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A developer's guide to containers

A developer’s guide to containers

In recent years, containers have become an integral part of software development. A good grasp of containers and what they do is essential for most…

Lee Brandt Oct 27

In recent years, containers have become an integral part of software development. A good grasp of containers and what they do is essential for most developers. If containers still make you feel like, “I kinda know what containers are, but I’m still pretty fuzzy on what the big deal is,” this is the post for you.

What are containers?

A container is a runtime environment for applications. Clear as mud? Cool. 

Seriously, a container is a virtual environment that carries with it an operating system, utilities (like Tomcat, Apache, Node.js, Ruby, etc), and even your application’s code! It’s packaged all up and runs in an environment specifically designed to run the container. Docker is possibly the most popular version of a container platform, but there are others (like Podman and Rkt).

You create a container by telling the container runtime to build one based on an image. An image is just a container frozen in time. It is used to create new containers. For those coming from an object-oriented background, it might help to think of it like classes and objects: classes are blueprints for creating objects. 

Aren’t containers just lightweight virtual machines?

The most common comment I hear from developers is: “Aren’t containers just lightweight VMs?” It’s easy to “visualize” containers this way, but there are some distinct differences that have led to the popularity of containers.

Containers are more portable. Images (used to create containers) are meant to be moved around. They are meant to be uploaded to image registries and downloaded to create containers. While there are some really big container images (more than 10Gb!) most images tend to be around 300-600Mb. The size of your application is also a factor. Remember that your code is also part of the image.

Containers are less resource intensive. I don’t know about you, but I get decision anxiety when I create new VMs. I worry I won’t give it enough RAM, disk space, or processors, and the application will crash. It also means that whatever RAM I give it, those resources are no longer usable by the host machine.  With containers, this isn’t an issue. Containers run a bit like applications in that they use what resources they need. This means when nobody is using your application, your container releases almost all the resources it has been using back to the host machine. Just be aware this also means if there is a spike in traffic, the container will take as many resources as it needs. So you will need to put a cap on how much of the host machine’s resources the container can use, to keep a big spike from crashing the host machine.

How do I run a container?

Running a container will depend on which container runtime you choose. Docker is arguably the most used container runtime, so I’ll use that as an example.

When you first start up Docker, there will probably be no images on your host machine. You can check this by running:

docker image list

 This will list the images you have downloaded onto your machine. If you have no images on your machine, you will only see the headers:


To get an image from Docker’s image repository, Docker Hub, you run the following:

docker pull nginx:latest

This tells Docker to pull the `nginx` image tagged as `latest` from Docker Hub. You can omit the `:latest` if you want (Docker pulls the image tagged with `latest` by default). Running the `docker image list` command again will show:

nginx      latest  0c404972e130  6 days ago  135MB

The IMAGE ID, CREATED, and SIZE values may differ if the image has been updated on Docker Hub since the writing of this post, but you can see you have an image on your host machine, now.

The IMAGE ID, CREATED, and SIZE values may differ if the image has been updated on Docker Hub since the writing of this post, but you can see you have an image on your host machine, now.

To run a container from this image, run the command:

docker run -d nginx

This should spit out a nice long hash, like:


The container based on the Nginx image is now running. You can confirm by running:

docker container list

This will list only the running containers on your host machine, so your list should now look like this:

CONTAINER ID   IMAGE     COMMAND                    CREATED         STATUS         PORTS     NAMES
79536f1d2569   nginx     "/docker-entrypoint.…"     4 minutes ago   Up 4 minutes   80/tcp    stupefied_shannon

The output will likely wrap, but you can see that the `CONTAINER ID` is the first part of that long hash, the `IMAGE` is `nginx`, and the `COMMAND` is a `/docker-entrypoint…` command. This is the main command the container runs. Usually, a container runs only one process, and that’s the process that keeps the container running. If that command crashes, the container will stop. The rest is mainly ancillary information like when it was created, what its current status is, any ports the container may have open, and the name of the container. In this case, it’s a made-up name (made from an adjective and a famous computer scientist’s last name (these can get very funny).

To stop the container, run:

docker stop stupefied_shannon

Here, I used the name of the container to stop it, but you could also use the container’s ID. Not the whole big number, just enough of the digits to make it unique in the list of containers. So in this case, you could refer to the container as `7`, because no other containers in the list start with `7`. The first three or four digits are usually enough to make it unique.

To ensure it’s gone, run the container listing command with a `-a` switch; this lists all containers regardless of whether they are running.

docker container list -a

You can remove the image from your host with:

docker rmi nginx:latest


docker image list

Should now have you back to an empty list (assuming it was originally empty).

Why should I care about containers?

This sounds interesting, huh? But what do you do with it? Why would you want to use containers?

Lighter weight than VMs. VMs are great for some things, but being easy to move is not one of them. The lighter weight of containers makes them easier to destroy and recreate. 

Made for modern software architectures. It also means that microservice applications can deliver each microservice in its own container, making it look and act more like it’s on different machines. Using Docker Compose can even allow you to take action on the entire architecture with a single command!

No more, “It works on my machine.” Every programmer has probably said this (I know I have). It happens when a bug shows up on staging or worse, production, that doesn’t happen in the development environment. These can be tricky to debug and usually turn out to be some small utility that is installed on the dev machines, but not the staging or production machines. Since the container has all that it needs to run the application all bundled together, this isn’t a common problem.

Easy scaling. Spinning up new containers and load-balancing them is pretty easy. Using an orchestrator, like Kubernetes, can even make scaling (up and down) automatic.

Employability. The tooling around containers gets better every day and more and more companies are taking advantage of containers in their tech stacks, so there’s a good chance you will run into companies who need container expertise while job hunting.

Where can I learn more about containers?

Check out the blog for other great content about containers using Docker!

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