.. _docker:

Gentile introduction to Docker Container
========================================

   Docker container images is a lightweight, standalone, executable
   package of software that includes everything needed to run an
   application: code, runtime, system tools, system libraries, and
   settings.

Docker containers are built from ``images``. These images are
line-by-line instructions on how to build a container. Please take a
look at the example from: `here <https://github.com/Swarmies/sap-web/blob/develop/Dockerfile>`__ read and understand the
comment of each line on the ``Dockerfile`` to understand how the docker
container is being built.

The ``docker-compose.yml`` file is used to make it easier to put the
container together rather than using lower-level ``docker`` commends.

After having the docker container instruction in the compose file. You
can run:

.. code:: bash

   docker-compose build

You should see it build the container like the one below 

.. figure:: https://github.com/Swarmies/sap-web/blob/develop/Doc/img/dockerImg/docker-container-building.PNG?raw=true
   :alt: docker building

to build the containers.

After the container is built the image is also created locally with the
image id and all the necessary attributes to start the container. To see
this you can use the lower level ``docker ps`` command or the
``docker-compose ps`` command.

NOTE: you will not see anything since there is no container running at
this time. To bring the container up you can run the command below.

.. code:: bash

   docker-compose up

You should see a similar output to the one below.

.. figure:: https://github.com/Swarmies/sap-web/blob/develop/Doc/img/dockerImg/start-up.PNG?raw=true
   :alt: docker building

   docker building

NOW if you run the command

.. code:: bash

   docker-compose ps

You will see all the container that are running and their status. Like
this:

.. figure:: https://github.com/Swarmies/sap-web/blob/develop/Doc/img/dockerImg/docker-compose-ps.PNG?raw=true
   :alt: docker ps

   docker ps

From the image above we can see that the two containers (``db`` and
``svc``) are running perfectly. While the ``ui`` container is trying
``Restarting``.

If your containers are ``healthy`` you should see them.

.. figure:: https://github.com/Swarmies/sap-web/blob/develop/Doc/img/dockerImg/properly_running.PNG?raw=true
   :alt: docker containers running properly

   docker containers running properly

Container Resource
------------------

While containers are isolated from your host machine. There are
resources that they use and share with the host machine. For instance,
container share volume (memory), ports (network) etc.

There are different network configurations in docker-compose files.
Which defines the interaction among containers and also with the host
machine. The process of specifying this is called mapping.

This is a vast topic in itself. For network configuration the two widely
used are `host network <https://docs.docker.com/network/host/>`__ and
`bridge network <https://docs.docker.com/network/bridge/>`__. For
volume, we have shared volume and mount.
`Here <https://docs.docker.com/storage/volumes/>`__ is more information.

There are different commands to manage this resource both on the host
side and in the container.