Stories about Software


How to Create Good Abstractions: Oracles and Magic Boxes

Oracles In Math

When I was a freshman in college, I took a class called “Great Theoretical Ideas In Computer Science”, taught by Professor Steven Rudich. It was in this class that I began to understand how fundamentally interwoven math and computer science are and to think of programming as a logical extension of math. We learned about concepts related to Game Theory, Cryptography, Logic Gates and P, NP, NP-Hard and NP-Complete problems. It is this last set that inspires today’s post.

This is not one of my “Practical Math” posts, so I will leave a more detailed description of these problems for another time, but I was introduced to a fascinating concept here: that you can make powerful statements about solutions to problems without actually having a solution to the problem. The mechanism for this was an incredibly cool-sounding concept called “The Problem Solving Oracle”. In the world of P and NP, we could make statements like “If I had a problem solving oracle that could solve problem X, then I know I could solve problem Y in polynomial (order n squared, n cubed, etc) time.” Don’t worry if you don’t understand the timing and particulars of the oracle — that doesn’t matter here. The important concept is “I don’t know how to solve X, but if I had a machine that could solve it, then I know some things about the solutions to these other problems.”

It was a powerful concept that intrigued me at the time, but more with grand visions of fast factoring and solving other famous math problems and making some kind of name for myself in the field of computer science related mathematics. Obviously, you’re not reading the blog of Fields Medal winning mathematician, Erik Dietrich, so my reach might have exceeded my grasp there. However, concepts like this don’t really fall out of my head so much as kind of marinate and lie dormant, to be re-appropriated for some other use.

Magic Boxes: Oracles For Programmers

One of the most important things that we do as developers and especially as designers/architects is to manage and isolate complexity. This is done by a variety of means, many of which have impressive-sounding terms like “loosely coupled”, “inverted control”, and “layered architecture”. But at their core, all of these approaches and architectural concepts have a basic underlying purpose: breaking problems apart into isolated and tackle-able smaller problems. To think of this another way, good architecture is about saying “assume that everything else in the world does its job perfectly — what’s necessary for your little corner of the world to do the same?”

That is why the Single Responsibility Principle is one of the most oft-cited principles in software design. This principle, in a way, says “divide up your problems until they’re as small as they can be without being non-trivial”. But it also implies “and just assume that everyone else is handling their business.”

Consider this recent post by John Sonmez, where he discusses deconstructing code into algorithms and coordinators (things responsible for getting the algorithms their inputs, more or less). As an example, he takes a “Calculator” class where the algorithms of calculation are mixed up with the particulars of storage and separates these concerns. This separates the very independent calculation algorithm from the coordinating storage details (which are of no consequence to calculations anyway) in support of his point, but it also provides the more general service of dividing up the problem at hand into smaller segments that take one another granted.

Another way to think of this is that his “Calculator_Mockless” class has two (easily testable) dependencies that it can trust to do their jobs. Going back to my undergrad days, Calculator_Mockless has two Oracles: one that performs calculations and the other that stores stuff. How do these things do their work? Calculator_Mockless doesn’t know or care about that; it just provides useful progress and feedback under the assumption that they do. This is certainly reminiscent of the criteria for “Oracle”, an assumption of functionality that allows further reasoning. However, “Oracle” has sort of a theoretical connotation in the math sense that I don’t intend to convey, so I’m going to adopt a new term for this concept in programming: “Magic Box”. John’s Calculator_Mockless says “I have two magic boxes — one for performing calculations and one for storing histories — and given these boxes that do those things, here’s how I’m going to proceed.”

How Spaghetti Code Is Born

It’s one thing to recognize the construction of Magic Boxes in the code, but how do you go about building and using them? Or, better yet, go about thinking in terms of them from the get-go? It’s a fairly sophisticated and not always intuitive method for deconstructing programming problems.

nullTo see what I mean, think of being assigned a hypothetical task to read an XML file full of names (right), remove any entries missing information, alphabetize the list by last name, and print out “First Last” with “President” pre-pended onto the string. So, for the picture here, the first line of the output should be “President Grover Cleveland”. You’ve got your assignment, now, quick, go – start picturing the code in your head!

