We were at the airport getting ready to go through security. A deep baritone voice shouted, “Everybody must take their shoes off and put them in the bin.” Hearing the instruction I told my son and daughter to take their shoes off and put them in the bin. When we got in line for the X-Ray machine, another man looked at my kids and said “Oh they don’t need to take their shoes off.” My wife and I looked at each other puzzled, “But the man over there said everyone take their shoes off.” “Oh, everyone except children under 12” he responded, as if that was the universal definition of “everybody”. I tell this story to highlight the idea that the words we choose to use matter a great deal when trying to convey an idea, thought or concept. Nowhere is this more true than the world of computing.

Immutable Infrastructure is one of those operational concepts that has been very popular, at least in conference talks. The idea isn’t particularly new, I remember building “golden images” in the 90’s. But there’s no doubt that the web, the rate of change and the tooling to support it has put the core concepts en vogue again. But is what we’re doing really immutable? I feel like it’s not. And while it may be a simple argument over words, we use the benefits of immutability in our arguments without any of the consequences that design choice incurs.

I often hear the argument that configuration management is on its way out, now that we’re ready to usher in an era of “immutable” infrastructure. You don’t push out new configurations, you build new images with the new configuration baked in and replace the existing nodes. How do we define configuration? That answer is simultaneously as concreate and maleable. I define configuration as

The applications, libraries, users, data and settings that are necessary to deliver the intended functionality of an application.

That’s a fairly broad definition, but so is configuration! Configuration management is the process (or absence of process) for managing the components in this list. Therefore if any one of these items is modified, that constitutes not just a change to your configuration, but a changer to your infrastructure as well.

Since we’ve defined configuration, what do we mean by immutability? (Or what do we as an industry mean by it) The traditional definition is

Not subject or susceptible to change or variation in form or quality or nature.

In the industry we boil it down to the basic meaning of “once it’s set, it never changes. A string is often immutable in programming languages. Though we give strings the appearance of mutability, in reality it’s a parlor trick to simplify development. But if you tell a developer that strings are immutable, it conveys a specific set of rules and the consequences for those rules.

What doe these definitions mean in practice? Let’s pretend it’s a normal Tuesday. There’s a 60% chance there’s a new OpenSSL package out and you need to update it. Rolling out a new OpenSSL package by creating a new image for your systems seems like a reasonable methodology. Now there’s a known good configuration of our system that we can replicate like-for-like in the environment. If you’re particularly good at it, getting the change rolled out takes you 30 minutes. (Making the change, pushing it, kicking off the image build process and then replacing, while dialing down traffic) For the rest of us mere mortals, it’s probably closer to a couple of hours. But regardless of time, immutable infrastructure wins!

Now lets pretend we’re in our testing environment. This obviously has a different set of nodes it communicates with vs production, so our configuration is different. We don’t want to maintain to separate images, one for production one for testing because that would rob us of our feeling of certainty about the images being the same. Of course we solve this with service discovery! Now instead of baking this configuration into the application, our nodes can use tools like Consul and Eureka to find the nodes it needs to communicate with. The image remains the same, but the applications configured on the image are neatly updated for reflect their running environment.

But isn’t that a change? And the definition of immutable was that the server doesn’t change. Are we more concerned that OpenSSL stays on the same version than we are about what database server an instance is talking to? I’m sure in the halls of Google, Netflix and LinkedIn, a point release of a library could have catastrophic consequences. But if you asked most of the industry “What frightens you more? Updating to the latest version of OpenSSL or updating worker_threads from 4 to 40?” I imagine most of us would choose the latter with absolutely zero context around what worker_threads is. Let’s wave our magic wand though and say service discovery has also relieved us of this particular concern. Let’s move on to something more basic, like user management.

In testing environments I have widely different access policies than I do for my production systems. I also have a completely different profile of users. In production, operations and a few developers are largely the only people that have access. In testing, development, QA and even product may have a login. How does that get managed? Do I shove that into service discovery as well? I could run LDAP in my environment, but that pushes my issue from “How do I manage users and keys?” to, “How do I manage access policy definitions for the LDAP configuration?”

This is all just to say that I’m incredibly confused about the Immutable Infrastructure conversation. In practice it doesn’t solve a whole host of concerns. Instead it pushes them around into a layer of the system that is often ill suited to the task. Or worse, the idealology simply ignores the failures caused by configuration changes and decides that “Immutable Infrastructure” is actually “Immutable Infrastructure, except for the most dangerous parts of the system”.

This doesn’t even tackle the idea that configuration management is still the best tool for…..wait for it…managing configuration, even if you’re using immutable infrastructure. Docker and Packer both transport us back to the early 90’s in their approach to defining configuration. It be a shame if the death of configuration management was as eminent as some people claim.

So what am I missing? Is there a piece of the puzzle that I’m not aware of? Am I being too pedantic in my definition of things? Or is there always an unexpressed qualifier when we say “immutable”?

Maybe words don’t matter.