I bet you weren't expecting any sane developer to make this argument!

Well, to be fair, the title of this post is somewhat intentionally provocative¹. But anyway, you're here now; so it served its purpose. And make no mistake: I am going to defend global state—just possibly not in quite the way you might be expecting.

What we mean by global state

First, some background. If you're reading this, there's a good chance you're a developer so I probably don't have to explain this; but I think there are at least two forms of global state that come to mind in software. One form is global variables--values that are accessible and can be changed, globally, anywhere, by all of the code in a project. The other is constants--values that may be accessible to potentially every part of a program, but cannot be changed.

These aren't the only kinds of global state, though. I would argue that globality--apparently that's actually a word⁵--is a continuous (as opposed to discrete or binary) property. Meaning: it isn't just variables and constants; there are things in between as well. There are some values that will definitely change (e.g., a global cache), there are some that might (e.g., a feature flag), others that probably won't (static configuration values), and so on. There is also the question of a variable's impact, which is not the same as its scope, or how widely it happens to be accessible. One bit of state might define the language or dialect in which an entire UI is presented, for example, while another may simply indicate whether or not to display a certain dialog when an application loads.

So there's some sort of spectrum here (you know how I like spectrums), or at the very least there's more than just one or two kinds of global state.

What's wrong with global state?

Global state has got a pretty bad rap in most of the software development community. For one, use of global state can make it difficult to reason about a section of code: what's happening locally may be clear, but the far-reaching ramifications of the code probably aren't. Global state also tends to fuel the proliferation of implicit dependencies between software components, making it increasingly difficult to "pull apart" those components down the line and resulting in code that less testable and less reusable.

There are plenty more reasons. I'm not going to claim they're invalid. But a recent tweet by my old teammate Scott Muc got me thinking on this topic, and it compelled me to consider the dissenting opinion².

Sometimes convention based frameworks feel like it's just 0 param methods using hidden global variables.

— Scott Muc (@ScottMuc) April 22, 2013

Embedded in Scott's statement is the shared understanding that global variables are A Bad Thing³. When everyone takes that for granted, then you can say things like "blah blah blah global variables" and your audience will intuitively grasp that global variables is just a proxy for something bad. Similarly, if I said, "Feature X is the goto of Language Y," you would understand I'm saying something bad about Feature X.

Now, regarding the kinds of global state I described earlier: I would guess that most of us are much more sympathetic to constants than to global variables. Constants, at least don't cause the same action-at-a-distance confusion that variables can. However, they still exhibit the other drawback I mentioned of global state: that dependence may spread through a code base, inhibiting reusability.

So we like constants more than global variables, but we're not exactly thrilled about either of them. What does this imply about the spectrum of globality? Do we start at "good" and gradually move towards "bad" the more scope and impact some bit of global state has?

More to the point: if I acknowledge the drawbacks of global state, why do I also like convention-based frameworks? What might I disagree with about Scott's tweet?⁴

Deciding what should be global

I think the real question with global state comes down to considering a section of code, and deciding what should be flexible. That may sound obvious on one hand—variables are things that change, so we use variables where we want to allow flexibility—but on the other, I suspect the idea of making this decision feels somewhat foreign to lots of developers. Many of us have come to accept, almost without thinking, that flexibility is an inherent good. Invariant, inflexible aspects of a software system are perceived as weaknesses or poor design. Flexibility, modularity, reusability: these are all sort of rolled up into one big bundle of attributes we automatically ascribe to any "good" or "well-written" software.

But fundamentally, we human beings are limited in our mental capacity. Infinitely flexible systems are not in our best interest, for the simple reason that flexibility demands our attention⁶. And so choosing the scope of a variable is really a matter of deciding at what point some information should require our attention, and at what point we should be able to take it for granted.

There's a great section of Gödel, Escher, Bach by Douglas Hofstadter that I think captures this idea quite beautifully. It is a chapter in which some characters tinker with a magical TV set that can alter the reality of a football game they're watching. By turning some knobs, they can see players alternately catching the ball or fumbling, scoring a touchdown or getting blocked, etc. One character fiddles with the knobs very aggressively and the game changes much more dramatically, to everyone's surprise.

Hofstadter goes on to compare the functioning of these knobs on the TV to "constants," "parameters," and "variables"--terms that should be quite familiar to any software developer:

We build up our mental representation of a situation layer by layer. The lowest layer establishes the deepest aspect of the context—sometimes being so low that it cannot vary at all. For instance, the three-dimensionality of our world is so ingrained that most of us never would imagine letting it slip mentally. It is a constant constant. Then there are layers which establish temporarily, though not permanently, fixed aspects of situations, which could be called background assumptions—things which, in the back of your mind, you know can vary, but which most of the time you unquestioningly accept as unchanging aspects. These could still be called "constants". For instance, when you go to a football game, the rules of the game are constants of that sort. Then there are "parameters": you think of them as more variable, but you temporarily hold them constant. At a football game, parameters might include the weather, the opposing team, and so forth. There could be—and probably are—several layers of parameters. Finally, we reach the "shakiest" aspects of your mental representation of the situation—the variables. These are things such as [a player] stepping out of bounds, which are mentally "loose" and which you don't mind letting slip away from their real values, for a short moment.

