That app you love, part 2: Immutable but flexible - What settings matter?

Welcome to the second installment of That App You Love, a blog series in which I show you how to you can make almost any app into a first-class cloud citizen. If you want to start from the beginning, jump back and check out Part 1: Making a connection.

In our last post, we met my ZNC container, good ol’ znc-cluster-app - but don’t fret about ZNC because we’re really talking about That App You Love - whatever it happens to be. You may recall that we got ZNC up and running using two commands - docker pull and docker run. And in a world where your local system happens to be a cloud computing environment, and containers never crash, you are good to go.

But we don’t live in that world, and neither does That App You Love. So we’ve got some work to do. For starters, we need to understand what is safe to “bake into” our image, versus what we should be able to adjust when the container is actually running.

In my case, I’m looking at ZNC, and I’ve determined that even though there are some other settings I could use, the minimum necessary information I need to configure ZNC is as follows:

• A port number
• SSL - yes / no?
• IPv6 - yes / no?
• A username
• A password
• Set up an IRC network - yes / no?

How did I determine that? ZNC happens to have a nice installer that you can invoke with znc --makeconf. I ran it, accepted every possible default, and noted the values I had to manually supply.

Technically Optional, Actually Non-Negotiable

Looking over those six required settings, I can see straight away that there are some that I don’t need to make optional in a running container:

• Port number - the port number that a container uses internally is almost always remapped to a different port on the host, so there’s not much point in making this setting configurable here.
• SSL - I can’t think of any reason why I wouldn’t want to run my app securely. I’m sticking this setting in the “on” position.
• IPv6 - I want this container to work everywhere, and IPv6 support is not available on all cloud platforms. So for now, I’ll pin this to “off”.
• Set up an IRC network? - This is a convenience function that enables me to set up networks before I actually run ZNC. Since I can set these up later in the app itself, I’m going to set this one to “no”.

In all four of these cases, I’m making decisions based on two things: how I want to use the app, and what my target platforms can support. So - what’s left?

Unique Snowflake Options

This series is called That App You Love, after all, not That App I Love. And in order to make ZNC as universally lovable as possible, we can look back over the variables and see what can and should be configurable: username and password.

But how do we expose those variables to user in an “immutable” container?

1. If your ZNC container is still running from Part 1, stop it by running:

   sudo docker stop znc

2. Now fire it up again with this command, but edit this line with whatever values you like for USERNAME and PASSWORD (and don’t forget to change 9999 to something else if you need to):

   sudo docker run -p 9999:6697 -d -e ZNC_USER=USERNAME -e ZNC_PASS=PASSWORD --name=znc nhripps/znc-cluster-app

   Note: it’s generally bad form to pass a password in clear text to a program. Don’t panic! We’ll do this in a much more secure way by the time this series is done.

3. Click this link (with port number change if necessary) to see the ZNC web console again:
   https://localhost:9999/
4. Log in. This time admin/admin won’t work unless you really drew a blank on creative values for USERNAME and PASSWORD and went with what I did.

But wait - how did our immutable container suddenly configure itself differently?

Environment Variables: The Ace Up My Sleeve

If you’ve got previous experience with docker containers, you are probably really unimpressed right now. “Dude,” you say, “all you did was exposed those two values as environment variables. That’s like, containers 101.” And you are correct! Environment variables are a major asset in our quest to cloud-enable That App You Love.

But the big question is: How did I actually link those environment variables to the app? And an even bigger question that we’ll answer later on - how did I link them in a way that can persist across multiple generations of containers?

Armed with the knowledge of which variables we want to lock in and which variables we want to expose for customization, we’ll head down the rabbit hole of configuring our container in the next part of this series. See you next time for Part 3: Every Setting In Its Place!

This series will run every Tuesday and Thursday until we've accomplished our goals, so stay tuned in, subscribe, and thanks for reading!

About the Author

Hi there! My name is N. Harrison Ripps, and I am an engineer and people manager on the Containers team at Red Hat. Together with the greater open source community, our team has taken the combination of the docker container format and Google’s Kubernetes orchestration system, and then extended this framework with a number of administrator- and developer-friendly features that we call the OpenShift Container Platform.