What did you picture? What did you say to yourself? Was it something like “Well, I’d read the file in using the XDoc API and I’d probably use an IList<> implementer instead of IEnumerable<> to store these things since that makes sorting easier, and I’d probably do a foreach loop for the person in people in the document and while I was doing that write to the list I’ve created, and then it’d probably be better to check for the name attributes in advance than taking exceptions because that’d be more efficient and speaking of efficiency, it’d probably be best to append the president as I was reading them in rather than iterating through the whole loop again afterward, but then again we have to iterate through again to write them to the console since we don’t know where in the list it will fall in the first pass, but that’s fine since it’s still linear time in the file size, and…”

And slow down there, Sparky! You’re making my head hurt. Let’s back up a minute and count how many magic boxes we’ve built so far. I’m thinking zero — what do you think? Zero too? Good. So, what did we do there instead? Well, we embarked on kind of a willy-nilly, scattershot, and most importantly, procedural approach to this problem. We thought only in terms of the basic sequence of runtime operations and thus the concepts kind of all got jumbled together. We were thinking about exception handling while thinking about console output. We were thinking about file reading while thinking about sorting strings. We were thinking about runtime optimization while we were thinking about the XDocument API. We were thinking about the problem as a monolithic mass of all of its smaller components and thus about to get started laying the bedrock for some seriously weirdly coupled spaghetti code.

Cut that Spaghetti and Hide It in a Magic Box

Instead of doing that, let’s take a deep breath and consider what mini-problems exist in this little assignment. There’s the issue of reading files to find people. There’s the issue of sorting people. There’s the issue of pre-pending text onto the names of people. There’s the issue of writing people to console output. There’s the issue of modeling the people (a series of string tuples, a series of dynamic types, a series of anonymous types, a series of structs, etc?). Notice that we’re not solving anything — just stating problems. Also notice that we’re not talking at all about exception handling, O-notation or runtime optimization. We already have some problems to solve that are actually in the direct path of solving our main problem without inventing solutions to problems that no one yet has.

So what to tackle first? Well, since every problem we’ve mentioned has the word “people” in it and the “people” problem makes no mention of anything else, we probably ought to consider defining that concept first (reasoning this way will tell you what the most abstract concepts in your code base are — the ones that other things depend on while depending on nothing themselves). Let’s do that (TDD process and artifacts that I would normally use elided):

Well, that was pretty easy. So, what’s next? Well, the context of our mini-application involves getting people objects, doing things to them, and then outputting them. Chronologically, it would make the most sense to figure out how to do the file reads at this point. But “chronologically” is another word for “procedurally” and we want to build abstractions that we assemble like building blocks rather than steps in a recipe. Another, perhaps more advantageous way would be to tackle the next simplest task or to let the business decide which functionality should come next (please note, I’m not saying that there would be anything wrong with implementing the file I/O at this point — only that the rationale “it’s what happens first sequentially in the running code” is not a good rationale).

Let’s go with the more “agile” approach and assume that the users want a stripped down, minimal set of functionality as quickly as possible. This means that you’re going to create a full skeleton of the application and do your best to avoid throw-away code. The thing of paramount importance is having something to deliver to your users, so you’re going to want to focus mainly on displaying something to the user’s medium of choice: the console. So what to do? Well, imagine that you have a magic box that generates people for you and another one that somehow gets you those people. What would you do with them? How ’bout this:

Where does that enumeration come from? Well, that’s a problem to deal with once we’ve handled the console writing problem, which is our top priority. If we had a demo in 30 seconds, we could simply write a main that instantiated a “PersonConsoleWriter” and fed it some dummy data. Here’s a dead simple application that prints, well, nothing, but it’s functional from an output perspective:

What to do next? Well, we probably want to give this thing some actual people to shove through the pipeline or our customers won’t be very impressed. We could new up some people inline, but that’s not very impressive or helpful. Instead, let’s assume that we have a magic box that will fetch people out of the ether for us. Where do they come from? Who knows, and who cares — not main’s problem. Take a look:

Alright — that’s pretty magic box-ish. The only way it could be more so is if we just defined an interface, but that’s overkill for the moment and I’m following YAGNI. We can add an interface if thing later needs to pull customers out of a web service or something. At this point, if we had to do a demo, we could simply return the empty enumeration instead of throwing an exception or we could dummy up some people for the demo here. And the important thing to note is that now the thing that’s supposed to be generating people is the thing that’s generating the people — we just have to sort out the correct internal implementation later. Let’s take a look at main:

Well, that’s starting to look pretty good. We get people from the people reader and feed them to the people console writer. At this point, it becomes pretty trivial to add sorting:

