What’s docker? I think a lot of folks that read my blog will already know what Docker is, however, I’ll give a brief explanation anyway. Docker is a wrapper around Linux Containers (LXC) written in Go. It uses some REST API’s to communicate with the docker daemon to be able to start up containers and relies on IPTables for networking along with the docker bridge.
Docker is a powerful tool because the same image I generate locally on my machine can be deployed into production, or the code can be committed and be made part of your CI/CD Pipeline. Other local development environment tools such as vagrant use virtual machines which are based on Xen, KVM, or VMWare. Unfortunately, building a vagrant box doesn’t really translate to building an image that can be deployed to AWS or Google Cloud. There are products that facilitate this like Packer (From HashiCorp as well), however, it’s a bit of a stretch to get developers to start using this, much less to start using it to bring up local environments.
|Host OS||Relatively, this is your machine or the server that has the container running on it.|
|Guest OS||This usually is reserved for virtual machines, but it’s fair to use the term when describing the operating system the container image is based on.|
|Ephemeral||Temporary or short lived.|
|Immutable||Indicates a lack of data persistence. No data is stored on something that is immutable. The underlying term actually means ‘replaceable’.|
Dockerfile The Dockerfile basically describes how to build your image. Dockerfiles are relatively simple. In fact, here’s the entire reference for Dockerfile, take a quick glance over it. You can do simple things like exposing ports to the host OS (making things externally accessible), you can attach volumes that can add persistence to Docker (Docker and Linux Containers are ephemeral and immutable by nature), you can add existing directories, etc… The Dockerfile is the quintessential for building custom images that run on Docker containers.
docker-compose Docker has an accompanying application called docker-compose. If you’re using OSX then the install for Docker already comes with it, if you’re using Linux or Windows you’ll need to download it separately.
Rarely do we ever just run one application by itself. Frequently we need to run an application, a data source, and any other piece of infrastructure it may be dependent on. In this example I’m going to use a real Dockerfile and docker-compose that I wrote for a FOSS project I’ve been working on. The docker-compose is yaml based and represents various container and images pairs as services.
Building our Dockerfile The framework I’m using is called Django which is python based. All I need is an OS with Python 3. All publicly available Docker images are based on some type of OS with all the non-essentials removed.
ENV sets an environment variable in Key Value format. If you had multiple you’d use multiple ENV’s.
RUN This indicates executes a command using the default shell. We’re making the preliminary directory called code in the root of the guest OS (note the prefixed slash).
WORKDIR Changes the working directory to
/code. The commands using this context are specific to
ADD Copies a file to your Docker image. You might’ve noticed there’s a command called
COPYthat is remarkably similar to
ADD. There’s two key differences.
- Add allows the
<src>to be a url.
- Add will decompress recognized compressed files.
- Add allows the
RUN I need pip to process my requirements.txt. We’re executing a normal shell command here, just as if we were sitting at the terminal.
ADD We’re now copying my existing code repository to Docker image.
That’s it! If all we wanted to do was build the image and run the container then Django would come up. You’d simply:
docker build .
This indicates to docker: Build my image, search for the Dockerfile in this directory (.)
docker create <image id>
At this point your docker container would run but would not be externally accessible (we never specified
version specifies which version of the docker-compose reference to use.
services Each top level is essentially a different container. Services are special because they can resolve each other by their service name. For instance, I can reach db from web by
db/image In this case I’m telling docker-compose to use the Docker Hub image for postgres. Notice there’s no label selected, Docker will assume latest.
db/environment I’m setting an environment variable in a Key: Value format. The postgres page on Docker Hub has some nice instructions on features they’ve put into the postgres image.
web/build I’m indicating to docker-compose to run a docker build in ‘.’. This will build my Docker image. If I had multiple custom images I’d just place a Dockerfile in different directories.
web/command this is the command to run when the container starts. Notice I lost my context behind
WORKDIRin the Dockerfile. There’s a way to fix this, but I wanted to point it out.
web/volumes I’m telling docker-compose that I want to mount /code as a volume. Changes I make in my code will be reflected into the container AND vice versa. I’ll get into this later.
web/ports This is the exposed port on the Host OS to the Guest OS.
web/depends_on This tells docker-compose that it can’t start the web container until the db container is online.
Using docker-compose. Before we were able to use the
docker command to build, create, run etc… Now that we’re using docker-compose though, we have to use the
docker-compose command. If you don’t, you can still reach your containers but they’ll be without some of the essential networking that docker-compose gives you.
You’ll need to create your code folder and generate a Django app, but after that simply run
docker-compose up -d. This launches docker-compose in daemon mode. Leave out
-d when you’re troubleshooting. You can run
docker-compose ps to get the names of your containers and run commands on them like this:
docker-compose run -it <container name> <command>.
Technically we’re all done, but I wanted to show you something neat. Remember how I said in docker-compose you can reference services by name and they’ll resolve? In case it didn’t click, here’s an excerpt from my Django settings.py:
My host is using ‘db’. That’s all for today, hopefully that’s helped get the gears spinning. There’s a lot more you can do with docker-compose and docker, so be sure to explore!