The question I get most often about containerization is: "What is a container?".
Let's make it simple, with my 2 favorite definitions:
1. From a logical point of view, it's a single and packaged unit of software.
2. From a technical point of view, it's an isolated process.
↓
Let's make it simple, with my 2 favorite definitions:
1. From a logical point of view, it's a single and packaged unit of software.
2. From a technical point of view, it's an isolated process.
↓
From a logical point of view, more understandable for a human, a container can be considered as a single unit of working software, with all its configurations and dependencies, packaged.
In this way, it can run quickly and reliably from one computer to another.
In this way, it can run quickly and reliably from one computer to another.
A container has its own dedicated filesystem, provided by an image.
The image contains:
-dependencies
-configuration
-binaries
-environment variables
-other important data related to the container to be instantiated.
Many containers can be run from a single image
The image contains:
-dependencies
-configuration
-binaries
-environment variables
-other important data related to the container to be instantiated.
Many containers can be run from a single image
To run a container, we need an image
Technically, a container is first created, then, started.
What often happens, is that when we create a container, we also want to start it.
To do this with a single command, we can use the 'docker run' command.
Technically, a container is first created, then, started.
What often happens, is that when we create a container, we also want to start it.
To do this with a single command, we can use the 'docker run' command.
If we use the 'docker run' command, specifying the image at the end of the command, Docker will look for this image on the current machine.
If the image is not on our filesystem, Docker looks for it in the public registry (Docker Hub), and if it finds it, it downloads the image.
If the image is not on our filesystem, Docker looks for it in the public registry (Docker Hub), and if it finds it, it downloads the image.
From a technical point of view, the container is just a process, that runs on the machine, isolated from the other processes.
To do this, Docker uses Linux features.
- Namespaces
- Cgroups
- Union FS (Filesystem)
To do this, Docker uses Linux features.
- Namespaces
- Cgroups
- Union FS (Filesystem)
NAMESPACES
The Docker Engine uses Namespaces, to provide a layer of isolation, such as on Linux.
Each aspect of a container runs on a separate namespace.
Its access is isolated and limited.
When you run a container, Docker creates Namespaces for that container.
The Docker Engine uses Namespaces, to provide a layer of isolation, such as on Linux.
Each aspect of a container runs on a separate namespace.
Its access is isolated and limited.
When you run a container, Docker creates Namespaces for that container.
The namespaces used by Docker for containers are:
PID: for the process ID
NET: to Manage Network interfaces
IPC: to manage access to interprocess
MNT: to manage filesystem mount points
UTS: to isolate kernel/version identifiers
PID: for the process ID
NET: to Manage Network interfaces
IPC: to manage access to interprocess
MNT: to manage filesystem mount points
UTS: to isolate kernel/version identifiers
CGROUPS
It's a Linux feature to limit the resources for a set of processes.
Docker relies on Cgroups to limit the usage of resources for containers.
It's used to limit and monitor system resources:
- CPU
- memory
- network bandwidth
- disk
It's a Linux feature to limit the resources for a set of processes.
Docker relies on Cgroups to limit the usage of resources for containers.
It's used to limit and monitor system resources:
- CPU
- memory
- network bandwidth
- disk
UNION FS
It's a filesystem service.
It implements a Union Mount for different file systems, It allows files/dirs of separate file systems, to be transparently overlaid, forming a single coherent one.
Docker engine uses Union FS to provide the building blocks for containers
It's a filesystem service.
It implements a Union Mount for different file systems, It allows files/dirs of separate file systems, to be transparently overlaid, forming a single coherent one.
Docker engine uses Union FS to provide the building blocks for containers
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