But, if we were so inclined, we could easily look at main and say “I want a magic box that I hand a person collection to and it gives me back a sorted person collection, and that could be stubbed out as follows:

The same kind of logic could also be applied for the “pre-pend the word ‘President'” requirement. That could pretty trivially go into the console writer or you could abstract it out. So, what about the file logic? I’m not going to bother with it in this post, and do you know why? You’ve created enough magic boxes here — decoupled the program enough — that it practically writes itself. You use an XDocument to pop all Person nodes and read their attributes into First and Last name, skipping any nodes that don’t have both. With Linq to XML, how many lines of code do you think you need? Even without shortcuts, the jump from our stubbed implementation to the real one is small. And that’s the power of this approach — deconstruct problems using magic boxes until they’re all pretty simple.

Also notice another interesting benefit which is that the problems of runtime optimization and exception handling now become easier to sort out. The exception handling and expensive file read operations can be isolated to a single class and console writes, sorting, and other business processing need not be affected. You’re able to isolate difficult problems to have a smaller surface area in the code and to be handled as requirements and situation dictate rather than polluting the entire codebase with them.

Pulling Back to the General Case

I have studiously avoided discussion of how tests or TDD would factor into all of this, but if you’re at all familiar with testable code, it should be quite apparent that this methodology will result in testable components (or endpoints of the system, like file readers and console writers). There is a deep parallel between building magic boxes and writing testsable code — so much so that “build magic boxes” is great advice for how to make your code testable. The only leap from simply building boxes to writing testable classes is to remember to demand your dependencies in constructors, methods and setters, rather than instantiating them or going out and finding them. So if PersonStorageReader uses an XDocument to do its thing, pass that XDocument into its constructor.

But this concept of magic boxes runs deeper than maintainable code, TDD, or any of the other specific clean coding/design principles. It’s really about chunking problems into manageable bites. If you’re even writing code in a method and you find yourself thinking “ok, first I’ll do X, and then-” STOP! Don’t do anything yet! Instead, first ask yourself “if I had a magic box that did X so I didn’t have to worry about it here and now, what would that look like?” You don’t necessarily need to abstract every possible detail out of every possible place, but the exercise of simply considering it is beneficial. I promise. It will help you start viewing code elements as solvers of different problems and collaborators in a broader solution, instead of methodical, plodding steps in a gigantic recipe. It will also help you practice writing tighter, more discoverable and usable APIs since your first conception of them would be “what would I most like to be a client of right here?”

So do the lazy and idealistic thing – imagine that you have a magic box that will solve all of your problems for you. The leap from “magic box” to “collaborating interface” to “implemented functionality” is much simpler and faster than it seems when you’re isolating your problems. The alternative is to have a system that is one gigantic, procedural “magic box” of spaghetti and the “magic” part is that it works at all.

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Gene Hughson

Visualizing structure and collaboration is fundamental to design and this walks through that process very well…great post.

Erik Dietrich

Thanks for the kind words — I’m glad you liked the post. 🙂

Paul Matovich

I think doing TDD helps pull you into this mind set. You are thinging “I have this problem… What parts can I write that I can test?”

Erik Dietrich

I definitely agree with this. TDD is a very natural fit with this way of thinking. In fact, I already have post queued up for next week that walks through deconstructing problems from the perspective of TDD. It’s the calculating bowling scores code kata.


[…] complex, it’s easy for my mind to start swimming with possibilities, as mentioned in the magic boxes post. In that post, I talked about thinking in abstractions and breaking problems apart and alluded to […]


[…] which players could see which moves were available for a given piece. The emailer cited a couple of old posts of mine in which I touched on abstractions and the use of TDD. He is at the point of having […]

Peter DiSalvo

I read this post whenever my designs begin to stray toward chaos. I’ve lost count of how many times I’ve read this. I’m making “If I had a magic box…” my mantra.

Erik Dietrich

Glad to hear it! It’s nice to get the feedback that this helps someone besides me, too. Taking this approach really clarifies design as I’m writing code.


Just read this following a link from your TDD and Modeling a Chess Game post. This helped clarify thoughts that I’ve had about the Single Responsibility Principle. I especially appreciate the mention that while you could create an interface for the Person Storage Reader, but that it isn’t needed at this point and could easily be extracted out later if the need arises. Great post!

Erik Dietrich

Thanks — glad if it was helpful. It’s always helped clarify things for me to think in terms of “what would it be handy to shove off to some other class?”