A Terminology #
- Container
A container is a running instance based on a particular container image. Each container can be distinguished by a unique container ID.
- Control groups
Control groups, also called
cgroups, are a Linux kernel feature that allows aggregating or partitioning tasks (processes) and all their children into hierarchically-organized groups, to manage their resource limits.- Docker Open Source Engine
Docker Open Source Engine is a server-client type application that performs all tasks related to containers. Docker Open Source Engine comprises the following:
Daemon: The server side of Docker Open Source Engine, which manages all Docker objects (images, containers, network connections used by containers, etc.).
REST API: Applications can use this API to communicate directly with the daemon.
CLI Client: Enables you to communicate with the daemon. If the daemon is running on a different machine than the CLI client, the CLI client can communicate by using network sockets or the REST API provided by Docker Open Source Engine.
- Dockerfile
A Dockerfile provides instructions on how to build a container image. Docker Open Source Engine reads instructions in the Dockerfile and builds a new image according to the instructions.
- Image
An image is a read-only template used to create a container. A Docker image is made by a series of layers built one over the other. Each layer corresponds to a permanent change, for example, an update of an application. The changes are stored in a file called a Dockerfile. For more details, see the official Docker documentation.
- Container Image
A container image is an unchangeable, static file that includes executable code so it can run an isolated process on IT infrastructure. The image is comprised of system libraries, system tools, and other platform settings a program needs to run on a containerization platform. A container image is compiled from file system layers built on top of a parent or base image.
- Base Image
A base image is an image that does not have a parent image. In a Dockerfile, a base image is identified by the
FROM scratchdirective.- Parent Image
The image that served the basis for another container image. In other words, if an image is not a base image, it is derived from a parent image. In a Dockerfile, the
FROMdirective is pointing to the parent image. Most Docker containers are created using parent images.- Namespaces
Docker Open Source Engine uses Linux namespaces for its containers, which isolates resources reserved for particular containers.
- Orchestration
In a production environment, you typically need a cluster with many containers on each cluster node. The containers must cooperate and you need a framework that enables you to automatically manage the containers. The act of automatic container management is called container orchestration and is typically handled by Kubernetes.
- Registry
A registry is storage for already created images. It typically contains several repositories There are two types of registries:
public registry: Any (usually registered) user can download and use images. A typical example of a public registry is Docker Hub.
private registry: Access is restricted to particular users, or from a particular private network.
- Repository
A repository is storage for images in a registry.