# Docker
Commercial product.
* Evolution of the concept of Solaris Containers and Linux Containers (LXC).
* Released with an OSS license at 2013.
Main concept: Container.
* Build with an application base (disconnected from the infrastructure).
* Composed of multiple functionalities (*namespaces*) that cooperate with each other.
* 1 Container - 1 Application (that can however make *spawn/fork*).
Containers contain all that is necessary to run an application, regardless of the hardware.
* Configurations, dependencies, auxiliary data, etc.
## Concepts
* **Image:** the data of the container (application, libraries, images, etc).
* **Container:** The instance of a **running application**.
* Composed of one or more *images*, each image adds a functionality.
* **Engine:** Software that executes the *containers*.
* **Registry:** Repository of Docker images.
* **Control Plane:** Infrastructure responsible for managing images and containers.
## A container can run on VMs
## Architecture
## Docker Registry
A central repository that stores and delivers images.
* Indexed by ***name*** and ***tag***.
* Can be private or public (Docker Hub).
Clients can push images to the registry.
* Local client.
Docker Engine creates a pull of the images and executes the container.
* Runs on the server.
Layers are *hashed* and indexed which allows for re-utilization.
* An image pull only needs to download the layers that do not exist locally.
## Workflow
Look for an image.
```bash
$ docker search nginx
```
Image pull.
```bash
$ docker pull nginx
```
List local images.
```bash
$ docker image ls
```
Run a container.
```bash
$ docker run -d -name frontend-server nginx
```
## Dockerfile
A sequence of commands that prepare the container for execution.
* List of dependencies.
* List of commands to execute inside the container.
* Set of commands to execute to initiate the container.
Used locally or distributed in a registry.
Can define versions of the same software.
```docker
FROM debian:stretch-slim
LABEL maintainer="NGINX Docker Maintainers "
ENV NGINX_VERSION 1.15.9-1~stretch
ENV NJS_VERSION 1.15.9.0.2.8-1~stretch
RUN set -x \ && apt-get update \ && apt-get install --no-install-
recommends --no-install- suggests -y gnupg1 apt-transport-https ca-
certificates \ && \
...
RUN ln -sf /dev/stdout /var/log/nginx/access.log \ && ln -sf
/dev/stderr /var/log/nginx/error.log
EXPOSE 80
STOPSIGNAL SIGTERM
CMD ["nginx", "-g", "daemon off;"]
```
## Images
*Containers* have their initial data in images.
* Data, application, libraries.
* Have an *"entrypoint"* that is executed when the container is initiated.
Composed of multiple layers.
* Base image.
* Various images, alter the content (with differences).
* Use *filesystems* by layer (overlayfs, aufs, btrfs, ZFS).
*Read-only* or non-persistent.
* Act as a base from where to initialize the *container*.
* Multiple *containers* can share the same image.
### Layers
It is important to keep images small.
* Do not base images in complete distributions, prefer distros specially designed for *containerization* (ex. Alpine).
Available in the host.
* */var/lib/Docker/overlay2/\*
* */merged*: filesystem view from inside the *container*.
* */diff*: differences to the base.
