Overview of images
TOC
Understanding containers and images
Containers and images are important concepts to understand when you set out to create and manage containerized software. An image holds a set of software that is ready to run, while a container is a running instance of a container image. Those different versions are represented by different tags on the same image name.
Images
Containers in Alauda Container Platform are based on OCI- or 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 have access only to resources defined in the image unless granted additional access at creation time. By deploying the same image in multiple containers across multiple hosts and load balancing between them, Alauda Container Platform can provide redundancy and horizontal scaling for a service packaged into an image.
You can use the nerdctl or docker CLI directly to build images, but Alauda Container Platform also supplies builder images that assist with creating new images by adding your code or configuration to existing images.
Because applications develop over time, a single image name can actually refer to many different versions of the same image. Each different image is referred to uniquely by its hash, a long hexadecimal number such as fd44297e2ddb050ec4f…, which is usually shortened to 12 characters, such as fd44297e2ddb.
Image registry
An image registry is a content server that can store and serve container images. For example:
A registry contains a collection of one or more image repositories, which contain one or more tagged images. Alauda Container Platform can supply its own image registry for managing custom container images.
Image repository
An image repository is a collection of related container images and tags identifying them. For example, the Alauda Container Platform Jenkins images are in the repository:
Image tags
An image tag is a label applied to a container image in a repository that distinguishes a specific image from other images in an image stream. Typically, the tag represents a version number of some sort. For example, here :v3 .11.59-2 is the tag:
You can add additional tags to an image. For example, an image might be assigned the tags :v3 .11.59-2 and :latest .
Image IDs
An image ID is a SHA (Secure Hash Algorithm) code that can be used to pull an image. A SHA image ID cannot change. A specific SHA identifier always references the exact same container image content. For example:
Containers
The basic units of Alauda Container Platform 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 word container is defined as a specific running or paused instance of a container image.
Many application instances can be running in containers on a single host without visibility into each others' processes, files, network, and so on. Typically, each container provides a single service, often called a micro-service, such as a web server or a database, though containers can be used for arbitrary workloads.
The Linux kernel has been incorporating capabilities for container technologies for years. The Docker project developed a convenient management interface for Linux containers on a host. More recently, the Open Container Initiative has developed open standards for container formats and container runtimes. Alauda Container Platform and Kubernetes add the ability to orchestrate OCI- and Docker-formatted containers across multi-host installations.
Though you do not directly interact with container runtimes when using Alauda Container Platform, understanding their capabilities and terminology is important for understanding their role in Alauda Container Platform and how your applications function inside of containers.