar in many respects. However, they differ in the scale each possesses because Docker is, at its core, a container runtime, while Kubernetes is a full platform designed to accommodate the running and administering of containers from multiple container runtimes. As an indicator of the difference between the two, consider that Kubernetes is large enough that Docker is just one of the multiple container runtimes that Kubernetes supports.

The market for container registry providers is always evolving, with players both entering and exiting the market at any given time. Nonetheless, the providers listed here have built lasting reputations in this market:

• Alibaba: Alibaba Group embraces the goal of “making it easier to do business anywhere.” Alibaba’s Cloud Container Registry enables the replication of images between instances (operating entities) that belong to the same account. Alibaba Cloud Container Registry is designed to manage images throughout their respective lifecycles.

• Amazon: Amazon Web Services (AWS) operates Amazon Elastic Container Registry (ECR), incorporating the use of Amazon Inspector, which manages the vulnerability scanning of images. ECR also enables cross-account and cross-regional replication, which allows easier access to images.

• DockerHub: Docker runs its own image resource known as DockerHub, which offers free access to 100,000+ images shared by Docker users, software vendors and open source projects. A paid Docker container registry service is also available for private use. DockerHub uses a Docker command-line interface (Docker CLI) that enables key operations like login, push and pull.

• GitHub: Now owned by Microsoft, GitHub was started in 2007. The platform has won wide support from developers, who appreciate its ability to foster code-sharing, collaboration and code management. (It should be noted that while often mistaken for open source software, GitHub is indeed proprietary software.)

• GitLab: Often confused with GitHub, GitLab was founded in 2014. The GitLab Container Registry is a secure private registry for container images. It can be used to both produce and publish images. The Gitlab Container Registry works with open source software. The latest version (now in Beta) features enhanced sorting and filtering capabilities.

• Google: Google Cloud’s container registry “Artifact Registry” is integrated with Google Cloud’s tooling and runtimes and supports the use of automated pipelines. Google was also the originator of Kubernetes, with the project now being managed through the efforts of contributors and the trademark being held by the Cloud Native Computing Foundation. Public cloud providers now each offer their own version of Kubernetes as a managed service.

• IBM: IBM Cloud® Container Registry enables users to easily start their own registry and begin pushing private images to the registry for use with IBM Cloud Kubernetes Service. Users benefit from the privacy of having their own, fully managed registry. The service, which can be test-driven without cost, features pay-as-you-go pricing, so users only pay for what they consume.

• Microsoft: Microsoft’s Azure platform includes Azure Container Registry (ACR), which contains Docker images and Open Container Initiative (OCI) images and supports OCI artifacts and Helm charts. ACR’s connected registry feature (part of the program’s premium service tier) creates an on-premises or remote replica that synchronizes container images and OCI artifacts with the cloud-based ACR. ACRs can accommodate both Windows and Linux images. One way to interact with computer programs is by writing and inputting lines of text, which are called command lines. When an operating system uses this method of interaction, it is said to be incorporating a command-line interface (CLI). Although most computer users now rely on graphical user interfaces (GUIs), CLI use is still favored by some, especially by those who wish to automate programs by writing scripts. Azure provides users with the flexibility to use the popular Docker CLI to run key container image operations like login, push and pull.

• Red Hat: Red Hat has promoted Linux-based programming since its inception in 1993. Its container registry, OpenShift Container Platform (OCP), lets users automatically gain access to image repositories on demand and gives them a standard location to push the images that result from their application builds.