Really, I probably could have skipped everything I've written so far and just started with that Hofstadter quote. But what's done is done, so let's move on now.

The relationship between explicit and global

It's very common for developers coming from more "conventional" backgrounds (har har) to be initially perplexed by the "magic"⁷ of convention-based frameworks such as Rails. This is a natural reaction for many of us with previous experience working on big balls of mud, with enough global state to give us nightmares. But when you consider the opposite approach—explicit configuration at every level—I think what you'll find is a lot of boilerplate⁸ and an indefensible number of YAGNI violations.

Which brings me to an objection I've been sensing through the ether while writing this. Somewhere out there, one of you is yelling at the screen: Global values and implicit values are not the same thing! If it seems to you that I've been conflating the ideas of "global state" with "hidden information" until now, that's probably fair. But now chew on this for a second: the more global something is, the less explicit it should have to be.

Here's one of probably millions of examples I could give: when you walk up to someone in the U.S.[^sorry-non-american-readers], I'm guessing you don't start the conversation by asking, "Do you speak English?" The vast majority of people here speak English—it is effectively a global value in our country—which means we can be implicit in that knowledge when we interact. It saves time, and removes one potential variable of consideration--What language(s) does this individual speak?--from our minds, reducing cognitive load.

The pitfalls of always being explicit

Of course, implicitly understood global state unavoidably involves a learning curve. There is no way you can just walk into a foreign culture and expect to internally understand that culture without making a serious time investment, for example. But we all know that. It's why language immersion is widely accepted (last I checked) as by far the most effective way to become fluent in a natural language.

Imagine someone from another culture who "masters" English, perfectly grasping the vocabulary and all of the grammatical rules of the language, then visits the United States and becomes frustrated by all of our idioms ("beat a dead horse", "break a leg", etc.), cultural references ("good thinking, Sherlock"), and other random phrases whose meanings aren't obvious at the surface level. To such a person I'd say: "Look, it just takes time." That's the cost of really learning the language.

But let's take a step back and imagine what life would be like if we insisted on being explicit about everything instead. I might have an exchange like this with a coworker:

Coworker: How's Kathryn doing?
Me: Which Kathryn? [Kathryn is my wife's name.]

Or you might have this odd encounter with a stranger on the sidewalk:

You: What time is it?
Stranger: In which time zone?

A doctor might be vexed by an overly-explicit patient:

Doctor: How much do you weigh?
Patient: On which planet?

These things are what Hofstadter called "constants"; we may be able to imagine them changing—in some cases much more readily than others (e.g., changing time zones vs. changing planets)--but, generally speaking, we expect them not to. At the very least, they're certainly not going to change mid-conversation.

More importantly, there are very sensible defaults for the "ambiguous" values in these examples. If someone refers to "Kathryn" in a conversation with me, he or she is almost certainly referring to the Kathryn most important to me: my wife. If you ask someone what time it is, you're very likely asking about the time zone where you currently are. And in almost every conversation, when referring to properties of objects, we're specifically talking about how things are on Earth. All of this can remain implicit; we don't have to think about these things at all.

Hidden global variables can be our friends

I believe many of us have been trained to be overly explicit in developing software, preferring dependency injection and local configuration at every level of depth throughout a system. The classic example, which has received some criticism, is the common practice of writing any time-related code with an explicit dependency on an external Time or Clock object, making something as seemingly universal as time configurable. Adam Keys has an excellent blog post on this point, in which I think he gets to the heart of my argument pretty concisely:

Even adding one optional parameter to a method carries "mass".

The "mass" is, of course, the cognitive load I mentioned of having to think about things that are flexible and non-global. My gut tells me that this predilection is particularly egregious in the Java and .NET communities, but I've seen it in Ruby land and elsewhere as well. And what this does is force undue attention on all of this configuration, much of which, most of the time, we ought to be taking for granted so that our minds can concentrate on more interesting issues.

This is why I enjoy working with convention-based frameworks. They shed much of this extra "mass," enabling developers to take things for granted that would otherwise weigh us down.

So yes, maybe at its core convention over configuration is just a fancy name for hidden global variables. But they're carefully chosen variables, which—after an admittedly necessary learning curve—establish a deeper level of shared knowledge, freeing us to be more implicit and less explicit. And just like in real life, that's often a good thing.