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Introduction to Docker

#docker

What is a container?

What is it?

This is a package that contains code (could be written in many languages), a .env file. If this is a package, it is portable among developers

A docker Image is a image based on Linux

Where could i find it?

Docker repositories. Here you will be able to find many containers, also there are private repos. From package managers to complete operative systems.

What is a Image?

This is a packages that contains dependencies and code that will be shared

What is a container?

A container is a bunch of layers, each layer represents an image. Generally the first one is a Linux image, an Alpine Linux image

Why Alpine is the default Linux Image

This image is lighter than other Linux images

Does Docker virtualize?

Docker is a kind of virtualization. But What is the difference with a VM?

A complete system is conformed by Hardware -> Kernel -> Software or Applications.

Docker Container Virtual Machines (VM)
This virtualizes only applications, and docker uses the kernel from the real machine's operative system This virtualizes the applications and the kernel. Kernel is the reason why virtual machines are heavier than a container

Commands

Common Commands

These are some commands used commonly in docker development. We will see all of them in this chapter

Note

It is important to note when you download a image for first time, probably it takes some time to be downloaded, but when you already have one image (for example, node:latest) and you just download the same but in different version/tag, it will take less time... but why?

It is simple, you already have the layers needed to make useful that images, in the first time that you pull an image, it downloaded all needed layers.

Create a container based on pulled images. Mongo DB example

Understanding the creation of containers

These are the steps to follow for creating a MongoDb container:

  1. Pull an image of db from docker hub
  2. docker create mongo (for example) this will return an id
  3. run docker start <id returned> by the previous step
  4. docker ps returns a table with the next data:
    1. CONTAINER ID: A shorter form of the original ID, but it works same
    2. IMAGE: The image that container is using (database, OS, node, php, etc)
    3. COMMAND: Command used by the container to be executed by its own self
    4. CREATED: How long was created
    5. STATUS: If it is up or down and how much time it has been in that status
    6. PORTS: Port that is used by the container
    7. NAMES: Container name
  5. docker stop <id>: Stops the container

To see all containers (inclusive all stopped): docker ps -a

Now. Why Can not I use this container that I created?, it is because the port is not mapping yet. but, What does this mean?

Port Mapping

Concept

This practice consists in make a reference of a port inside of the container using real port of the host. For example:

PortMappingConcept1.png

Also this could be applied for Databases:

PortMappingConcept2.png

Commands to put in practice

Docker create & Docker logs

  1. docker create -p27017:27017 --name mongodb mongo: -p parameter is the meaning of publish, but because of put this in practical terms, we will think this means port (Port Mapping), this parameter will be followed by the port reference (the real port of our machine) and then the port inside of the container
  2. docker start mongodb
  3. docker ps

Here will see something like this:

CONTAINER ID   IMAGE     COMMAND                  CREATED         STATUS         PORTS                                             NAMES
594dfb346b4c   mongo     "docker-entrypoint.s…"   5 minutes ago   Up 2 seconds   0.0.0.0:27017->27017/tcp, [::]:27017->27017/tcp   mongodb

In the section of PORTS you will see that port 27017 is doing a reference of 27017 within container mongodb:

0.0.0.0:27017->27017/tcp
Note

We can let docker choose the port mapping for us, but this is not recommended in most of the cases, except for little development environments. This could be done setting only the port inside of the container for whatever service that we ran with docker (for this example is mongo and its port 27017, for other services we recommend you check what port corresponds for each service).

docker create -p27017 --name mongodb mongo

Now, we can use the port to connect us with the Database. To see if our docker container has been executed successfully we can see the logs:

docker logs mongodb

But instantly it will exit us from the logs, to make that this command keeps us listening the logs, we will use --follow or -f parameter

Docker run

Also, we have short all of these steps with one command: docker run. This command is in charge of three key points:

  1. Pulling an unexistent image from docker hub (id it does not exist)
  2. Create the container
  3. Run the created container
docker run mongodb

This will start correctly and will show us the logs, but when we press ctrl+c, it will stop the logs, but also the container. To avoid this, we will execute the command with the -d parameter (from the meaning of detach from the logs).

docker run -d mongodb

We can set all parameters from docker create command for docker run:

docker run -p27017:27017 --name mongodb -d mongo
References

Some references used for this page are the next: