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Containers

Running your first container

Run the nginx container by:

docker pull nginx
docker run nginx

When you run this command, the following happens (assuming you are using the default DockerHub registry configuration):

  1. Docker pulls the nginx image from DockerHub.
  2. Docker creates a new container from the nginx image. The nginx image is a ready-to-run container image that encapsulates the NGINX web server software, along with its dependencies and configuration. When the image is used to create a container, it provides a fully functional NGINX server environment, without the need to install and configure nginx on your machine.
  3. Docker allocates a dedicated read-write filesystem to the container (which is completely different and isolated from the host machine fs). This allows a running container to create or modify files and directories in its local filesystem.
  4. Docker creates a virtual network interface to connect the container to the network. This includes assigning an IP address to the container. By default, containers can connect to external networks using the host machine's network connection.
  5. Docker starts the container.
  6. When you type CTRL+c the container stops but is not removed. You can start it again or remove it. When a container is removed, its file system is deleted.

Container management and lifecycle

To see your running containers, type

docker container ls

or add -a flag to list also stopped containers:

$ docker container ls -a
CONTAINER ID   IMAGE     COMMAND                  CREATED              STATUS                      PORTS     NAMES
d841a2fe07f9   nginx     "/docker-entrypoint..."   About a minute ago   Exited (0) 14 seconds ago             funny_blackburn

In the above output:

  • d841a2fe07f9 is the container ID - a unique identifier assigned to each running container in Docker.
  • /docker-entrypoint... is the (beginning) of the actual linux command that has run to initiate the process of the container.
  • funny_blackburn is a random alphabetical name that docker assigned to the container.

Try it yourself

Pull and run the container hello-world.

  1. What is the status of the container after some moments of running?
  2. Use docker images hello-world to get some information about the image from which the container has run. What is the image size?
  3. What is the command used to launch the container hello-world?

Override the default command

When the nginx image was built (we will build our own images soon), Docker allows us to specify a default command that defines what is executed when a container is started from the image. However, you can override the default execution command by providing a new command as arguments when running the docker run command.

To override the default command, simply append the desired command to the end of the docker run command.

Let's say you want to run the same above nginx container, but you want to modify the default command so nginx is running in debug mode. You can override the default command by:

docker run nginx nginx-debug -g 'daemon off;'

In the above example, the nginx container will be initiated using the command nginx-debug -g 'daemon off;'.

Here is another very useful example:

docker pull ubuntu
docker run -it ubuntu /bin/bash

The command starts a new Docker container using the Ubuntu image and launches an interactive terminal session within the container.

Here's what each part of the command does:

  1. docker run instructs Docker to create and start a new container.
  2. -it is a combination of two flags: -i keeps STDIN open for the container, and -t allocates a pseudo-TTY to allow interaction with the container's terminal.
  3. ubuntu refers to the Ubuntu image, which is pulled from Docker Hub if not already available locally.
  4. /bin/bash specifies the command to be executed within the container, in this case, launching a Bash shell.

When you run this command, a new container based on the Ubuntu image is created, and you are provided with an interactive, fresh and beloved bash terminal session inside the container. This allows you to directly interact with the Ubuntu environment, run commands, and perform operations within the isolated containerized environment.

Feel free to go wild, you are within a container :-)

✏️ Playing with the running container

In your open Ubuntu container terminal session:

  1. What is the current user?
  2. What is the hostname?
  3. Is the container connected to the internet? Can you ping google.com? Oh, don't have the ping command? Install it inside the container!
  4. What is the user's home directory?
  5. How many processes are running in the container? What could that indicate?
  6. Do you have docker installed in the container?

Interacting with containers

In addition to starting a new container with an interactive terminal session using docker run -it, you can also interact with running containers using the docker exec command.

The docker exec command allows you to execute a command inside a running container. Here's the basic syntax:

docker exec [OPTIONS] CONTAINER COMMAND [ARG...]

Let's see it in action...

Start a new nginx container and keep it running. Give it a meaningful name instead the one Docker generates:

docker run --name my-nginx nginx

Make sure the container is up and running. Since the running container occupying your current terminal session, open up another terminal session and execute:

$ docker container ls
CONTAINER ID   IMAGE     COMMAND                  CREATED              STATUS              PORTS     NAMES
89cf04f27c04   nginx     "/docker-entrypoint.…"   About a minute ago   Up About a minute   80/tcp    my-nginx

Now say we want to debug the running nginx container, and perform some maintenance tasks, or executing specific commands within the containerized environment, we can achieve it by:

docker exec -it my-nginx /bin/bash

And you're in... You can execute any command you want within the running my-nginx container.

Tip: if you don't know the container name, you can exec a command also using the container id:

docker exec -it 89cf04f27c04 /bin/bash

✏️ Playing with the Nginx container

How many running processes does the container run? Hint: you can use the docker top command. The first process is the nginx master process, and the rest are workers that should serve incoming requests to the webserver.

You are told that the nginx configuration file is located under /etc/nginx/nginx.conf.

Install nano in the container, and edit the nginx.conf as follows:

- worker_processes  auto;
+ worker_processes  1;

Save the file. Stop and start the container again, waa the number of workers changed?

Inspecting a container

The docker inspect command is used to retrieve detailed information about Docker objects such as containers, images, networks, and volumes. It provides a comprehensive JSON representation of the specified object, including its configuration, network settings, mounted volumes, and more.

The basic syntax for the docker inspect command is:

docker inspect [OPTIONS] OBJECT

Where OBJECT represents the name or ID of the Docker object you want to inspect.

Inspect your running container by:

$ docker inspect my-nginx
....

Running containers in the background

When running containers with Docker, you have the option to run them in the background, also known as detached mode. This allows containers to run independently of your current terminal session, freeing up your terminal for other tasks.

To run a container in the background, you can use the -d or --detach flag with the docker run command.

Let's run another nginx container:

$ docker run -d --name my-nginx-2 nginx
310f1c48e402648ce4db41817dd76027d4528e481b25e985296fccc83421ddcb

When a container is running in the background, Docker assigns a unique container ID and displays it as output. You can use this ID to reference and manage the container later.

To view the list of running containers, you can use the docker ps command. This command lists all the running containers along with their respective container IDs, names, and other information.

Since my-nginx-2 is running in the background, the docker logs command can help you to view the logs generated by a running Docker container. It allows you to retrieve and display the standard output (stdout) and standard error (stderr) logs generated by the container's processes.

$ docker logs my-nginx-2
/docker-entrypoint.sh: /docker-entrypoint.d/ is not empty, will attempt to perform configuration
/docker-entrypoint.sh: Looking for shell scripts in /docker-entrypoint.d/
....

If you want a real-time view, add the -f (--follow) flag.

$ docker logs my-nginx-2 -f
...

Killing, stopping and removing containers

The docker container stop command is used to stop one or more running containers in Docker. It gracefully stops the containers by sending a SIGTERM signal to the main process running inside each container and then waits for a specified timeout (default is 10 seconds) before forcefully terminating them with a SIGKILL signal if needed.

docker container stop my-nginx-2

The docker container kill command is used to forcefully terminate one or more running containers in Docker. It immediately sends a SIGKILL signal to the main process running inside each container, causing them to stop abruptly without any graceful shutdown.

Stopped or killed containers can be restarted using the docker container start command, which resumes their execution from the point where they were stopped or killed.

Restarting a stopped container

When you restart a stopped container using the docker container start command, it resumes execution from the point where it was previously stopped. The container retains its configuration and any changes made inside the container's file system prior to stopping.

Restarting a killed container (you'll barely use it...)

When you restart a killed container using the docker container start command, it starts fresh from the beginning, as if it were a newly created container. It's essentially starting a new instance of the container based on the original image and configuration. The container does not retain any previous state or changes made inside the container before it was killed.

It's worth noting that the state of a container (stopped or killed) does not affect the Docker images associated with it. Docker images remain unchanged and can be used to create and start new containers as needed.

The docker container rm command is used to remove one or more stopped or killed containers from Docker. It allows you to delete containers that are no longer needed, freeing up disk space and cleaning up resources. It's important to note that the containers you wish to remove must be in a stopped state. If you attempt to remove a running container, you will encounter an error.

docker container stop my-nginx my-nginx-2
docker container rm my-nginx my-nginx-2
docker contianer ls -a

Tip: The --rm flag in the docker run command is used to automatically remove the container when it exits or stops running. It can be handy when you don't want to retain the container after it has served its purpose.

docker pull busybox
docker run --rm busybox wget google.com

Start containers automatically

In Docker, the container restart policy determines the actions to be taken by the Docker daemon when a container exits or encounters an error. The restart policy can be specified during container creation using the --restart flag with the docker run command.

Flag Description
no Do not automatically restart the container. (the default)
on-failure[:max-retries] Restart the container if it exits due to an error, which manifests as a non-zero exit code. Optionally, limit the number of times the Docker daemon attempts to restart the container using the :max-retries option.
always Always restart the container if it stops. If it is manually stopped, it is restarted only when Docker daemon restarts or the container itself is manually restarted. (See the second bullet listed in restart policy details)
unless-stopped Similar to always, except that when the container is stopped (manually or otherwise), it is not restarted even after Docker daemon restarts.

The following example starts a nginx container and configures it to always restart unless it is explicitly stopped or Docker is restarted.

$ docker run -d --restart unless-stopped nginx

Published ports

By default, when you run a container using the docker run command, the container doesn't expose any of its ports to the outside world. To make a port available to services outside of Docker, or to Docker containers running on a different network, use the --publish or -p flag. This creates a firewall rule in the container, mapping a container port to a port on the host machine to the outside world.

Here are some examples:

Flag value Description
-p 8080:80 Map TCP port 80 in the container to port 8080 on the host machine.
-p 192.168.1.100:8080:80 Map TCP port 80 in the container to port 8080 on the host machine for connections to host IP 192.168.1.100.
-p 8080:80/udp Map UDP port 80 in the container to port 8080 on the host machine.
-p 8080:80/tcp -p 8080:80/udp Map TCP port 80 in the container to TCP port 8080 on the host machine, and map UDP port 80 in the container to UDP port 8080 on the host machine.

To run an nginx container with port 80 exposed (the default port in which nginx is listening port incoming requests), you can use the docker run command with the -p or --publish option. Here's the command:

docker run --name nginx3 -p 8080:80 nginx

-p 8080:80 maps port 80 in the container to port 8080 in the host machine.

You can then access the nginx web server by opening a web browser and navigating to http://localhost:8080.

More docker networking topics will be covered in next chapters.

Explore the running nginx container logs, can you see a log indicating your request you've just performed from the web browser? Try to run the container without the -p flag and check that the nginx container is not accessible.

Set environment variables for containers

When running a container using the docker run command, you can specify environment variables using the -e or --env flag. For example:

docker run -d -e MY_VAR=my_value --name nginx4 nginx

Container configuration files

Under the /var/lib/docker/containers directory, you will find subdirectories corresponding to each container running on your host machine. Each container has its own unique directory, named after the container ID. Inside each container's directory, you can find the following files and directories:

  1. config.v2.json: This file contains the configuration settings for the container, including details like the image used, environment variables, container networking, and more.
  2. hostconfig.json: This file stores the configuration settings related to the container's host-specific configuration, such as port bindings, volume mounts, resource limits, and more.
  3. hostname: This file contains the hostname assigned to the container.
  4. resolv.conf: This file represents the container's DNS configuration (will be discussed in further modules).
  5. mounts/: This directory contains information about the mounted volumes associated with the container (will be discussed in further modules).
  6. networks/: This directory stores information about the container's network interfaces, such as IP addresses, network settings, and more (will be discussed in further modules).
  7. *.log: Depending on your container's logging configuration, you may find log files in this directory. By default, Docker uses the JSON-file logging driver, and log files are stored in /var/lib/docker/containers/<container_id>/<container_id>.log.

Self-check questions

Enter the interactive self-check page

Exercises

✏️ Communication between containers and the internet

Run two ubuntu containers named ubuntu1 and ubuntu2. Use the -it flags to make an interactive interaction with the running containers using a tty terminal.

Your goal is to be able to successfully ping the ubuntu1 container from ubuntu2, i.e. to verify communication between the two containers. Install ping if needed, inspect the containers to discover their IP addresses.

✏️ Availability test service

In this exercise, you will run a bash script which continuously monitor the availability of a pre-determined set of servers using the ping command. It sends the ping results to an InfluxDB database and displays results in Grafana dashboard.

InfluxDB is a time-series database designed to handle high volumes of timestamped data. The database is listening on port 8086, which should be published to the host machine.

Grafana is an open-source platform for data visualization and monitoring The server is listening on port 3000, which should be published to the host machine as well.

No need to install the InfluxDB or Grafana on your system, as they will be run as containers in pre-built images. Before you run the availability test script, let's run the InfluxDB container and the Grafana container.

  1. Run the InfluxDB container first. You should run a specific image version: influxdb:1.8.10. In the docker run command, define the INFLUXDB_ADMIN_USER and INFLUXDB_ADMIN_PASSWORD with a custom admin user and password of your choice. Furthermore, define INFLUXDB_HTTP_AUTH_ENABLED to be equal to true.

  2. In order to write data to InfluxDB, we need to create a database first. In another new terminal session, create a database named hosts_metrics in which the test data will be stored. To do so, perform the following POST HTTP request:

curl -X POST 'http://localhost:8086/query' -u <your-username>:<your-password> --data-urlencode "q=CREATE DATABASE hosts_metrics"

Upon successful creation, you'll get the following output: {"results":[{"statement_id":0}]}.

  1. Next, run the Grafana container: docker run -d --name=grafana -p 3000:3000 grafana/grafana. After initialization, visit your server in http://localhost:3000.

    1. Default username and password is admin.
    2. Now we want to integrate InfluxDB as a data source in Grafana. On the left menu, under Connections button, click Data sources.
    3. In the opened configuration console, choose Add data source, then choose InfluxDB.
    4. In the data source setting page, under HTTP section URL field, enter influxDB url: http://<influc-db-container-ip>:8086.
    5. In Auth section, turn on the Basic auth toggle, and enter the InfluxDB username and password in the appropriate fields below.
    6. In InfluxDB Details section, under Database enter your db name: hosts_metrics.
    7. Finally, click Save & Test, and make sure you get the Data source is working message.

  2. Now you are ready to run the availability test script. The code is available under availability_agent/agent.sh in our shared repo. In the agent.sh file, change the variables according to your username and password:

    DB_USERNAME="<your-username>"
DB_PASSWORD="<your-password>"

  3. Run the script by:

    cd availability_agent
bash agent.sh

The running script should start to monitor the hosts specified in the hosts file, and write the results into InfluxDB.

  1. Let's visualize the results in Grafana:
  2. In the grafana server, click the Explore button. In the exploration panel, build a graph of the test results over time. Your graph should look similar to the bellow screenshot.
    availabilityMonitor