The basic units of Azure Red Hat OpenShift applications are called containers. Linux container technologies are lightweight mechanisms for isolating running processes so that they are limited to interacting with only their designated resources.


The Linux kernel has been incorporating capabilities for container technologies for years. More recently the Docker project has developed a convenient management interface for Linux containers on a host. Azure Red Hat OpenShift and Kubernetes add the ability to orchestrate Docker-formatted containers across multi-host installations.

Though you do not directly interact with the Docker CLI or service when using Azure Red Hat OpenShift, understanding their capabilities and terminology is important for understanding their role in Azure Red Hat OpenShift and how your applications function inside of containers. The docker RPM is available as part of RHEL 7, as well as CentOS and Fedora, so you can experiment with it separately from Azure Red Hat OpenShift. Refer to the article Get Started with Docker Formatted Container Images on Red Hat Systems for a guided introduction.


Containers in Azure Red Hat OpenShift are based on Docker-formatted container images. An image is a binary that includes all of the requirements for running a single container, as well as metadata describing its needs and capabilities.

You can think of it as a packaging technology. Containers only have access to resources defined in the image unless you give the container additional access when creating it. By deploying the same image in multiple containers across multiple hosts and load balancing between them, Azure Red Hat OpenShift can provide redundancy and horizontal scaling for a service packaged into an image.

You can use the Docker CLI directly to build images, but Azure Red Hat OpenShift also supplies builder images that assist with creating new images by adding your code or configuration to existing images.


Image Version Tag Policy


Do not use the latest tag for any official Azure Red Hat OpenShift images. These are images that start with openshift3/. latest can refer to a number of versions, such as 3.10, or 3.11.

How you tag the images dictates the updating policy. The more specific you are, the less frequently the image will be updated. Use the following to determine your chosen Azure Red Hat OpenShift images policy:


The vX.Y tag points to X.Y.Z-<number>. For example, if the registry-console image is updated to v3.11, it points to the newest 3.11.Z-<number> tag, such as 3.11.1-8.


Similar to the vX.Y example above, the X.Y.Z tag points to the latest X.Y.Z-<number>. For example, 3.11.1 would point to 3.11.1-8


The tag is unique and does not change. When using this tag, the image does not update if an image is updated. For example, the 3.11.1-8 will always point to 3.11.1-8, even if an image is updated.

Container Image Registries

A container image registry is a service for storing and retrieving Docker-formatted container images. A registry contains a collection of one or more image repositories. Each image repository contains one or more tagged images. Docker provides its own registry, the Docker Hub, and you can also use private or third-party registries. Red Hat provides a registry at for subscribers. Azure Red Hat OpenShift can also supply its own internal registry for managing custom container images.


Docker Diagram